googletest/docs/gmock_cook_book.md

gMock Cookbook

You can find recipes for using gMock here. If you haven't yet, please read the dummy guide first to make sure you understand the basics.

{: .callout .note} Note: gMock lives in the testing name space. For readability, it is recommended to write using ::testing::Foo; once in your file before using the name Foo defined by gMock. We omit such using statements in this section for brevity, but you should do it in your own code.

Creating Mock Classes

Mock classes are defined as normal classes, using the MOCK_METHOD macro to generate mocked methods. The macro gets 3 or 4 parameters:

class MyMock {
 public:
  MOCK_METHOD(ReturnType, MethodName, (Args...));
  MOCK_METHOD(ReturnType, MethodName, (Args...), (Specs...));
};

The first 3 parameters are simply the method declaration, split into 3 parts. The 4th parameter accepts a closed list of qualifiers, which affect the generated method:

• const - Makes the mocked method a const method. Required if overriding a const method.
• override - Marks the method with override. Recommended if overriding a virtual method.
• noexcept - Marks the method with noexcept. Required if overriding a noexcept method.
• Calltype(...) - Sets the call type for the method (e.g. to STDMETHODCALLTYPE), useful in Windows.
• ref(...) - Marks the method with the reference qualification specified. Required if overriding a method that has reference qualifications. Eg ref(&) or ref(&&).

Dealing with unprotected commas

Unprotected commas, i.e. commas which are not surrounded by parentheses, prevent MOCK_METHOD from parsing its arguments correctly:

{: .bad}

class MockFoo {
 public:
  MOCK_METHOD(std::pair<bool, int>, GetPair, ());  // Won't compile!
  MOCK_METHOD(bool, CheckMap, (std::map<int, double>, bool));  // Won't compile!
};

Solution 1 - wrap with parentheses:

{: .good}

class MockFoo {
 public:
  MOCK_METHOD((std::pair<bool, int>), GetPair, ());
  MOCK_METHOD(bool, CheckMap, ((std::map<int, double>), bool));
};

Note that wrapping a return or argument type with parentheses is, in general, invalid C++. MOCK_METHOD removes the parentheses.

Solution 2 - define an alias:

{: .good}

class MockFoo {
 public:
  using BoolAndInt = std::pair<bool, int>;
  MOCK_METHOD(BoolAndInt, GetPair, ());
  using MapIntDouble = std::map<int, double>;
  MOCK_METHOD(bool, CheckMap, (MapIntDouble, bool));
};

Mocking Private or Protected Methods

You must always put a mock method definition (MOCK_METHOD) in a public: section of the mock class, regardless of the method being mocked being public, protected, or private in the base class. This allows ON_CALL and EXPECT_CALL to reference the mock function from outside of the mock class. (Yes, C++ allows a subclass to change the access level of a virtual function in the base class.) Example:

class Foo {
 public:
  ...
  virtual bool Transform(Gadget* g) = 0;

 protected:
  virtual void Resume();

 private:
  virtual int GetTimeOut();
};

class MockFoo : public Foo {
 public:
  ...
  MOCK_METHOD(bool, Transform, (Gadget* g), (override));

  // The following must be in the public section, even though the
  // methods are protected or private in the base class.
  MOCK_METHOD(void, Resume, (), (override));
  MOCK_METHOD(int, GetTimeOut, (), (override));
};

Mocking Overloaded Methods

You can mock overloaded functions as usual. No special attention is required:

class Foo {
  ...

  // Must be virtual as we'll inherit from Foo.
  virtual ~Foo();

  // Overloaded on the types and/or numbers of arguments.
  virtual int Add(Element x);
  virtual int Add(int times, Element x);

  // Overloaded on the const-ness of this object.
  virtual Bar& GetBar();
  virtual const Bar& GetBar() const;
};

class MockFoo : public Foo {
  ...
  MOCK_METHOD(int, Add, (Element x), (override));
  MOCK_METHOD(int, Add, (int times, Element x), (override));

  MOCK_METHOD(Bar&, GetBar, (), (override));
  MOCK_METHOD(const Bar&, GetBar, (), (const, override));
};

{: .callout .note} Note: if you don't mock all versions of the overloaded method, the compiler will give you a warning about some methods in the base class being hidden. To fix that, use using to bring them in scope:

class MockFoo : public Foo {
  ...
  using Foo::Add;
  MOCK_METHOD(int, Add, (Element x), (override));
  // We don't want to mock int Add(int times, Element x);
  ...
};

Mocking Class Templates

You can mock class templates just like any class.

template <typename Elem>
class StackInterface {
  ...
  // Must be virtual as we'll inherit from StackInterface.
  virtual ~StackInterface();

  virtual int GetSize() const = 0;
  virtual void Push(const Elem& x) = 0;
};

template <typename Elem>
class MockStack : public StackInterface<Elem> {
  ...
  MOCK_METHOD(int, GetSize, (), (override));
  MOCK_METHOD(void, Push, (const Elem& x), (override));
};

Mocking Non-virtual Methods

gMock can mock non-virtual functions to be used in Hi-perf dependency injection.

In this case, instead of sharing a common base class with the real class, your mock class will be unrelated to the real class, but contain methods with the same signatures. The syntax for mocking non-virtual methods is the same as mocking virtual methods (just don't add override):

// A simple packet stream class.  None of its members is virtual.
class ConcretePacketStream {
 public:
  void AppendPacket(Packet* new_packet);
  const Packet* GetPacket(size_t packet_number) const;
  size_t NumberOfPackets() const;
  ...
};

// A mock packet stream class.  It inherits from no other, but defines
// GetPacket() and NumberOfPackets().
class MockPacketStream {
 public:
  MOCK_METHOD(const Packet*, GetPacket, (size_t packet_number), (const));
  MOCK_METHOD(size_t, NumberOfPackets, (), (const));
  ...
};

Note that the mock class doesn't define AppendPacket(), unlike the real class. That's fine as long as the test doesn't need to call it.

Next, you need a way to say that you want to use ConcretePacketStream in production code, and use MockPacketStream in tests. Since the functions are not virtual and the two classes are unrelated, you must specify your choice at compile time (as opposed to run time).

One way to do it is to templatize your code that needs to use a packet stream. More specifically, you will give your code a template type argument for the type of the packet stream. In production, you will instantiate your template with ConcretePacketStream as the type argument. In tests, you will instantiate the same template with MockPacketStream. For example, you may write:

template <class PacketStream>
void CreateConnection(PacketStream* stream) { ... }

template <class PacketStream>
class PacketReader {
 public:
  void ReadPackets(PacketStream* stream, size_t packet_num);
};

Then you can use CreateConnection<ConcretePacketStream>() and PacketReader<ConcretePacketStream> in production code, and use CreateConnection<MockPacketStream>() and PacketReader<MockPacketStream> in tests.

  MockPacketStream mock_stream;
  EXPECT_CALL(mock_stream, ...)...;
  .. set more expectations on mock_stream ...
  PacketReader<MockPacketStream> reader(&mock_stream);
  ... exercise reader ...

Mocking Free Functions

It is not possible to directly mock a free function (i.e. a C-style function or a static method). If you need to, you can rewrite your code to use an interface (abstract class).

Instead of calling a free function (say, OpenFile) directly, introduce an interface for it and have a concrete subclass that calls the free function:

class FileInterface {
 public:
  ...
  virtual bool Open(const char* path, const char* mode) = 0;
};

class File : public FileInterface {
 public:
  ...
  bool Open(const char* path, const char* mode) override {
     return OpenFile(path, mode);
  }
};

Your code should talk to FileInterface to open a file. Now it's easy to mock out the function.

This may seem like a lot of hassle, but in practice you often have multiple related functions that you can put in the same interface, so the per-function syntactic overhead will be much lower.

If you are concerned about the performance overhead incurred by virtual functions, and profiling confirms your concern, you can combine this with the recipe for mocking non-virtual methods.

Alternatively, instead of introducing a new interface, you can rewrite your code to accept a std::function instead of the free function, and then use MockFunction to mock the std::function.

Old-Style MOCK_METHODn Macros

Before the generic MOCK_METHOD macro was introduced in 2018, mocks where created using a family of macros collectively called MOCK_METHODn. These macros are still supported, though migration to the new MOCK_METHOD is recommended.

The macros in the MOCK_METHODn family differ from MOCK_METHOD:

• The general structure is MOCK_METHODn(MethodName, ReturnType(Args)), instead of MOCK_METHOD(ReturnType, MethodName, (Args)).
• The number n must equal the number of arguments.
• When mocking a const method, one must use MOCK_CONST_METHODn.
• When mocking a class template, the macro name must be suffixed with _T.
• In order to specify the call type, the macro name must be suffixed with _WITH_CALLTYPE, and the call type is the first macro argument.

Old macros and their new equivalents:

Simple
Old	MOCK_METHOD1(Foo, bool(int))
New	MOCK_METHOD(bool, Foo, (int))
Const Method
Old	MOCK_CONST_METHOD1(Foo, bool(int))
New	MOCK_METHOD(bool, Foo, (int), (const))
Method in a Class Template
Old	MOCK_METHOD1_T(Foo, bool(int))
New	MOCK_METHOD(bool, Foo, (int))
Const Method in a Class Template
Old	MOCK_CONST_METHOD1_T(Foo, bool(int))
New	MOCK_METHOD(bool, Foo, (int), (const))
Method with Call Type
Old	MOCK_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))
New	MOCK_METHOD(bool, Foo, (int), (Calltype(STDMETHODCALLTYPE)))
Const Method with Call Type
Old	MOCK_CONST_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))
New	MOCK_METHOD(bool, Foo, (int), (const, Calltype(STDMETHODCALLTYPE)))
Method with Call Type in a Class Template
Old	MOCK_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))
New	MOCK_METHOD(bool, Foo, (int), (Calltype(STDMETHODCALLTYPE)))
Const Method with Call Type in a Class Template
Old	MOCK_CONST_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))
New	MOCK_METHOD(bool, Foo, (int), (const, Calltype(STDMETHODCALLTYPE)))

The Nice, the Strict, and the Naggy

If a mock method has no EXPECT_CALL spec but is called, we say that it's an "uninteresting call", and the default action (which can be specified using ON_CALL()) of the method will be taken. Currently, an uninteresting call will also by default cause gMock to print a warning.

However, sometimes you may want to ignore these uninteresting calls, and sometimes you may want to treat them as errors. gMock lets you make the decision on a per-mock-object basis.

Suppose your test uses a mock class MockFoo:

TEST(...) {
  MockFoo mock_foo;
  EXPECT_CALL(mock_foo, DoThis());
  ... code that uses mock_foo ...
}

If a method of mock_foo other than DoThis() is called, you will get a warning. However, if you rewrite your test to use NiceMock<MockFoo> instead, you can suppress the warning:

using ::testing::NiceMock;

TEST(...) {
  NiceMock<MockFoo> mock_foo;
  EXPECT_CALL(mock_foo, DoThis());
  ... code that uses mock_foo ...
}

NiceMock<MockFoo> is a subclass of MockFoo, so it can be used wherever MockFoo is accepted.

It also works if MockFoo's constructor takes some arguments, as NiceMock<MockFoo> "inherits" MockFoo's constructors:

using ::testing::NiceMock;

TEST(...) {
  NiceMock<MockFoo> mock_foo(5, "hi");  // Calls MockFoo(5, "hi").
  EXPECT_CALL(mock_foo, DoThis());
  ... code that uses mock_foo ...
}

The usage of StrictMock is similar, except that it makes all uninteresting calls failures:

using ::testing::StrictMock;

TEST(...) {
  StrictMock<MockFoo> mock_foo;
  EXPECT_CALL(mock_foo, DoThis());
  ... code that uses mock_foo ...

  // The test will fail if a method of mock_foo other than DoThis()
  // is called.
}

{: .callout .note} NOTE: NiceMock and StrictMock only affects uninteresting calls (calls of methods with no expectations); they do not affect unexpected calls (calls of methods with expectations, but they don't match). See Understanding Uninteresting vs Unexpected Calls.

There are some caveats though (sadly they are side effects of C++'s limitations):

1. NiceMock<MockFoo> and StrictMock<MockFoo> only work for mock methods defined using the MOCK_METHOD macro directly in the MockFoo class. If a mock method is defined in a base class of MockFoo, the "nice" or "strict" modifier may not affect it, depending on the compiler. In particular, nesting NiceMock and StrictMock (e.g. NiceMock<StrictMock<MockFoo> >) is not supported.
2. NiceMock<MockFoo> and StrictMock<MockFoo> may not work correctly if the destructor of MockFoo is not virtual. We would like to fix this, but it requires cleaning up existing tests.

Finally, you should be very cautious about when to use naggy or strict mocks, as they tend to make tests more brittle and harder to maintain. When you refactor your code without changing its externally visible behavior, ideally you shouldn't need to update any tests. If your code interacts with a naggy mock, however, you may start to get spammed with warnings as the result of your change. Worse, if your code interacts with a strict mock, your tests may start to fail and you'll be forced to fix them. Our general recommendation is to use nice mocks (not yet the default) most of the time, use naggy mocks (the current default) when developing or debugging tests, and use strict mocks only as the last resort.

Simplifying the Interface without Breaking Existing Code

Sometimes a method has a long list of arguments that is mostly uninteresting. For example:

class LogSink {
 public:
  ...
  virtual void send(LogSeverity severity, const char* full_filename,
                    const char* base_filename, int line,
                    const struct tm* tm_time,
                    const char* message, size_t message_len) = 0;
};

This method's argument list is lengthy and hard to work with (the message argument is not even 0-terminated). If we mock it as is, using the mock will be awkward. If, however, we try to simplify this interface, we'll need to fix all clients depending on it, which is often infeasible.

The trick is to redispatch the method in the mock class:

class ScopedMockLog : public LogSink {
 public:
  ...
  void send(LogSeverity severity, const char* full_filename,
                    const char* base_filename, int line, const tm* tm_time,
                    const char* message, size_t message_len) override {
    // We are only interested in the log severity, full file name, and
    // log message.
    Log(severity, full_filename, std::string(message, message_len));
  }

  // Implements the mock method:
  //
  //   void Log(LogSeverity severity,
  //            const string& file_path,
  //            const string& message);
  MOCK_METHOD(void, Log,
              (LogSeverity severity, const string& file_path,
               const string& message));
};

By defining a new mock method with a trimmed argument list, we make the mock class more user-friendly.

This technique may also be applied to make overloaded methods more amenable to mocking. For example, when overloads have been used to implement default arguments:

class MockTurtleFactory : public TurtleFactory {
 public:
  Turtle* MakeTurtle(int length, int weight) override { ... }
  Turtle* MakeTurtle(int length, int weight, int speed) override { ... }

  // the above methods delegate to this one:
  MOCK_METHOD(Turtle*, DoMakeTurtle, ());
};

This allows tests that don't care which overload was invoked to avoid specifying argument matchers:

ON_CALL(factory, DoMakeTurtle)
    .WillByDefault(Return(MakeMockTurtle()));

Alternative to Mocking Concrete Classes

Often you may find yourself using classes that don't implement interfaces. In order to test your code that uses such a class (let's call it Concrete), you may be tempted to make the methods of Concrete virtual and then mock it.

Try not to do that.

Making a non-virtual function virtual is a big decision. It creates an extension point where subclasses can tweak your class' behavior. This weakens your control on the class because now it's harder to maintain the class invariants. You should make a function virtual only when there is a valid reason for a subclass to override it.

Mocking concrete classes directly is problematic as it creates a tight coupling between the class and the tests - any small change in the class may invalidate your tests and make test maintenance a pain.

To avoid such problems, many programmers have been practicing "coding to interfaces": instead of talking to the Concrete class, your code would define an interface and talk to it. Then you implement that interface as an adaptor on top of Concrete. In tests, you can easily mock that interface to observe how your code is doing.

This technique incurs some overhead:

• You pay the cost of virtual function calls (usually not a problem).
• There is more abstraction for the programmers to learn.

However, it can also bring significant benefits in addition to better testability:

• Concrete's API may not fit your problem domain very well, as you may not be the only client it tries to serve. By designing your own interface, you have a chance to tailor it to your need - you may add higher-level functionalities, rename stuff, etc instead of just trimming the class. This allows you to write your code (user of the interface) in a more natural way, which means it will be more readable, more maintainable, and you'll be more productive.
• If Concrete's implementation ever has to change, you don't have to rewrite everywhere it is used. Instead, you can absorb the change in your implementation of the interface, and your other code and tests will be insulated from this change.

Some people worry that if everyone is practicing this technique, they will end up writing lots of redundant code. This concern is totally understandable. However, there are two reasons why it may not be the case:

• Different projects may need to use Concrete in different ways, so the best interfaces for them will be different. Therefore, each of them will have its own domain-specific interface on top of Concrete, and they will not be the same code.
• If enough projects want to use the same interface, they can always share it, just like they have been sharing Concrete. You can check in the interface and the adaptor somewhere near Concrete (perhaps in a contrib sub-directory) and let many projects use it.

You need to weigh the pros and cons carefully for your particular problem, but I'd like to assure you that the Java community has been practicing this for a long time and it's a proven effective technique applicable in a wide variety of situations. :-)

Delegating Calls to a Fake

Some times you have a non-trivial fake implementation of an interface. For example:

class Foo {
 public:
  virtual ~Foo() {}
  virtual char DoThis(int n) = 0;
  virtual void DoThat(const char* s, int* p) = 0;
};

class FakeFoo : public Foo {
 public:
  char DoThis(int n) override {
    return (n > 0) ? '+' :
           (n < 0) ? '-' : '0';
  }

  void DoThat(const char* s, int* p) override {
    *p = strlen(s);
  }
};

Now you want to mock this interface such that you can set expectations on it. However, you also want to use FakeFoo for the default behavior, as duplicating it in the mock object is, well, a lot of work.

When you define the mock class using gMock, you can have it delegate its default action to a fake class you already have, using this pattern:

class MockFoo : public Foo {
 public:
  // Normal mock method definitions using gMock.
  MOCK_METHOD(char, DoThis, (int n), (override));
  MOCK_METHOD(void, DoThat, (const char* s, int* p), (override));

  // Delegates the default actions of the methods to a FakeFoo object.
  // This must be called *before* the custom ON_CALL() statements.
  void DelegateToFake() {
    ON_CALL(*this, DoThis).WillByDefault([this](int n) {
      return fake_.DoThis(n);
    });
    ON_CALL(*this, DoThat).WillByDefault([this](const char* s, int* p) {
      fake_.DoThat(s, p);
    });
  }

 private:
  FakeFoo fake_;  // Keeps an instance of the fake in the mock.
};

With that, you can use MockFoo in your tests as usual. Just remember that if you don't explicitly set an action in an ON_CALL() or EXPECT_CALL(), the fake will be called upon to do it.:

using ::testing::_;

TEST(AbcTest, Xyz) {
  MockFoo foo;

  foo.DelegateToFake();  // Enables the fake for delegation.

  // Put your ON_CALL(foo, ...)s here, if any.

  // No action specified, meaning to use the default action.
  EXPECT_CALL(foo, DoThis(5));
  EXPECT_CALL(foo, DoThat(_, _));

  int n = 0;
  EXPECT_EQ(foo.DoThis(5), '+');  // FakeFoo::DoThis() is invoked.
  foo.DoThat("Hi", &n);  // FakeFoo::DoThat() is invoked.
  EXPECT_EQ(n, 2);
}

Some tips:

• If you want, you can still override the default action by providing your own ON_CALL() or using .WillOnce() / .WillRepeatedly() in EXPECT_CALL().

• In DelegateToFake(), you only need to delegate the methods whose fake implementation you intend to use.

• The general technique discussed here works for overloaded methods, but you'll need to tell the compiler which version you mean. To disambiguate a mock function (the one you specify inside the parentheses of ON_CALL()), use this technique; to disambiguate a fake function (the one you place inside Invoke()), use a static_cast to specify the function's type. For instance, if class Foo has methods char DoThis(int n) and bool DoThis(double x) const, and you want to invoke the latter, you need to write Invoke(&fake_, static_cast<bool (FakeFoo::*)(double) const>(&FakeFoo::DoThis)) instead of Invoke(&fake_, &FakeFoo::DoThis) (The strange-looking thing inside the angled brackets of static_cast is the type of a function pointer to the second DoThis() method.).

• Having to mix a mock and a fake is often a sign of something gone wrong. Perhaps you haven't got used to the interaction-based way of testing yet. Or perhaps your interface is taking on too many roles and should be split up. Therefore, don't abuse this. We would only recommend to do it as an intermediate step when you are refactoring your code.

Regarding the tip on mixing a mock and a fake, here's an example on why it may be a bad sign: Suppose you have a class System for low-level system operations. In particular, it does file and I/O operations. And suppose you want to test how your code uses System to do I/O, and you just want the file operations to work normally. If you mock out the entire System class, you'll have to provide a fake implementation for the file operation part, which suggests that System is taking on too many roles.

Instead, you can define a FileOps interface and an IOOps interface and split System's functionalities into the two. Then you can mock IOOps without mocking FileOps.

Delegating Calls to a Real Object

When using testing doubles (mocks, fakes, stubs, and etc), sometimes their behaviors will differ from those of the real objects. This difference could be either intentional (as in simulating an error such that you can test the error handling code) or unintentional. If your mocks have different behaviors than the real objects by mistake, you could end up with code that passes the tests but fails in production.

You can use the delegating-to-real technique to ensure that your mock has the same behavior as the real object while retaining the ability to validate calls. This technique is very similar to the delegating-to-fake technique, the difference being that we use a real object instead of a fake. Here's an example:

using ::testing::AtLeast;

class MockFoo : public Foo {
 public:
  MockFoo() {
    // By default, all calls are delegated to the real object.
    ON_CALL(*this, DoThis).WillByDefault([this](int n) {
      return real_.DoThis(n);
    });
    ON_CALL(*this, DoThat).WillByDefault([this](const char* s, int* p) {
      real_.DoThat(s, p);
    });
    ...
  }
  MOCK_METHOD(char, DoThis, ...);
  MOCK_METHOD(void, DoThat, ...);
  ...
 private:
  Foo real_;
};

...
  MockFoo mock;
  EXPECT_CALL(mock, DoThis())
      .Times(3);
  EXPECT_CALL(mock, DoThat("Hi"))
      .Times(AtLeast(1));
  ... use mock in test ...

With this, gMock will verify that your code made the right calls (with the right arguments, in the right order, called the right number of times, etc), and a real object will answer the calls (so the behavior will be the same as in production). This gives you the best of both worlds.

Delegating Calls to a Parent Class

Ideally, you should code to interfaces, whose methods are all pure virtual. In reality, sometimes you do need to mock a virtual method that is not pure (i.e, it already has an implementation). For example:

class Foo {
 public:
  virtual ~Foo();

  virtual void Pure(int n) = 0;
  virtual int Concrete(const char* str) { ... }
};

class MockFoo : public Foo {
 public:
  // Mocking a pure method.
  MOCK_METHOD(void, Pure, (int n), (override));
  // Mocking a concrete method.  Foo::Concrete() is shadowed.
  MOCK_METHOD(int, Concrete, (const char* str), (override));
};

Sometimes you may want to call Foo::Concrete() instead of MockFoo::Concrete(). Perhaps you want to do it as part of a stub action, or perhaps your test doesn't need to mock Concrete() at all (but it would be oh-so painful to have to define a new mock class whenever you don't need to mock one of its methods).

You can call Foo::Concrete() inside an action by:

...
  EXPECT_CALL(foo, Concrete).WillOnce([&foo](const char* str) {
    return foo.Foo::Concrete(str);
  });

or tell the mock object that you don't want to mock Concrete():

...
  ON_CALL(foo, Concrete).WillByDefault([&foo](const char* str) {
    return foo.Foo::Concrete(str);
  });

(Why don't we just write { return foo.Concrete(str); }? If you do that, MockFoo::Concrete() will be called (and cause an infinite recursion) since Foo::Concrete() is virtual. That's just how C++ works.)

Using Matchers

Matching Argument Values Exactly

You can specify exactly which arguments a mock method is expecting:

using ::testing::Return;
...
  EXPECT_CALL(foo, DoThis(5))
      .WillOnce(Return('a'));
  EXPECT_CALL(foo, DoThat("Hello", bar));

Using Simple Matchers

You can use matchers to match arguments that have a certain property:

using ::testing::NotNull;
using ::testing::Return;
...
  EXPECT_CALL(foo, DoThis(Ge(5)))  // The argument must be >= 5.
      .WillOnce(Return('a'));
  EXPECT_CALL(foo, DoThat("Hello", NotNull()));
      // The second argument must not be NULL.

A frequently used matcher is _, which matches anything:

  EXPECT_CALL(foo, DoThat(_, NotNull()));

Combining Matchers

You can build complex matchers from existing ones using AllOf(), AllOfArray(), AnyOf(), AnyOfArray() and Not():

using ::testing::AllOf;
using ::testing::Gt;
using ::testing::HasSubstr;
using ::testing::Ne;
using ::testing::Not;
...
  // The argument must be > 5 and != 10.
  EXPECT_CALL(foo, DoThis(AllOf(Gt(5),
                                Ne(10))));

  // The first argument must not contain sub-string "blah".
  EXPECT_CALL(foo, DoThat(Not(HasSubstr("blah")),
                          NULL));

Matchers are function objects, and parametrized matchers can be composed just like any other function. However because their types can be long and rarely provide meaningful information, it can be easier to express them with C++14 generic lambdas to avoid specifying types. For example,

using ::testing::Contains;
using ::testing::Property;

inline constexpr auto HasFoo = [](const auto& f) {
  return Property("foo", &MyClass::foo, Contains(f));
};
...
  EXPECT_THAT(x, HasFoo("blah"));

Casting Matchers

gMock matchers are statically typed, meaning that the compiler can catch your mistake if you use a matcher of the wrong type (for example, if you use Eq(5) to match a string argument). Good for you!

Sometimes, however, you know what you're doing and want the compiler to give you some slack. One example is that you have a matcher for long and the argument you want to match is int. While the two types aren't exactly the same, there is nothing really wrong with using a Matcher<long> to match an int - after all, we can first convert the int argument to a long losslessly before giving it to the matcher.

To support this need, gMock gives you the SafeMatcherCast<T>(m) function. It casts a matcher m to type Matcher<T>. To ensure safety, gMock checks that (let U be the type m accepts :

1. Type T can be implicitly cast to type U;
2. When both T and U are built-in arithmetic types (bool, integers, and floating-point numbers), the conversion from T to U is not lossy (in other words, any value representable by T can also be represented by U); and
3. When U is a reference, T must also be a reference (as the underlying matcher may be interested in the address of the U value).

The code won't compile if any of these conditions isn't met.

Here's one example:

using ::testing::SafeMatcherCast;

// A base class and a child class.
class Base { ... };
class Derived : public Base { ... };

class MockFoo : public Foo {
 public:
  MOCK_METHOD(void, DoThis, (Derived* derived), (override));
};

...
  MockFoo foo;
  // m is a Matcher<Base*> we got from somewhere.
  EXPECT_CALL(foo, DoThis(SafeMatcherCast<Derived*>(m)));

If you find SafeMatcherCast<T>(m) too limiting, you can use a similar function MatcherCast<T>(m). The difference is that MatcherCast works as long as you can static_cast type T to type U.

MatcherCast essentially lets you bypass C++'s type system (static_cast isn't always safe as it could throw away information, for example), so be careful not to misuse/abuse it.

Selecting Between Overloaded Functions

If you expect an overloaded function to be called, the compiler may need some help on which overloaded version it is.

To disambiguate functions overloaded on the const-ness of this object, use the Const() argument wrapper.

using ::testing::ReturnRef;

class MockFoo : public Foo {
  ...
  MOCK_METHOD(Bar&, GetBar, (), (override));
  MOCK_METHOD(const Bar&, GetBar, (), (const, override));
};

...
  MockFoo foo;
  Bar bar1, bar2;
  EXPECT_CALL(foo, GetBar())         // The non-const GetBar().
      .WillOnce(ReturnRef(bar1));
  EXPECT_CALL(Const(foo), GetBar())  // The const GetBar().
      .WillOnce(ReturnRef(bar2));

(Const() is defined by gMock and returns a const reference to its argument.)

To disambiguate overloaded functions with the same number of arguments but different argument types, you may need to specify the exact type of a matcher, either by wrapping your matcher in Matcher<type>(), or using a matcher whose type is fixed (TypedEq<type>, An<type>(), etc):

using ::testing::An;
using ::testing::Matcher;
using ::testing::TypedEq;

class MockPrinter : public Printer {
 public:
  MOCK_METHOD(void, Print, (int n), (override));
  MOCK_METHOD(void, Print, (char c), (override));
};

TEST(PrinterTest, Print) {
  MockPrinter printer;

  EXPECT_CALL(printer, Print(An<int>()));            // void Print(int);
  EXPECT_CALL(printer, Print(Matcher<int>(Lt(5))));  // void Print(int);
  EXPECT_CALL(printer, Print(TypedEq<char>('a')));   // void Print(char);

  printer.Print(3);
  printer.Print(6);
  printer.Print('a');
}

Performing Different Actions Based on the Arguments

When a mock method is called, the last matching expectation that's still active will be selected (think "newer overrides older"). So, you can make a method do different things depending on its argument values like this:

using ::testing::_;
using ::testing::Lt;
using ::testing::Return;
...
  // The default case.
  EXPECT_CALL(foo, DoThis(_))
      .WillRepeatedly(Return('b'));
  // The more specific case.
  EXPECT_CALL(foo, DoThis(Lt(5)))
      .WillRepeatedly(Return('a'));

Now, if foo.DoThis() is called with a value less than 5, 'a' will be returned; otherwise 'b' will be returned.

Matching Multiple Arguments as a Whole

Sometimes it's not enough to match the arguments individually. For example, we may want to say that the first argument must be less than the second argument. The With() clause allows us to match all arguments of a mock function as a whole. For example,

using ::testing::_;
using ::testing::Ne;
using ::testing::Lt;
...
  EXPECT_CALL(foo, InRange(Ne(0), _))
      .With(Lt());

says that the first argument of InRange() must not be 0, and must be less than the second argument.

The expression inside With() must be a matcher of type Matcher<std::tuple<A1, ..., An>>, where A1, ..., An are the types of the function arguments.

You can also write AllArgs(m) instead of m inside .With(). The two forms are equivalent, but .With(AllArgs(Lt())) is more readable than .With(Lt()).

You can use Args<k1, ..., kn>(m) to match the n selected arguments (as a tuple) against m. For example,

using ::testing::_;
using ::testing::AllOf;
using ::testing::Args;
using ::testing::Lt;
...
  EXPECT_CALL(foo, Blah)
      .With(AllOf(Args<0, 1>(Lt()), Args<1, 2>(Lt())));

says that Blah will be called with arguments x, y, and z where x < y < z. Note that in this example, it wasn't necessary to specify the positional matchers.

As a convenience and example, gMock provides some matchers for 2-tuples, including the Lt() matcher above. See Multi-argument Matchers for the complete list.

Note that if you want to pass the arguments to a predicate of your own (e.g. .With(Args<0, 1>(Truly(&MyPredicate)))), that predicate MUST be written to take a std::tuple as its argument; gMock will pass the n selected arguments as one single tuple to the predicate.

Using Matchers as Predicates

Have you noticed that a matcher is just a fancy predicate that also knows how to describe itself? Many existing algorithms take predicates as arguments (e.g. those defined in STL's <algorithm> header), and it would be a shame if gMock matchers were not allowed to participate.

Luckily, you can use a matcher where a unary predicate functor is expected by wrapping it inside the Matches() function. For example,

#include <algorithm>
#include <vector>

using ::testing::Matches;
using ::testing::Ge;

vector<int> v;
...
// How many elements in v are >= 10?
const int count = count_if(v.begin(), v.end(), Matches(Ge(10)));

Since you can build complex matchers from simpler ones easily using gMock, this gives you a way to conveniently construct composite predicates (doing the same using STL's <functional> header is just painful). For example, here's a predicate that's satisfied by any number that is >= 0, <= 100, and != 50:

using ::testing::AllOf;
using ::testing::Ge;
using ::testing::Le;
using ::testing::Matches;
using ::testing::Ne;
...
Matches(AllOf(Ge(0), Le(100), Ne(50)))

Using Matchers in googletest Assertions

See EXPECT_THAT in the Assertions Reference.

Using Predicates as Matchers

gMock provides a set of built-in matchers for matching arguments with expected values—see the Matchers Reference for more information. In case you find the built-in set lacking, you can use an arbitrary unary predicate function or functor as a matcher - as long as the predicate accepts a value of the type you want. You do this by wrapping the predicate inside the Truly() function, for example:

using ::testing::Truly;

int IsEven(int n) { return (n % 2) == 0 ? 1 : 0; }
...
  // Bar() must be called with an even number.
  EXPECT_CALL(foo, Bar(Truly(IsEven)));

Note that the predicate function / functor doesn't have to return bool. It works as long as the return value can be used as the condition in the statement if (condition) ....

Matching Arguments that Are Not Copyable

When you do an EXPECT_CALL(mock_obj, Foo(bar)), gMock saves away a copy of bar. When Foo() is called later, gMock compares the argument to Foo() with the saved copy of bar. This way, you don't need to worry about bar being modified or destroyed after the EXPECT_CALL() is executed. The same is true when you use matchers like Eq(bar), Le(bar), and so on.

But what if bar cannot be copied (i.e. has no copy constructor)? You could define your own matcher function or callback and use it with Truly(), as the previous couple of recipes have shown. Or, you may be able to get away from it if you can guarantee that bar won't be changed after the EXPECT_CALL() is executed. Just tell gMock that it should save a reference to bar, instead of a copy of it. Here's how:

using ::testing::Eq;
using ::testing::Lt;
...
  // Expects that Foo()'s argument == bar.
  EXPECT_CALL(mock_obj, Foo(Eq(std::ref(bar))));

  // Expects that Foo()'s argument < bar.
  EXPECT_CALL(mock_obj, Foo(Lt(std::ref(bar))));

Remember: if you do this, don't change bar after the EXPECT_CALL(), or the result is undefined.

Validating a Member of an Object

Often a mock function takes a reference to object as an argument. When matching the argument, you may not want to compare the entire object against a fixed object, as that may be over-specification. Instead, you may need to validate a certain member variable or the result of a certain getter method of the object. You can do this with Field() and Property(). More specifically,

Field(&Foo::bar, m)

is a matcher that matches a Foo object whose bar member variable satisfies matcher m.

Property(&Foo::baz, m)

is a matcher that matches a Foo object whose baz() method returns a value that satisfies matcher m.

For example:

Expression	Description
Field(&Foo::number, Ge(3))	Matches x where x.number >= 3.
Property(&Foo::name, StartsWith("John "))	Matches x where x.name() starts with "John ".

Note that in Property(&Foo::baz, ...), method baz() must take no argument and be declared as const. Don't use Property() against member functions that you do not own, because taking addresses of functions is fragile and generally not part of the contract of the function.

Field() and Property() can also match plain pointers to objects. For instance,

using ::testing::Field;
using ::testing::Ge;
...
Field(&Foo::number, Ge(3))

matches a plain pointer p where p->number >= 3. If p is NULL, the match will always fail regardless of the inner matcher.

What if you want to validate more than one members at the same time? Remember that there are AllOf() and AllOfArray().

Finally Field() and Property() provide overloads that take the field or property names as the first argument to include it in the error message. This can be useful when creating combined matchers.

using ::testing::AllOf;
using ::testing::Field;
using ::testing::Matcher;
using ::testing::SafeMatcherCast;

Matcher<Foo> IsFoo(const Foo& foo) {
  return AllOf(Field("some_field", &Foo::some_field, foo.some_field),
               Field("other_field", &Foo::other_field, foo.other_field),
               Field("last_field", &Foo::last_field, foo.last_field));
}

Validating the Value Pointed to by a Pointer Argument

C++ functions often take pointers as arguments. You can use matchers like IsNull(), NotNull(), and other comparison matchers to match a pointer, but what if you want to make sure the value pointed to by the pointer, instead of the pointer itself, has a certain property? Well, you can use the Pointee(m) matcher.

Pointee(m) matches a pointer if and only if m matches the value the pointer points to. For example:

using ::testing::Ge;
using ::testing::Pointee;
...
  EXPECT_CALL(foo, Bar(Pointee(Ge(3))));

expects foo.Bar() to be called with a pointer that points to a value greater than or equal to 3.

One nice thing about Pointee() is that it treats a NULL pointer as a match failure, so you can write Pointee(m) instead of

using ::testing::AllOf;
using ::testing::NotNull;
using ::testing::Pointee;
...
  AllOf(NotNull(), Pointee(m))

without worrying that a NULL pointer will crash your test.

Also, did we tell you that Pointee() works with both raw pointers and smart pointers (std::unique_ptr, std::shared_ptr, etc)?

What if you have a pointer to pointer? You guessed it - you can use nested Pointee() to probe deeper inside the value. For example, Pointee(Pointee(Lt(3))) matches a pointer that points to a pointer that points to a number less than 3 (what a mouthful...).

Defining a Custom Matcher Class

Most matchers can be simply defined using the MATCHER* macros, which are terse and flexible, and produce good error messages. However, these macros are not very explicit about the interfaces they create and are not always suitable, especially for matchers that will be widely reused.

For more advanced cases, you may need to define your own matcher class. A custom matcher allows you to test a specific invariant property of that object. Let's take a look at how to do so.

Imagine you have a mock function that takes an object of type Foo, which has an int bar() method and an int baz() method. You want to constrain that the argument's bar() value plus its baz() value is a given number. (This is an invariant.) Here's how we can write and use a matcher class to do so:

class BarPlusBazEqMatcher {
 public:
  using is_gtest_matcher = void;

  explicit BarPlusBazEqMatcher(int expected_sum)
      : expected_sum_(expected_sum) {}

  bool MatchAndExplain(const Foo& foo,
                       std::ostream* /* listener */) const {
    return (foo.bar() + foo.baz()) == expected_sum_;
  }

  void DescribeTo(std::ostream* os) const {
    *os << "bar() + baz() equals " << expected_sum_;
  }

  void DescribeNegationTo(std::ostream* os) const {
    *os << "bar() + baz() does not equal " << expected_sum_;
  }
 private:
  const int expected_sum_;
};

::testing::Matcher<const Foo&> BarPlusBazEq(int expected_sum) {
  return BarPlusBazEqMatcher(expected_sum);
}

...
  Foo foo;
  EXPECT_THAT(foo, BarPlusBazEq(5))...;

Matching Containers

Sometimes an STL container (e.g. list, vector, map, ...) is passed to a mock function and you may want to validate it. Since most STL containers support the == operator, you can write Eq(expected_container) or simply expected_container to match a container exactly.

Sometimes, though, you may want to be more flexible (for example, the first element must be an exact match, but the second element can be any positive number, and so on). Also, containers used in tests often have a small number of elements, and having to define the expected container out-of-line is a bit of a hassle.

You can use the ElementsAre() or UnorderedElementsAre() matcher in such cases:

using ::testing::_;
using ::testing::ElementsAre;
using ::testing::Gt;
...
  MOCK_METHOD(void, Foo, (const vector<int>& numbers), (override));
...
  EXPECT_CALL(mock, Foo(ElementsAre(1, Gt(0), _, 5)));

The above matcher says that the container must have 4 elements, which must be 1, greater than 0, anything, and 5 respectively.

If you instead write:

using ::testing::_;
using ::testing::Gt;
using ::testing::UnorderedElementsAre;
...
  MOCK_METHOD(void, Foo, (const vector<int>& numbers), (override));
...
  EXPECT_CALL(mock, Foo(UnorderedElementsAre(1, Gt(0), _, 5)));

It means that the container must have 4 elements, which (under some permutation) must be 1, greater than 0, anything, and 5 respectively.

As an alternative you can place the arguments in a C-style array and use ElementsAreArray() or UnorderedElementsAreArray() instead:

using ::testing::ElementsAreArray;
...
  // ElementsAreArray accepts an array of element values.
  const int expected_vector1[] = {1, 5, 2, 4, ...};
  EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector1)));

  // Or, an array of element matchers.
  Matcher<int> expected_vector2[] = {1, Gt(2), _, 3, ...};
  EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector2)));

In case the array needs to be dynamically created (and therefore the array size cannot be inferred by the compiler), you can give ElementsAreArray() an additional argument to specify the array size:

using ::testing::ElementsAreArray;
...
  int* const expected_vector3 = new int[count];
  ... fill expected_vector3 with values ...
  EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector3, count)));

Use Pair when comparing maps or other associative containers.

{% raw %}

using ::testing::UnorderedElementsAre;
using ::testing::Pair;
...
  absl::flat_hash_map<string, int> m = {{"a", 1}, {"b", 2}, {"c", 3}};
  EXPECT_THAT(m, UnorderedElementsAre(
      Pair("a", 1), Pair("b", 2), Pair("c", 3)));

{% endraw %}

Tips:

• ElementsAre*() can be used to match any container that implements the STL iterator pattern (i.e. it has a const_iterator type and supports begin()/end()), not just the ones defined in STL. It will even work with container types yet to be written - as long as they follows the above pattern.
• You can use nested ElementsAre*() to match nested (multi-dimensional) containers.
• If the container is passed by pointer instead of by reference, just write Pointee(ElementsAre*(...)).
• The order of elements matters for ElementsAre*(). If you are using it with containers whose element order are undefined (such as a std::unordered_map) you should use UnorderedElementsAre.

Sharing Matchers

Under the hood, a gMock matcher object consists of a pointer to a ref-counted implementation object. Copying matchers is allowed and very efficient, as only the pointer is copied. When the last matcher that references the implementation object dies, the implementation object will be deleted.

Therefore, if you have some complex matcher that you want to use again and again, there is no need to build it every time. Just assign it to a matcher variable and use that variable repeatedly! For example,

using ::testing::AllOf;
using ::testing::Gt;
using ::testing::Le;
using ::testing::Matcher;
...
  Matcher<int> in_range = AllOf(Gt(5), Le(10));
  ... use in_range as a matcher in multiple EXPECT_CALLs ...

Matchers must have no side-effects

{: .callout .warning} WARNING: gMock does not guarantee when or how many times a matcher will be invoked. Therefore, all matchers must be purely functional: they cannot have any side effects, and the match result must not depend on anything other than the matcher's parameters and the value being matched.

This requirement must be satisfied no matter how a matcher is defined (e.g., if it is one of the standard matchers, or a custom matcher). In particular, a matcher can never call a mock function, as that will affect the state of the mock object and gMock.

Setting Expectations

Knowing When to Expect

ON_CALL is likely the single most under-utilized construct in gMock.

There are basically two constructs for defining the behavior of a mock object: ON_CALL and EXPECT_CALL. The difference? ON_CALL defines what happens when a mock method is called, but doesn't imply any expectation on the method being called. EXPECT_CALL not only defines the behavior, but also sets an expectation that the method will be called with the given arguments, for the given number of times (and in the given order when you specify the order too).

Since EXPECT_CALL does more, isn't it better than ON_CALL? Not really. Every EXPECT_CALL adds a constraint on the behavior of the code under test. Having more constraints than necessary is baaad - even worse than not having enough constraints.

This may be counter-intuitive. How could tests that verify more be worse than tests that verify less? Isn't verification the whole point of tests?

The answer lies in what a test should verify. A good test verifies the contract of the code. If a test over-specifies, it doesn't leave enough freedom to the implementation. As a result, changing the implementation without breaking the contract (e.g. refactoring and optimization), which should be perfectly fine to do, can break such tests. Then you have to spend time fixing them, only to see them broken again the next time the implementation is changed.

Keep in mind that one doesn't have to verify more than one property in one test. In fact, it's a good style to verify only one thing in one test. If you do that, a bug will likely break only one or two tests instead of dozens (which case would you rather debug?). If you are also in the habit of giving tests descriptive names that tell what they verify, you can often easily guess what's wrong just from the test log itself.

So use ON_CALL by default, and only use EXPECT_CALL when you actually intend to verify that the call is made. For example, you may have a bunch of ON_CALLs in your test fixture to set the common mock behavior shared by all tests in the same group, and write (scarcely) different EXPECT_CALLs in different TEST_Fs to verify different aspects of the code's behavior. Compared with the style where each TEST has many EXPECT_CALLs, this leads to tests that are more resilient to implementational changes (and thus less likely to require maintenance) and makes the intent of the tests more obvious (so they are easier to maintain when you do need to maintain them).

If you are bothered by the "Uninteresting mock function call" message printed when a mock method without an EXPECT_CALL is called, you may use a NiceMock instead to suppress all such messages for the mock object, or suppress the message for specific methods by adding EXPECT_CALL(...).Times(AnyNumber()). DO NOT suppress it by blindly adding an EXPECT_CALL(...), or you'll have a test that's a pain to maintain.

Ignoring Uninteresting Calls

If you are not interested in how a mock method is called, just don't say anything about it. In this case, if the method is ever called, gMock will perform its default action to allow the test program to continue. If you are not happy with the default action taken by gMock, you can override it using DefaultValue<T>::Set() (described here) or ON_CALL().

Please note that once you expressed interest in a particular mock method (via EXPECT_CALL()), all invocations to it must match some expectation. If this function is called but the arguments don't match any EXPECT_CALL() statement, it will be an error.

Disallowing Unexpected Calls

If a mock method shouldn't be called at all, explicitly say so:

using ::testing::_;
...
  EXPECT_CALL(foo, Bar(_))
      .Times(0);

If some calls to the method are allowed, but the rest are not, just list all the expected calls:

using ::testing::AnyNumber;
using ::testing::Gt;
...
  EXPECT_CALL(foo, Bar(5));
  EXPECT_CALL(foo, Bar(Gt(10)))
      .Times(AnyNumber());

A call to foo.Bar() that doesn't match any of the EXPECT_CALL() statements will be an error.

Understanding Uninteresting vs Unexpected Calls

Uninteresting calls and unexpected calls are different concepts in gMock. Very different.

A call x.Y(...) is uninteresting if there's not even a single EXPECT_CALL(x, Y(...)) set. In other words, the test isn't interested in the x.Y() method at all, as evident in that the test doesn't care to say anything about it.

A call x.Y(...) is unexpected if there are some EXPECT_CALL(x, Y(...))s set, but none of them matches the call. Put another way, the test is interested in the x.Y() method (therefore it explicitly sets some EXPECT_CALL to verify how it's called); however, the verification fails as the test doesn't expect this particular call to happen.

An unexpected call is always an error, as the code under test doesn't behave the way the test expects it to behave.

By default, an uninteresting call is not an error, as it violates no constraint specified by the test. (gMock's philosophy is that saying nothing means there is no constraint.) However, it leads to a warning, as it might indicate a problem (e.g. the test author might have forgotten to specify a constraint).

In gMock, NiceMock and StrictMock can be used to make a mock class "nice" or "strict". How does this affect uninteresting calls and unexpected calls?

A nice mock suppresses uninteresting call warnings. It is less chatty than the default mock, but otherwise is the same. If a test fails with a default mock, it will also fail using a nice mock instead. And vice versa. Don't expect making a mock nice to change the test's result.

A strict mock turns uninteresting call warnings into errors. So making a mock strict may change the test's result.

Let's look at an example:

TEST(...) {
  NiceMock<MockDomainRegistry> mock_registry;
  EXPECT_CALL(mock_registry, GetDomainOwner("google.com"))
          .WillRepeatedly(Return("Larry Page"));

  // Use mock_registry in code under test.
  ... &mock_registry ...
}

The sole EXPECT_CALL here says that all calls to GetDomainOwner() must have "google.com" as the argument. If GetDomainOwner("yahoo.com") is called, it will be an unexpected call, and thus an error. Having a nice mock doesn't change the severity of an unexpected call.

So how do we tell gMock that GetDomainOwner() can be called with some other arguments as well? The standard technique is to add a "catch all" EXPECT_CALL:

  EXPECT_CALL(mock_registry, GetDomainOwner(_))
        .Times(AnyNumber());  // catches all other calls to this method.
  EXPECT_CALL(mock_registry, GetDomainOwner("google.com"))
        .WillRepeatedly(Return("Larry Page"));

Remember that _ is the wildcard matcher that matches anything. With this, if GetDomainOwner("google.com") is called, it will do what the second EXPECT_CALL says; if it is called with a different argument, it will do what the first EXPECT_CALL says.

Note that the order of the two EXPECT_CALLs is important, as a newer EXPECT_CALL takes precedence over an older one.

For more on uninteresting calls, nice mocks, and strict mocks, read "The Nice, the Strict, and the Naggy".

Ignoring Uninteresting Arguments

If your test doesn't care about the parameters (it only cares about the number or order of calls), you can often simply omit the parameter list:

  // Expect foo.Bar( ... ) twice with any arguments.
  EXPECT_CALL(foo, Bar).Times(2);

  // Delegate to the given method whenever the factory is invoked.
  ON_CALL(foo_factory, MakeFoo)
      .WillByDefault(&BuildFooForTest);

This functionality is only available when a method is not overloaded; to prevent unexpected behavior it is a compilation error to try to set an expectation on a method where the specific overload is ambiguous. You can work around this by supplying a simpler mock interface than the mocked class provides.

This pattern is also useful when the arguments are interesting, but match logic is substantially complex. You can leave the argument list unspecified and use SaveArg actions to save the values for later verification. If you do that, you can easily differentiate calling the method the wrong number of times from calling it with the wrong arguments.

Expecting Ordered Calls

Although an EXPECT_CALL() statement defined later takes precedence when gMock tries to match a function call with an expectation, by default calls don't have to happen in the order EXPECT_CALL() statements are written. For example, if the arguments match the matchers in the second EXPECT_CALL(), but not those in the first and third, then the second expectation will be used.

If you would rather have all calls occur in the order of the expectations, put the EXPECT_CALL() statements in a block where you define a variable of type InSequence:

using ::testing::_;
using ::testing::InSequence;

  {
    InSequence s;

    EXPECT_CALL(foo, DoThis(5));
    EXPECT_CALL(bar, DoThat(_))
        .Times(2);
    EXPECT_CALL(foo, DoThis(6));
  }

In this example, we expect a call to foo.DoThis(5), followed by two calls to bar.DoThat() where the argument can be anything, which are in turn followed by a call to foo.DoThis(6). If a call occurred out-of-order, gMock will report an error.

Expecting Partially Ordered Calls

Sometimes requiring everything to occur in a predetermined order can lead to brittle tests. For example, we may care about A occurring before both B and C, but aren't interested in the relative order of B and C. In this case, the test should reflect our real intent, instead of being overly constraining.

gMock allows you to impose an arbitrary DAG (directed acyclic graph) on the calls. One way to express the DAG is to use the After clause of EXPECT_CALL.

Another way is via the InSequence() clause (not the same as the InSequence class), which we borrowed from jMock 2. It's less flexible than After(), but more convenient when you have long chains of sequential calls, as it doesn't require you to come up with different names for the expectations in the chains. Here's how it works:

If we view EXPECT_CALL() statements as nodes in a graph, and add an edge from node A to node B wherever A must occur before B, we can get a DAG. We use the term "sequence" to mean a directed path in this DAG. Now, if we decompose the DAG into sequences, we just need to know which sequences each EXPECT_CALL() belongs to in order to be able to reconstruct the original DAG.

So, to specify the partial order on the expectations we need to do two things: first to define some Sequence objects, and then for each EXPECT_CALL() say which Sequence objects it is part of.

Expectations in the same sequence must occur in the order they are written. For example,

using ::testing::Sequence;
...
  Sequence s1, s2;

  EXPECT_CALL(foo, A())
      .InSequence(s1, s2);
  EXPECT_CALL(bar, B())
      .InSequence(s1);
  EXPECT_CALL(bar, C())
      .InSequence(s2);
  EXPECT_CALL(foo, D())
      .InSequence(s2);

specifies the following DAG (where s1 is A -> B, and s2 is A -> C -> D):

       +---> B
       |
  A ---|
       |
       +---> C ---> D

This means that A must occur before B and C, and C must occur before D. There's no restriction about the order other than these.

Controlling When an Expectation Retires

When a mock method is called, gMock only considers expectations that are still active. An expectation is active when created, and becomes inactive (aka retires) when a call that has to occur later has occurred. For example, in

using ::testing::_;
using ::testing::Sequence;
...
  Sequence s1, s2;

  EXPECT_CALL(log, Log(WARNING, _, "File too large."))      // #1
      .Times(AnyNumber())
      .InSequence(s1, s2);
  EXPECT_CALL(log, Log(WARNING, _, "Data set is empty."))   // #2
      .InSequence(s1);
  EXPECT_CALL(log, Log(WARNING, _, "User not found."))      // #3
      .InSequence(s2);

as soon as either #2 or #3 is matched, #1 will retire. If a warning "File too large." is logged after this, it will be an error.

Note that an expectation doesn't retire automatically when it's saturated. For example,

using ::testing::_;
...
  EXPECT_CALL(log, Log(WARNING, _, _));                     // #1
  EXPECT_CALL(log, Log(WARNING, _, "File too large."));     // #2

says that there will be exactly one warning with the message "File too large.". If the second warning contains this message too, #2 will match again and result in an upper-bound-violated error.

If this is not what you want, you can ask an expectation to retire as soon as it becomes saturated:

using ::testing::_;
...
  EXPECT_CALL(log, Log(WARNING, _, _));                     // #1
  EXPECT_CALL(log, Log(WARNING, _, "File too large."))      // #2
      .RetiresOnSaturation();

Here #2 can be used only once, so if you have two warnings with the message "File too large.", the first will match #2 and the second will match #1 - there will be no error.

Using Actions

Returning References from Mock Methods

If a mock function's return type is a reference, you need to use ReturnRef() instead of Return() to return a result:

using ::testing::ReturnRef;

class MockFoo : public Foo {
 public:
  MOCK_METHOD(Bar&, GetBar, (), (override));
};
...
  MockFoo foo;
  Bar bar;
  EXPECT_CALL(foo, GetBar())
      .WillOnce(ReturnRef(bar));
...

Returning Live Values from Mock Methods

The Return(x) action saves a copy of x when the action is created, and always returns the same value whenever it's executed. Sometimes you may want to instead return the live value of x (i.e. its value at the time when the action is executed.). Use either ReturnRef() or ReturnPointee() for this purpose.

If the mock function's return type is a reference, you can do it using ReturnRef(x), as shown in the previous recipe ("Returning References from Mock Methods"). However, gMock doesn't let you use ReturnRef() in a mock function whose return type is not a reference, as doing that usually indicates a user error. So, what shall you do?

Though you may be tempted, DO NOT use std::ref():

using ::testing::Return;

class MockFoo : public Foo {
 public:
  MOCK_METHOD(int, GetValue, (), (override));
};
...
  int x = 0;
  MockFoo foo;
  EXPECT_CALL(foo, GetValue())
      .WillRepeatedly(Return(std::ref(x)));  // Wrong!
  x = 42;
  EXPECT_EQ(foo.GetValue(), 42);

Unfortunately, it doesn't work here. The above code will fail with error:

Value of: foo.GetValue()
  Actual: 0
Expected: 42

The reason is that Return(*value*) converts value to the actual return type of the mock function at the time when the action is created, not when it is executed. (This behavior was chosen for the action to be safe when value is a proxy object that references some temporary objects.) As a result, std::ref(x) is converted to an int value (instead of a const int&) when the expectation is set, and Return(std::ref(x)) will always return 0.

ReturnPointee(pointer) was provided to solve this problem specifically. It returns the value pointed to by pointer at the time the action is executed:

using ::testing::ReturnPointee;
...
  int x = 0;
  MockFoo foo;
  EXPECT_CALL(foo, GetValue())
      .WillRepeatedly(ReturnPointee(&x));  // Note the & here.
  x = 42;
  EXPECT_EQ(foo.GetValue(), 42);  // This will succeed now.

Combining Actions

Want to do more than one thing when a function is called? That's fine. DoAll() allows you to do a sequence of actions every time. Only the return value of the last action in the sequence will be used.

using ::testing::_;
using ::testing::DoAll;

class MockFoo : public Foo {
 public:
  MOCK_METHOD(bool, Bar, (int n), (override));
};
...
  EXPECT_CALL(foo, Bar(_))
      .WillOnce(DoAll(action_1,
                      action_2,
                      ...
                      action_n));

Verifying Complex Arguments

If you want to verify that a method is called with a particular argument but the match criteria is complex, it can be difficult to distinguish between cardinality failures (calling the method the wrong number of times) and argument match failures. Similarly, if you are matching multiple parameters, it may not be easy to distinguishing which argument failed to match. For example:

  // Not ideal: this could fail because of a problem with arg1 or arg2, or maybe
  // just the method wasn't called.
  EXPECT_CALL(foo, SendValues(_, ElementsAre(1, 4, 4, 7), EqualsProto( ... )));

You can instead save the arguments and test them individually:

  EXPECT_CALL(foo, SendValues)
      .WillOnce(DoAll(SaveArg<1>(&actual_array), SaveArg<2>(&actual_proto)));
  ... run the test
  EXPECT_THAT(actual_array, ElementsAre(1, 4, 4, 7));
  EXPECT_THAT(actual_proto, EqualsProto( ... ));

Mocking Side Effects

Sometimes a method exhibits its effect not via returning a value but via side effects. For example, it may change some global state or modify an output argument. To mock side effects, in general you can define your own action by implementing ::testing::ActionInterface.

If all you need to do is to change an output argument, the built-in SetArgPointee() action is convenient:

using ::testing::_;
using ::testing::SetArgPointee;

class MockMutator : public Mutator {
 public:
  MOCK_METHOD(void, Mutate, (bool mutate, int* value), (override));
  ...
}
...
  MockMutator mutator;
  EXPECT_CALL(mutator, Mutate(true, _))
      .WillOnce(SetArgPointee<1>(5));

In this example, when mutator.Mutate() is called, we will assign 5 to the int variable pointed to by argument #1 (0-based).

SetArgPointee() conveniently makes an internal copy of the value you pass to it, removing the need to keep the value in scope and alive. The implication however is that the value must have a copy constructor and assignment operator.

If the mock method also needs to return a value as well, you can chain SetArgPointee() with Return() using DoAll(), remembering to put the Return() statement last:

using ::testing::_;
using ::testing::DoAll;
using ::testing::Return;
using ::testing::SetArgPointee;

class MockMutator : public Mutator {
 public:
  ...
  MOCK_METHOD(bool, MutateInt, (int* value), (override));
}
...
  MockMutator mutator;
  EXPECT_CALL(mutator, MutateInt(_))
      .WillOnce(DoAll(SetArgPointee<0>(5),
                      Return(true)));

Note, however, that if you use the ReturnOKWith() method, it will override the values provided by SetArgPointee() in the response parameters of your function call.

If the output argument is an array, use the SetArrayArgument<N>(first, last) action instead. It copies the elements in source range [first, last) to the array pointed to by the N-th (0-based) argument:

using ::testing::NotNull;
using ::testing::SetArrayArgument;

class MockArrayMutator : public ArrayMutator {
 public:
  MOCK_METHOD(void, Mutate, (int* values, int num_values), (override));
  ...
}
...
  MockArrayMutator mutator;
  int values[5] = {1, 2, 3, 4, 5};
  EXPECT_CALL(mutator, Mutate(NotNull(), 5))
      .WillOnce(SetArrayArgument<0>(values, values + 5));

This also works when the argument is an output iterator:

using ::testing::_;
using ::testing::SetArrayArgument;

class MockRolodex : public Rolodex {
 public:
  MOCK_METHOD(void, GetNames, (std::back_insert_iterator<vector<string>>),
              (override));
  ...
}
...
  MockRolodex rolodex;
  vector<string> names = {"George", "John", "Thomas"};
  EXPECT_CALL(rolodex, GetNames(_))
      .WillOnce(SetArrayArgument<0>(names.begin(), names.end()));

Changing a Mock Object's Behavior Based on the State

If you expect a call to change the behavior of a mock object, you can use ::testing::InSequence to specify different behaviors before and after the call:

using ::testing::InSequence;
using ::testing::Return;

...
  {
     InSequence seq;
     EXPECT_CALL(my_mock, IsDirty())
         .WillRepeatedly(Return(true));
     EXPECT_CALL(my_mock, Flush());
     EXPECT_CALL(my_mock, IsDirty())
         .WillRepeatedly(Return(false));
  }
  my_mock.FlushIfDirty();

This makes my_mock.IsDirty() return true before my_mock.Flush() is called and return false afterwards.

If the behavior change is more complex, you can store the effects in a variable and make a mock method get its return value from that variable:

using ::testing::_;
using ::testing::SaveArg;
using ::testing::Return;

ACTION_P(ReturnPointee, p) { return *p; }
...
  int previous_value = 0;
  EXPECT_CALL(my_mock, GetPrevValue)
      .WillRepeatedly(ReturnPointee(&previous_value));
  EXPECT_CALL(my_mock, UpdateValue)
      .WillRepeatedly(SaveArg<0>(&previous_value));
  my_mock.DoSomethingToUpdateValue();

Here my_mock.GetPrevValue() will always return the argument of the last UpdateValue() call.

Setting the Default Value for a Return Type

If a mock method's return type is a built-in C++ type or pointer, by default it will return 0 when invoked. Also, in C++ 11 and above, a mock method whose return type has a default constructor will return a default-constructed value by default. You only need to specify an action if this default value doesn't work for you.

Sometimes, you may want to change this default value, or you may want to specify a default value for types gMock doesn't know about. You can do this using the ::testing::DefaultValue class template:

using ::testing::DefaultValue;

class MockFoo : public Foo {
 public:
  MOCK_METHOD(Bar, CalculateBar, (), (override));
};


...
  Bar default_bar;
  // Sets the default return value for type Bar.
  DefaultValue<Bar>::Set(default_bar);

  MockFoo foo;

  // We don't need to specify an action here, as the default
  // return value works for us.
  EXPECT_CALL(foo, CalculateBar());

  foo.CalculateBar();  // This should return default_bar.

  // Unsets the default return value.
  DefaultValue<Bar>::Clear();

Please note that changing the default value for a type can make your tests hard to understand. We recommend you to use this feature judiciously. For example, you may want to make sure the Set() and Clear() calls are right next to the code that uses your mock.

Setting the Default Actions for a Mock Method

You've learned how to change the default value of a given type. However, this may be too coarse for your purpose: perhaps you have two mock methods with the same return type and you want them to have different behaviors. The ON_CALL() macro allows you to customize your mock's behavior at the method level:

using ::testing::_;
using ::testing::AnyNumber;
using ::testing::Gt;
using ::testing::Return;
...
  ON_CALL(foo, Sign(_))
      .WillByDefault(Return(-1));
  ON_CALL(foo, Sign(0))
      .WillByDefault(Return(0));
  ON_CALL(foo, Sign(Gt(0)))
      .WillByDefault(Return(1));

  EXPECT_CALL(foo, Sign(_))
      .Times(AnyNumber());

  foo.Sign(5);   // This should return 1.
  foo.Sign(-9);  // This should return -1.
  foo.Sign(0);   // This should return 0.

As you may have guessed, when there are more than one ON_CALL() statements, the newer ones in the order take precedence over the older ones. In other words, the last one that matches the function arguments will be used. This matching order allows you to set up the common behavior in a mock object's constructor or the test fixture's set-up phase and specialize the mock's behavior later.

Note that both ON_CALL and EXPECT_CALL have the same "later statements take precedence" rule, but they don't interact. That is, EXPECT_CALLs have their own precedence order distinct from the ON_CALL precedence order.

Using Functions/Methods/Functors/Lambdas as Actions

If the built-in actions don't suit you, you can use an existing callable (function, std::function, method, functor, lambda) as an action.

using ::testing::_; using ::testing::Invoke;

class MockFoo : public Foo {
 public:
  MOCK_METHOD(int, Sum, (int x, int y), (override));
  MOCK_METHOD(bool, ComplexJob, (int x), (override));
};

int CalculateSum(int x, int y) { return x + y; }
int Sum3(int x, int y, int z) { return x + y + z; }

class Helper {
 public:
  bool ComplexJob(int x);
};

...
  MockFoo foo;
  Helper helper;
  EXPECT_CALL(foo, Sum(_, _))
      .WillOnce(&CalculateSum)
      .WillRepeatedly(Invoke(NewPermanentCallback(Sum3, 1)));
  EXPECT_CALL(foo, ComplexJob(_))
      .WillOnce(Invoke(&helper, &Helper::ComplexJob))
      .WillOnce([] { return true; })
      .WillRepeatedly([](int x) { return x > 0; });

  foo.Sum(5, 6);         // Invokes CalculateSum(5, 6).
  foo.Sum(2, 3);         // Invokes Sum3(1, 2, 3).
  foo.ComplexJob(10);    // Invokes helper.ComplexJob(10).
  foo.ComplexJob(-1);    // Invokes the inline lambda.

The only requirement is that the type of the function, etc must be compatible with the signature of the mock function, meaning that the latter's arguments (if it takes any) can be implicitly converted to the corresponding arguments of the former, and the former's return type can be implicitly converted to that of the latter. So, you can invoke something whose type is not exactly the same as the mock function, as long as it's safe to do so - nice, huh?

Note that:

• The action takes ownership of the callback and will delete it when the action itself is destructed.

• If the type of a callback is derived from a base callback type C, you need to implicitly cast it to C to resolve the overloading, e.g.

  using ::testing::Invoke;
  ...
    ResultCallback<bool>* is_ok = ...;
    ... Invoke(is_ok) ...;  // This works.
  
    BlockingClosure* done = new BlockingClosure;
    ... Invoke(implicit_cast<Closure*>(done)) ...;  // The cast is necessary.
  

Using Functions with Extra Info as Actions

The function or functor you call using Invoke() must have the same number of arguments as the mock function you use it for. Sometimes you may have a function that takes more arguments, and you are willing to pass in the extra arguments yourself to fill the gap. You can do this in gMock using callbacks with pre-bound arguments. Here's an example:

using ::testing::Invoke;

class MockFoo : public Foo {
 public:
  MOCK_METHOD(char, DoThis, (int n), (override));
};

char SignOfSum(int x, int y) {
  const int sum = x + y;
  return (sum > 0) ? '+' : (sum < 0) ? '-' : '0';
}

TEST_F(FooTest, Test) {
  MockFoo foo;

  EXPECT_CALL(foo, DoThis(2))
      .WillOnce(Invoke(NewPermanentCallback(SignOfSum, 5)));
  EXPECT_EQ(foo.DoThis(2), '+');  // Invokes SignOfSum(5, 2).
}

Invoking a Function/Method/Functor/Lambda/Callback Without Arguments

Invoke() passes the mock function's arguments to the function, etc being invoked such that the callee has the full context of the call to work with. If the invoked function is not interested in some or all of the arguments, it can simply ignore them.

Yet, a common pattern is that a test author wants to invoke a function without the arguments of the mock function. She could do that using a wrapper function that throws away the arguments before invoking an underlining nullary function. Needless to say, this can be tedious and obscures the intent of the test.

There are two solutions to this problem. First, you can pass any callable of zero args as an action. Alternatively, use InvokeWithoutArgs(), which is like Invoke() except that it doesn't pass the mock function's arguments to the callee. Here's an example of each:

using ::testing::_;
using ::testing::InvokeWithoutArgs;

class MockFoo : public Foo {
 public:
  MOCK_METHOD(bool, ComplexJob, (int n), (override));
};

bool Job1() { ... }
bool Job2(int n, char c) { ... }

...
  MockFoo foo;
  EXPECT_CALL(foo, ComplexJob(_))
      .WillOnce([] { Job1(); });
      .WillOnce(InvokeWithoutArgs(NewPermanentCallback(Job2, 5, 'a')));

  foo.ComplexJob(10);  // Invokes Job1().
  foo.ComplexJob(20);  // Invokes Job2(5, 'a').

Note that:

• The action takes ownership of the callback and will delete it when the action itself is destructed.

• If the type of a callback is derived from a base callback type C, you need to implicitly cast it to C to resolve the overloading, e.g.

  using ::testing::InvokeWithoutArgs;
  ...
    ResultCallback<bool>* is_ok = ...;
    ... InvokeWithoutArgs(is_ok) ...;  // This works.
  
    BlockingClosure* done = ...;
    ... InvokeWithoutArgs(implicit_cast<Closure*>(done)) ...;
    // The cast is necessary.
  

Invoking an Argument of the Mock Function

Sometimes a mock function will receive a function pointer, a functor (in other words, a "callable") as an argument, e.g.

class MockFoo : public Foo {
 public:
  MOCK_METHOD(bool, DoThis, (int n, (ResultCallback1<bool, int>* callback)),
              (override));
};

and you may want to invoke this callable argument:

using ::testing::_;
...
  MockFoo foo;
  EXPECT_CALL(foo, DoThis(_, _))
      .WillOnce(...);
      // Will execute callback->Run(5), where callback is the
      // second argument DoThis() receives.

{: .callout .note} NOTE: The section below is legacy documentation from before C++ had lambdas:

Arghh, you need to refer to a mock function argument but C++ has no lambda (yet), so you have to define your own action. :-( Or do you really?

Well, gMock has an action to solve exactly this problem:

InvokeArgument<N>(arg_1, arg_2, ..., arg_m)

will invoke the N-th (0-based) argument the mock function receives, with arg_1, arg_2, ..., and arg_m. No matter if the argument is a function pointer, a functor, or a callback. gMock handles them all.

With that, you could write:

using ::testing::_;
using ::testing::InvokeArgument;
...
  EXPECT_CALL(foo, DoThis(_, _))
      .WillOnce(InvokeArgument<1>(5));
      // Will execute callback->Run(5), where callback is the
      // second argument DoThis() receives.

What if the callable takes an argument by reference? No problem - just wrap it inside std::ref():

  ...
  MOCK_METHOD(bool, Bar,
              ((ResultCallback2<bool, int, const Helper&>* callback)),
              (override));
  ...
  using ::testing::_;
  using ::testing::InvokeArgument;
  ...
  MockFoo foo;
  Helper helper;
  ...
  EXPECT_CALL(foo, Bar(_))
      .WillOnce(InvokeArgument<0>(5, std::ref(helper)));
      // std::ref(helper) guarantees that a reference to helper, not a copy of
      // it, will be passed to the callback.

What if the callable takes an argument by reference and we do not wrap the argument in std::ref()? Then InvokeArgument() will make a copy of the argument, and pass a reference to the copy, instead of a reference to the original value, to the callable. This is especially handy when the argument is a temporary value:

  ...
  MOCK_METHOD(bool, DoThat, (bool (*f)(const double& x, const string& s)),
              (override));
  ...
  using ::testing::_;
  using ::testing::InvokeArgument;
  ...
  MockFoo foo;
  ...
  EXPECT_CALL(foo, DoThat(_))
      .WillOnce(InvokeArgument<0>(5.0, string("Hi")));
      // Will execute (*f)(5.0, string("Hi")), where f is the function pointer
      // DoThat() receives.  Note that the values 5.0 and string("Hi") are
      // temporary and dead once the EXPECT_CALL() statement finishes.  Yet
      // it's fine to perform this action later, since a copy of the values
      // are kept inside the InvokeArgument action.

Ignoring an Action's Result

Sometimes you have an action that returns something, but you need an action that returns void (perhaps you want to use it in a mock function that returns void, or perhaps it needs to be used in DoAll() and it's not the last in the list). IgnoreResult() lets you do that. For example:

using ::testing::_;
using ::testing::DoAll;
using ::testing::IgnoreResult;
using ::testing::Return;

int Process(const MyData& data);
string DoSomething();

class MockFoo : public Foo {
 public:
  MOCK_METHOD(void, Abc, (const MyData& data), (override));
  MOCK_METHOD(bool, Xyz, (), (override));
};

  ...
  MockFoo foo;
  EXPECT_CALL(foo, Abc(_))
      // .WillOnce(Invoke(Process));
      // The above line won't compile as Process() returns int but Abc() needs
      // to return void.
      .WillOnce(IgnoreResult(Process));
  EXPECT_CALL(foo, Xyz())
      .WillOnce(DoAll(IgnoreResult(DoSomething),
                      // Ignores the string DoSomething() returns.
                      Return(true)));

Note that you cannot use IgnoreResult() on an action that already returns void. Doing so will lead to ugly compiler errors.

Selecting an Action's Arguments

Say you have a mock function Foo() that takes seven arguments, and you have a custom action that you want to invoke when Foo() is called. Trouble is, the custom action only wants three arguments:

using ::testing::_;
using ::testing::Invoke;
...
  MOCK_METHOD(bool, Foo,
              (bool visible, const string& name, int x, int y,
               (const map<pair<int, int>>), double& weight, double min_weight,
               double max_wight));
...
bool IsVisibleInQuadrant1(bool visible, int x, int y) {
  return visible && x >= 0 && y >= 0;
}
...
  EXPECT_CALL(mock, Foo)
      .WillOnce(Invoke(IsVisibleInQuadrant1));  // Uh, won't compile. :-(

To please the compiler God, you need to define an "adaptor" that has the same signature as Foo() and calls the custom action with the right arguments:

using ::testing::_;
using ::testing::Invoke;
...
bool MyIsVisibleInQuadrant1(bool visible, const string& name, int x, int y,
                            const map<pair<int, int>, double>& weight,
                            double min_weight, double max_wight) {
  return IsVisibleInQuadrant1(visible, x, y);
}
...
  EXPECT_CALL(mock, Foo)
      .WillOnce(Invoke(MyIsVisibleInQuadrant1));  // Now it works.

But isn't this awkward?

gMock provides a generic action adaptor, so you can spend your time minding more important business than writing your own adaptors. Here's the syntax:

WithArgs<N1, N2, ..., Nk>(action)

creates an action that passes the arguments of the mock function at the given indices (0-based) to the inner action and performs it. Using WithArgs, our original example can be written as:

using ::testing::_;
using ::testing::Invoke;
using ::testing::WithArgs;
...
  EXPECT_CALL(mock, Foo)
      .WillOnce(WithArgs<0, 2, 3>(Invoke(IsVisibleInQuadrant1)));  // No need to define your own adaptor.

For better readability, gMock also gives you:

• WithoutArgs(action) when the inner action takes no argument, and
• WithArg<N>(action) (no s after Arg) when the inner action takes one argument.

As you may have realized, InvokeWithoutArgs(...) is just syntactic sugar for WithoutArgs(Invoke(...)).

Here are more tips:

• The inner action used in WithArgs and friends does not have to be Invoke() -- it can be anything.
• You can repeat an argument in the argument list if necessary, e.g. WithArgs<2, 3, 3, 5>(...).
• You can change the order of the arguments, e.g. WithArgs<3, 2, 1>(...).
• The types of the selected arguments do not have to match the signature of the inner action exactly. It works as long as they can be implicitly converted to the corresponding arguments of the inner action. For example, if the 4-th argument of the mock function is an int and my_action takes a double, WithArg<4>(my_action) will work.

Ignoring Arguments in Action Functions

The selecting-an-action's-arguments recipe showed us one way to make a mock function and an action with incompatible argument lists fit together. The downside is that wrapping the action in WithArgs<...>() can get tedious for people writing the tests.

If you are defining a function (or method, functor, lambda, callback) to be used with Invoke*(), and you are not interested in some of its arguments, an alternative to WithArgs is to declare the uninteresting arguments as Unused. This makes the definition less cluttered and less fragile in case the types of the uninteresting arguments change. It could also increase the chance the action function can be reused. For example, given

 public:
  MOCK_METHOD(double, Foo, double(const string& label, double x, double y),
              (override));
  MOCK_METHOD(double, Bar, (int index, double x, double y), (override));

instead of

using ::testing::_;
using ::testing::Invoke;

double DistanceToOriginWithLabel(const string& label, double x, double y) {
  return sqrt(x*x + y*y);
}
double DistanceToOriginWithIndex(int index, double x, double y) {
  return sqrt(x*x + y*y);
}
...
  EXPECT_CALL(mock, Foo("abc", _, _))
      .WillOnce(Invoke(DistanceToOriginWithLabel));
  EXPECT_CALL(mock, Bar(5, _, _))
      .WillOnce(Invoke(DistanceToOriginWithIndex));

you could write

using ::testing::_;
using ::testing::Invoke;
using ::testing::Unused;

double DistanceToOrigin(Unused, double x, double y) {
  return sqrt(x*x + y*y);
}
...
  EXPECT_CALL(mock, Foo("abc", _, _))
      .WillOnce(Invoke(DistanceToOrigin));
  EXPECT_CALL(mock, Bar(5, _, _))
      .WillOnce(Invoke(DistanceToOrigin));

Sharing Actions

Just like matchers, a gMock action object consists of a pointer to a ref-counted implementation object. Therefore copying actions is also allowed and very efficient. When the last action that references the implementation object dies, the implementation object will be deleted.

If you have some complex action that you want to use again and again, you may not have to build it from scratch every time. If the action doesn't have an internal state (i.e. if it always does the same thing no matter how many times it has been called), you can assign it to an action variable and use that variable repeatedly. For example:

using ::testing::Action;
using ::testing::DoAll;
using ::testing::Return;
using ::testing::SetArgPointee;
...
  Action<bool(int*)> set_flag = DoAll(SetArgPointee<0>(5),
                                      Return(true));
  ... use set_flag in .WillOnce() and .WillRepeatedly() ...

However, if the action has its own state, you may be surprised if you share the action object. Suppose you have an action factory IncrementCounter(init) which creates an action that increments and returns a counter whose initial value is init, using two actions created from the same expression and using a shared action will exhibit different behaviors. Example:

  EXPECT_CALL(foo, DoThis())
      .WillRepeatedly(IncrementCounter(0));
  EXPECT_CALL(foo, DoThat())
      .WillRepeatedly(IncrementCounter(0));
  foo.DoThis();  // Returns 1.
  foo.DoThis();  // Returns 2.
  foo.DoThat();  // Returns 1 - Blah() uses a different
                 // counter than Bar()'s.

versus

using ::testing::Action;
...
  Action<int()> increment = IncrementCounter(0);
  EXPECT_CALL(foo, DoThis())
      .WillRepeatedly(increment);
  EXPECT_CALL(foo, DoThat())
      .WillRepeatedly(increment);
  foo.DoThis();  // Returns 1.
  foo.DoThis();  // Returns 2.
  foo.DoThat();  // Returns 3 - the counter is shared.

Testing Asynchronous Behavior

One oft-encountered problem with gMock is that it can be hard to test asynchronous behavior. Suppose you had a EventQueue class that you wanted to test, and you created a separate EventDispatcher interface so that you could easily mock it out. However, the implementation of the class fired all the events on a background thread, which made test timings difficult. You could just insert sleep() statements and hope for the best, but that makes your test behavior nondeterministic. A better way is to use gMock actions and Notification objects to force your asynchronous test to behave synchronously.

class MockEventDispatcher : public EventDispatcher {
  MOCK_METHOD(bool, DispatchEvent, (int32), (override));
};

TEST(EventQueueTest, EnqueueEventTest) {
  MockEventDispatcher mock_event_dispatcher;
  EventQueue event_queue(&mock_event_dispatcher);

  const int32 kEventId = 321;
  absl::Notification done;
  EXPECT_CALL(mock_event_dispatcher, DispatchEvent(kEventId))
      .WillOnce([&done] { done.Notify(); });

  event_queue.EnqueueEvent(kEventId);
  done.WaitForNotification();
}

In the example above, we set our normal gMock expectations, but then add an additional action to notify the Notification object. Now we can just call Notification::WaitForNotification() in the main thread to wait for the asynchronous call to finish. After that, our test suite is complete and we can safely exit.

{: .callout .note} Note: this example has a downside: namely, if the expectation is not satisfied, our test will run forever. It will eventually time-out and fail, but it will take longer and be slightly harder to debug. To alleviate this problem, you can use WaitForNotificationWithTimeout(ms) instead of WaitForNotification().

Misc Recipes on Using gMock

Mocking Methods That Use Move-Only Types

C++11 introduced move-only types. A move-only-typed value can be moved from one object to another, but cannot be copied. std::unique_ptr<T> is probably the most commonly used move-only type.

Mocking a method that takes and/or returns move-only types presents some challenges, but nothing insurmountable. This recipe shows you how you can do it. Note that the support for move-only method arguments was only introduced to gMock in April 2017; in older code, you may find more complex workarounds for lack of this feature.

Let’s say we are working on a fictional project that lets one post and share snippets called “buzzes”. Your code uses these types:

enum class AccessLevel { kInternal, kPublic };

class Buzz {
 public:
  explicit Buzz(AccessLevel access) { ... }
  ...
};

class Buzzer {
 public:
  virtual ~Buzzer() {}
  virtual std::unique_ptr<Buzz> MakeBuzz(StringPiece text) = 0;
  virtual bool ShareBuzz(std::unique_ptr<Buzz> buzz, int64_t timestamp) = 0;
  ...
};

A Buzz object represents a snippet being posted. A class that implements the Buzzer interface is capable of creating and sharing Buzzes. Methods in Buzzer may return a unique_ptr<Buzz> or take a unique_ptr<Buzz>. Now we need to mock Buzzer in our tests.

To mock a method that accepts or returns move-only types, you just use the familiar MOCK_METHOD syntax as usual:

class MockBuzzer : public Buzzer {
 public:
  MOCK_METHOD(std::unique_ptr<Buzz>, MakeBuzz, (StringPiece text), (override));
  MOCK_METHOD(bool, ShareBuzz, (std::unique_ptr<Buzz> buzz, int64_t timestamp),
              (override));
};

Now that we have the mock class defined, we can use it in tests. In the following code examples, we assume that we have defined a MockBuzzer object named mock_buzzer_:

  MockBuzzer mock_buzzer_;

First let’s see how we can set expectations on the MakeBuzz() method, which returns a unique_ptr<Buzz>.

As usual, if you set an expectation without an action (i.e. the .WillOnce() or .WillRepeatedly() clause), when that expectation fires, the default action for that method will be taken. Since unique_ptr<> has a default constructor that returns a null unique_ptr, that’s what you’ll get if you don’t specify an action:

using ::testing::IsNull;
...
  // Use the default action.
  EXPECT_CALL(mock_buzzer_, MakeBuzz("hello"));

  // Triggers the previous EXPECT_CALL.
  EXPECT_THAT(mock_buzzer_.MakeBuzz("hello"), IsNull());

If you are not happy with the default action, you can tweak it as usual; see Setting Default Actions.

If you just need to return a move-only value, you can use it in combination with WillOnce. For example:

  EXPECT_CALL(mock_buzzer_, MakeBuzz("hello"))
      .WillOnce(Return(std::make_unique<Buzz>(AccessLevel::kInternal)));
  EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("hello"));

Quiz time! What do you think will happen if a Return action is performed more than once (e.g. you write ... .WillRepeatedly(Return(std::move(...)));)? Come think of it, after the first time the action runs, the source value will be consumed (since it’s a move-only value), so the next time around, there’s no value to move from -- you’ll get a run-time error that Return(std::move(...)) can only be run once.

If you need your mock method to do more than just moving a pre-defined value, remember that you can always use a lambda or a callable object, which can do pretty much anything you want:

  EXPECT_CALL(mock_buzzer_, MakeBuzz("x"))
      .WillRepeatedly([](StringPiece text) {
        return std::make_unique<Buzz>(AccessLevel::kInternal);
      });

  EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("x"));
  EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("x"));

Every time this EXPECT_CALL fires, a new unique_ptr<Buzz> will be created and returned. You cannot do this with Return(std::make_unique<...>(...)).

That covers returning move-only values; but how do we work with methods accepting move-only arguments? The answer is that they work normally, although some actions will not compile when any of method's arguments are move-only. You can always use Return, or a lambda or functor:

  using ::testing::Unused;

  EXPECT_CALL(mock_buzzer_, ShareBuzz(NotNull(), _)).WillOnce(Return(true));
  EXPECT_TRUE(mock_buzzer_.ShareBuzz(std::make_unique<Buzz>(AccessLevel::kInternal)),
              0);

  EXPECT_CALL(mock_buzzer_, ShareBuzz(_, _)).WillOnce(
      [](std::unique_ptr<Buzz> buzz, Unused) { return buzz != nullptr; });
  EXPECT_FALSE(mock_buzzer_.ShareBuzz(nullptr, 0));

Many built-in actions (WithArgs, WithoutArgs,DeleteArg, SaveArg, ...) could in principle support move-only arguments, but the support for this is not implemented yet. If this is blocking you, please file a bug.

A few actions (e.g. DoAll) copy their arguments internally, so they can never work with non-copyable objects; you'll have to use functors instead.

Legacy workarounds for move-only types

Support for move-only function arguments was only introduced to gMock in April of 2017. In older code, you may encounter the following workaround for the lack of this feature (it is no longer necessary - we're including it just for reference):

class MockBuzzer : public Buzzer {
 public:
  MOCK_METHOD(bool, DoShareBuzz, (Buzz* buzz, Time timestamp));
  bool ShareBuzz(std::unique_ptr<Buzz> buzz, Time timestamp) override {
    return DoShareBuzz(buzz.get(), timestamp);
  }
};

The trick is to delegate the ShareBuzz() method to a mock method (let’s call it DoShareBuzz()) that does not take move-only parameters. Then, instead of setting expectations on ShareBuzz(), you set them on the DoShareBuzz() mock method:

  MockBuzzer mock_buzzer_;
  EXPECT_CALL(mock_buzzer_, DoShareBuzz(NotNull(), _));

  // When one calls ShareBuzz() on the MockBuzzer like this, the call is
  // forwarded to DoShareBuzz(), which is mocked.  Therefore this statement
  // will trigger the above EXPECT_CALL.
  mock_buzzer_.ShareBuzz(std::make_unique<Buzz>(AccessLevel::kInternal), 0);

Making the Compilation Faster

Believe it or not, the vast majority of the time spent on compiling a mock class is in generating its constructor and destructor, as they perform non-trivial tasks (e.g. verification of the expectations). What's more, mock methods with different signatures have different types and thus their constructors/destructors need to be generated by the compiler separately. As a result, if you mock many different types of methods, compiling your mock class can get really slow.

If you are experiencing slow compilation, you can move the definition of your mock class' constructor and destructor out of the class body and into a .cc file. This way, even if you #include your mock class in N files, the compiler only needs to generate its constructor and destructor once, resulting in a much faster compilation.

Let's illustrate the idea using an example. Here's the definition of a mock class before applying this recipe:

// File mock_foo.h.
...
class MockFoo : public Foo {
 public:
  // Since we don't declare the constructor or the destructor,
  // the compiler will generate them in every translation unit
  // where this mock class is used.

  MOCK_METHOD(int, DoThis, (), (override));
  MOCK_METHOD(bool, DoThat, (const char* str), (override));
  ... more mock methods ...
};

After the change, it would look like:

// File mock_foo.h.
...
class MockFoo : public Foo {
 public:
  // The constructor and destructor are declared, but not defined, here.
  MockFoo();
  virtual ~MockFoo();

  MOCK_METHOD(int, DoThis, (), (override));
  MOCK_METHOD(bool, DoThat, (const char* str), (override));
  ... more mock methods ...
};

and

// File mock_foo.cc.
#include "path/to/mock_foo.h"

// The definitions may appear trivial, but the functions actually do a
// lot of things through the constructors/destructors of the member
// variables used to implement the mock methods.
MockFoo::MockFoo() {}
MockFoo::~MockFoo() {}

Forcing a Verification

When it's being destroyed, your friendly mock object will automatically verify that all expectations on it have been satisfied, and will generate googletest failures if not. This is convenient as it leaves you with one less thing to worry about. That is, unless you are not sure if your mock object will be destroyed.

How could it be that your mock object won't eventually be destroyed? Well, it might be created on the heap and owned by the code you are testing. Suppose there's a bug in that code and it doesn't delete the mock object properly - you could end up with a passing test when there's actually a bug.

Using a heap checker is a good idea and can alleviate the concern, but its implementation is not 100% reliable. So, sometimes you do want to force gMock to verify a mock object before it is (hopefully) destructed. You can do this with Mock::VerifyAndClearExpectations(&mock_object):

TEST(MyServerTest, ProcessesRequest) {
  using ::testing::Mock;

  MockFoo* const foo = new MockFoo;
  EXPECT_CALL(*foo, ...)...;
  // ... other expectations ...

  // server now owns foo.
  MyServer server(foo);
  server.ProcessRequest(...);

  // In case that server's destructor will forget to delete foo,
  // this will verify the expectations anyway.
  Mock::VerifyAndClearExpectations(foo);
}  // server is destroyed when it goes out of scope here.

{: .callout .tip} Tip: The Mock::VerifyAndClearExpectations() function returns a bool to indicate whether the verification was successful (true for yes), so you can wrap that function call inside a ASSERT_TRUE() if there is no point going further when the verification has failed.

Do not set new expectations after verifying and clearing a mock after its use. Setting expectations after code that exercises the mock has undefined behavior. See Using Mocks in Tests for more information.

Using Checkpoints

Sometimes you might want to test a mock object's behavior in phases whose sizes are each manageable, or you might want to set more detailed expectations about which API calls invoke which mock functions.

A technique you can use is to put the expectations in a sequence and insert calls to a dummy "checkpoint" function at specific places. Then you can verify that the mock function calls do happen at the right time. For example, if you are exercising the code:

  Foo(1);
  Foo(2);
  Foo(3);

and want to verify that Foo(1) and Foo(3) both invoke mock.Bar("a"), but Foo(2) doesn't invoke anything, you can write:

using ::testing::MockFunction;

TEST(FooTest, InvokesBarCorrectly) {
  MyMock mock;
  // Class MockFunction<F> has exactly one mock method.  It is named
  // Call() and has type F.
  MockFunction<void(string check_point_name)> check;
  {
    InSequence s;

    EXPECT_CALL(mock, Bar("a"));
    EXPECT_CALL(check, Call("1"));
    EXPECT_CALL(check, Call("2"));
    EXPECT_CALL(mock, Bar("a"));
  }
  Foo(1);
  check.Call("1");
  Foo(2);
  check.Call("2");
  Foo(3);
}

The expectation spec says that the first Bar("a") call must happen before checkpoint "1", the second Bar("a") call must happen after checkpoint "2", and nothing should happen between the two checkpoints. The explicit checkpoints make it clear which Bar("a") is called by which call to Foo().

Mocking Destructors

Sometimes you want to make sure a mock object is destructed at the right time, e.g. after bar->A() is called but before bar->B() is called. We already know that you can specify constraints on the order of mock function calls, so all we need to do is to mock the destructor of the mock function.

This sounds simple, except for one problem: a destructor is a special function with special syntax and special semantics, and the MOCK_METHOD macro doesn't work for it:

MOCK_METHOD(void, ~MockFoo, ());  // Won't compile!

The good news is that you can use a simple pattern to achieve the same effect. First, add a mock function Die() to your mock class and call it in the destructor, like this:

class MockFoo : public Foo {
  ...
  // Add the following two lines to the mock class.
  MOCK_METHOD(void, Die, ());
  ~MockFoo() override { Die(); }
};

(If the name Die() clashes with an existing symbol, choose another name.) Now, we have translated the problem of testing when a MockFoo object dies to testing when its Die() method is called:

  MockFoo* foo = new MockFoo;
  MockBar* bar = new MockBar;
  ...
  {
    InSequence s;

    // Expects *foo to die after bar->A() and before bar->B().
    EXPECT_CALL(*bar, A());
    EXPECT_CALL(*foo, Die());
    EXPECT_CALL(*bar, B());
  }

And that's that.

Using gMock and Threads

In a unit test, it's best if you could isolate and test a piece of code in a single-threaded context. That avoids race conditions and dead locks, and makes debugging your test much easier.

Yet most programs are multi-threaded, and sometimes to test something we need to pound on it from more than one thread. gMock works for this purpose too.

Remember the steps for using a mock:

1. Create a mock object foo.
2. Set its default actions and expectations using ON_CALL() and EXPECT_CALL().
3. The code under test calls methods of foo.
4. Optionally, verify and reset the mock.
5. Destroy the mock yourself, or let the code under test destroy it. The destructor will automatically verify it.

If you follow the following simple rules, your mocks and threads can live happily together:

• Execute your test code (as opposed to the code being tested) in one thread. This makes your test easy to follow.
• Obviously, you can do step #1 without locking.
• When doing step #2 and #5, make sure no other thread is accessing foo. Obvious too, huh?
• #3 and #4 can be done either in one thread or in multiple threads - anyway you want. gMock takes care of the locking, so you don't have to do any - unless required by your test logic.

If you violate the rules (for example, if you set expectations on a mock while another thread is calling its methods), you get undefined behavior. That's not fun, so don't do it.

gMock guarantees that the action for a mock function is done in the same thread that called the mock function. For example, in

  EXPECT_CALL(mock, Foo(1))
      .WillOnce(action1);
  EXPECT_CALL(mock, Foo(2))
      .WillOnce(action2);

if Foo(1) is called in thread 1 and Foo(2) is called in thread 2, gMock will execute action1 in thread 1 and action2 in thread 2.

gMock does not impose a sequence on actions performed in different threads (doing so may create deadlocks as the actions may need to cooperate). This means that the execution of action1 and action2 in the above example may interleave. If this is a problem, you should add proper synchronization logic to action1 and action2 to make the test thread-safe.

Also, remember that DefaultValue<T> is a global resource that potentially affects all living mock objects in your program. Naturally, you won't want to mess with it from multiple threads or when there still are mocks in action.

Controlling How Much Information gMock Prints

When gMock sees something that has the potential of being an error (e.g. a mock function with no expectation is called, a.k.a. an uninteresting call, which is allowed but perhaps you forgot to explicitly ban the call), it prints some warning messages, including the arguments of the function, the return value, and the stack trace. Hopefully this will remind you to take a look and see if there is indeed a problem.

Sometimes you are confident that your tests are correct and may not appreciate such friendly messages. Some other times, you are debugging your tests or learning about the behavior of the code you are testing, and wish you could observe every mock call that happens (including argument values, the return value, and the stack trace). Clearly, one size doesn't fit all.

You can control how much gMock tells you using the --gmock_verbose=LEVEL command-line flag, where LEVEL is a string with three possible values:

• info: gMock will print all informational messages, warnings, and errors (most verbose). At this setting, gMock will also log any calls to the ON_CALL/EXPECT_CALL macros. It will include a stack trace in "uninteresting call" warnings.
• warning: gMock will print both warnings and errors (less verbose); it will omit the stack traces in "uninteresting call" warnings. This is the default.
• error: gMock will print errors only (least verbose).

Alternatively, you can adjust the value of that flag from within your tests like so:

  ::testing::FLAGS_gmock_verbose = "error";

If you find gMock printing too many stack frames with its informational or warning messages, remember that you can control their amount with the --gtest_stack_trace_depth=max_depth flag.

Now, judiciously use the right flag to enable gMock serve you better!

Gaining Super Vision into Mock Calls

You have a test using gMock. It fails: gMock tells you some expectations aren't satisfied. However, you aren't sure why: Is there a typo somewhere in the matchers? Did you mess up the order of the EXPECT_CALLs? Or is the code under test doing something wrong? How can you find out the cause?

Won't it be nice if you have X-ray vision and can actually see the trace of all EXPECT_CALLs and mock method calls as they are made? For each call, would you like to see its actual argument values and which EXPECT_CALL gMock thinks it matches? If you still need some help to figure out who made these calls, how about being able to see the complete stack trace at each mock call?

You can unlock this power by running your test with the --gmock_verbose=info flag. For example, given the test program:

#include "gmock/gmock.h"

using ::testing::_;
using ::testing::HasSubstr;
using ::testing::Return;

class MockFoo {
 public:
  MOCK_METHOD(void, F, (const string& x, const string& y));
};

TEST(Foo, Bar) {
  MockFoo mock;
  EXPECT_CALL(mock, F(_, _)).WillRepeatedly(Return());
  EXPECT_CALL(mock, F("a", "b"));
  EXPECT_CALL(mock, F("c", HasSubstr("d")));

  mock.F("a", "good");
  mock.F("a", "b");
}

if you run it with --gmock_verbose=info, you will see this output:

[ RUN       ] Foo.Bar

foo_test.cc:14: EXPECT_CALL(mock, F(_, _)) invoked
Stack trace: ...

foo_test.cc:15: EXPECT_CALL(mock, F("a", "b")) invoked
Stack trace: ...

foo_test.cc:16: EXPECT_CALL(mock, F("c", HasSubstr("d"))) invoked
Stack trace: ...

foo_test.cc:14: Mock function call matches EXPECT_CALL(mock, F(_, _))...
    Function call: F(@0x7fff7c8dad40"a",@0x7fff7c8dad10"good")
Stack trace: ...

foo_test.cc:15: Mock function call matches EXPECT_CALL(mock, F("a", "b"))...
    Function call: F(@0x7fff7c8dada0"a",@0x7fff7c8dad70"b")
Stack trace: ...

foo_test.cc:16: Failure
Actual function call count doesn't match EXPECT_CALL(mock, F("c", HasSubstr("d")))...
         Expected: to be called once
           Actual: never called - unsatisfied and active
[  FAILED  ] Foo.Bar

Suppose the bug is that the "c" in the third EXPECT_CALL is a typo and should actually be "a". With the above message, you should see that the actual F("a", "good") call is matched by the first EXPECT_CALL, not the third as you thought. From that it should be obvious that the third EXPECT_CALL is written wrong. Case solved.

If you are interested in the mock call trace but not the stack traces, you can combine --gmock_verbose=info with --gtest_stack_trace_depth=0 on the test command line.

Running Tests in Emacs

If you build and run your tests in Emacs using the M-x google-compile command (as many googletest users do), the source file locations of gMock and googletest errors will be highlighted. Just press <Enter> on one of them and you'll be taken to the offending line. Or, you can just type `C-x`` to jump to the next error.

To make it even easier, you can add the following lines to your ~/.emacs file:

(global-set-key "\M-m"  'google-compile)  ; m is for make
(global-set-key [M-down] 'next-error)
(global-set-key [M-up]  '(lambda () (interactive) (next-error -1)))

Then you can type M-m to start a build (if you want to run the test as well, just make sure foo_test.run or runtests is in the build command you supply after typing M-m), or M-up/M-down to move back and forth between errors.

Extending gMock

Writing New Matchers Quickly

{: .callout .warning} WARNING: gMock does not guarantee when or how many times a matcher will be invoked. Therefore, all matchers must be functionally pure. See this section for more details.

The MATCHER* family of macros can be used to define custom matchers easily. The syntax:

MATCHER(name, description_string_expression) { statements; }

will define a matcher with the given name that executes the statements, which must return a bool to indicate if the match succeeds. Inside the statements, you can refer to the value being matched by arg, and refer to its type by arg_type.

The description string is a string-typed expression that documents what the matcher does, and is used to generate the failure message when the match fails. It can (and should) reference the special bool variable negation, and should evaluate to the description of the matcher when negation is false, or that of the matcher's negation when negation is true.

For convenience, we allow the description string to be empty (""), in which case gMock will use the sequence of words in the matcher name as the description.

For example:

MATCHER(IsDivisibleBy7, "") { return (arg % 7) == 0; }

allows you to write

  // Expects mock_foo.Bar(n) to be called where n is divisible by 7.
  EXPECT_CALL(mock_foo, Bar(IsDivisibleBy7()));

or,

  using ::testing::Not;
  ...
  // Verifies that a value is divisible by 7 and the other is not.
  EXPECT_THAT(some_expression, IsDivisibleBy7());
  EXPECT_THAT(some_other_expression, Not(IsDivisibleBy7()));

If the above assertions fail, they will print something like:

  Value of: some_expression
  Expected: is divisible by 7
    Actual: 27
  ...
  Value of: some_other_expression
  Expected: not (is divisible by 7)
    Actual: 21

where the descriptions "is divisible by 7" and "not (is divisible by 7)" are automatically calculated from the matcher name IsDivisibleBy7.

As you may have noticed, the auto-generated descriptions (especially those for the negation) may not be so great. You can always override them with a string expression of your own:

MATCHER(IsDivisibleBy7,
        absl::StrCat(negation ? "isn't" : "is", " divisible by 7")) {
  return (arg % 7) == 0;
}

Optionally, you can stream additional information to a hidden argument named result_listener to explain the match result. For example, a better definition of IsDivisibleBy7 is:

MATCHER(IsDivisibleBy7, "") {
  if ((arg % 7) == 0)
    return true;

  *result_listener << "the remainder is " << (arg % 7);
  return false;
}

With this definition, the above assertion will give a better message:

  Value of: some_expression
  Expected: is divisible by 7
    Actual: 27 (the remainder is 6)

You should let MatchAndExplain() print any additional information that can help a user understand the match result. Note that it should explain why the match succeeds in case of a success (unless it's obvious) - this is useful when the matcher is used inside Not(). There is no need to print the argument value itself, as gMock already prints it for you.

{: .callout .note} NOTE: The type of the value being matched (arg_type) is determined by the context in which you use the matcher and is supplied to you by the compiler, so you don't need to worry about declaring it (nor can you). This allows the matcher to be polymorphic. For example, IsDivisibleBy7() can be used to match any type where the value of (arg % 7) == 0 can be implicitly converted to a bool. In the Bar(IsDivisibleBy7()) example above, if method Bar() takes an int, arg_type will be int; if it takes an unsigned long, arg_type will be unsigned long; and so on.

Writing New Parameterized Matchers Quickly

Sometimes you'll want to define a matcher that has parameters. For that you can use the macro:

MATCHER_P(name, param_name, description_string) { statements; }

where the description string can be either "" or a string expression that references negation and param_name.

For example:

MATCHER_P(HasAbsoluteValue, value, "") { return abs(arg) == value; }

will allow you to write:

  EXPECT_THAT(Blah("a"), HasAbsoluteValue(n));

which may lead to this message (assuming n is 10):

  Value of: Blah("a")
  Expected: has absolute value 10
    Actual: -9

Note that both the matcher description and its parameter are printed, making the message human-friendly.

In the matcher definition body, you can write foo_type to reference the type of a parameter named foo. For example, in the body of MATCHER_P(HasAbsoluteValue, value) above, you can write value_type to refer to the type of value.

gMock also provides MATCHER_P2, MATCHER_P3, ..., up to MATCHER_P10 to support multi-parameter matchers:

MATCHER_Pk(name, param_1, ..., param_k, description_string) { statements; }

Please note that the custom description string is for a particular instance of the matcher, where the parameters have been bound to actual values. Therefore usually you'll want the parameter values to be part of the description. gMock lets you do that by referencing the matcher parameters in the description string expression.

For example,

using ::testing::PrintToString;
MATCHER_P2(InClosedRange, low, hi,
           absl::StrFormat("%s in range [%s, %s]", negation ? "isn't" : "is",
                           PrintToString(low), PrintToString(hi))) {
  return low <= arg && arg <= hi;
}
...
EXPECT_THAT(3, InClosedRange(4, 6));

would generate a failure that contains the message:

  Expected: is in range [4, 6]

If you specify "" as the description, the failure message will contain the sequence of words in the matcher name followed by the parameter values printed as a tuple. For example,

  MATCHER_P2(InClosedRange, low, hi, "") { ... }
  ...
  EXPECT_THAT(3, InClosedRange(4, 6));

would generate a failure that contains the text:

  Expected: in closed range (4, 6)

For the purpose of typing, you can view

MATCHER_Pk(Foo, p1, ..., pk, description_string) { ... }

as shorthand for

template <typename p1_type, ..., typename pk_type>
FooMatcherPk<p1_type, ..., pk_type>
Foo(p1_type p1, ..., pk_type pk) { ... }

When you write Foo(v1, ..., vk), the compiler infers the types of the parameters v1, ..., and vk for you. If you are not happy with the result of the type inference, you can specify the types by explicitly instantiating the template, as in Foo<long, bool>(5, false). As said earlier, you don't get to (or need to) specify arg_type as that's determined by the context in which the matcher is used.

You can assign the result of expression Foo(p1, ..., pk) to a variable of type FooMatcherPk<p1_type, ..., pk_type>. This can be useful when composing matchers. Matchers that don't have a parameter or have only one parameter have special types: you can assign Foo() to a FooMatcher-typed variable, and assign Foo(p) to a FooMatcherP<p_type>-typed variable.

While you can instantiate a matcher template with reference types, passing the parameters by pointer usually makes your code more readable. If, however, you still want to pass a parameter by reference, be aware that in the failure message generated by the matcher you will see the value of the referenced object but not its address.

You can overload matchers with different numbers of parameters:

MATCHER_P(Blah, a, description_string_1) { ... }
MATCHER_P2(Blah, a, b, description_string_2) { ... }

While it's tempting to always use the MATCHER* macros when defining a new matcher, you should also consider implementing the matcher interface directly instead (see the recipes that follow), especially if you need to use the matcher a lot. While these approaches require more work, they give you more control on the types of the value being matched and the matcher parameters, which in general leads to better compiler error messages that pay off in the long run. They also allow overloading matchers based on parameter types (as opposed to just based on the number of parameters).

Writing New Monomorphic Matchers

A matcher of argument type T implements the matcher interface for T and does two things: it tests whether a value of type T matches the matcher, and can describe what kind of values it matches. The latter ability is used for generating readable error messages when expectations are violated.

A matcher of T must declare a typedef like:

using is_gtest_matcher = void;

and supports the following operations:

// Match a value and optionally explain into an ostream.
bool matched = matcher.MatchAndExplain(value, maybe_os);
// where `value` is of type `T` and
// `maybe_os` is of type `std::ostream*`, where it can be null if the caller
// is not interested in there textual explanation.

matcher.DescribeTo(os);
matcher.DescribeNegationTo(os);
// where `os` is of type `std::ostream*`.

If you need a custom matcher but Truly() is not a good option (for example, you may not be happy with the way Truly(predicate) describes itself, or you may want your matcher to be polymorphic as Eq(value) is), you can define a matcher to do whatever you want in two steps: first implement the matcher interface, and then define a factory function to create a matcher instance. The second step is not strictly needed but it makes the syntax of using the matcher nicer.

For example, you can define a matcher to test whether an int is divisible by 7 and then use it like this:

using ::testing::Matcher;

class DivisibleBy7Matcher {
 public:
  using is_gtest_matcher = void;

  bool MatchAndExplain(int n, std::ostream*) const {
    return (n % 7) == 0;
  }

  void DescribeTo(std::ostream* os) const {
    *os << "is divisible by 7";
  }

  void DescribeNegationTo(std::ostream* os) const {
    *os << "is not divisible by 7";
  }
};

Matcher<int> DivisibleBy7() {
  return DivisibleBy7Matcher();
}

...
  EXPECT_CALL(foo, Bar(DivisibleBy7()));

You may improve the matcher message by streaming additional information to the os argument in MatchAndExplain():

class DivisibleBy7Matcher {
 public:
  bool MatchAndExplain(int n, std::ostream* os) const {
    const int remainder = n % 7;
    if (remainder != 0 && os != nullptr) {
      *os << "the remainder is " << remainder;
    }
    return remainder == 0;
  }
  ...
};

Then, EXPECT_THAT(x, DivisibleBy7()); may generate a message like this:

Value of: x
Expected: is divisible by 7
  Actual: 23 (the remainder is 2)

{: .callout .tip} Tip: for convenience, MatchAndExplain() can take a MatchResultListener* instead of std::ostream*.

Writing New Polymorphic Matchers

Expanding what we learned above to polymorphic matchers is now just as simple as adding templates in the right place.

class NotNullMatcher {
 public:
  using is_gtest_matcher = void;

  // To implement a polymorphic matcher, we just need to make MatchAndExplain a
  // template on its first argument.

  // In this example, we want to use NotNull() with any pointer, so
  // MatchAndExplain() accepts a pointer of any type as its first argument.
  // In general, you can define MatchAndExplain() as an ordinary method or
  // a method template, or even overload it.
  template <typename T>
  bool MatchAndExplain(T* p, std::ostream*) const {
    return p != nullptr;
  }

  // Describes the property of a value matching this matcher.
  void DescribeTo(std::ostream* os) const { *os << "is not NULL"; }

  // Describes the property of a value NOT matching this matcher.
  void DescribeNegationTo(std::ostream* os) const { *os << "is NULL"; }
};

NotNullMatcher NotNull() {
  return NotNullMatcher();
}

...

  EXPECT_CALL(foo, Bar(NotNull()));  // The argument must be a non-NULL pointer.

Legacy Matcher Implementation

Defining matchers used to be somewhat more complicated, in which it required several supporting classes and virtual functions. To implement a matcher for type T using the legacy API you have to derive from MatcherInterface<T> and call MakeMatcher to construct the object.

The interface looks like this:

class MatchResultListener {
 public:
  ...
  // Streams x to the underlying ostream; does nothing if the ostream
  // is NULL.
  template <typename T>
  MatchResultListener& operator<<(const T& x);

  // Returns the underlying ostream.
  std::ostream* stream();
};

template <typename T>
class MatcherInterface {
 public:
  virtual ~MatcherInterface();

  // Returns true if and only if the matcher matches x; also explains the match
  // result to 'listener'.
  virtual bool MatchAndExplain(T x, MatchResultListener* listener) const = 0;

  // Describes this matcher to an ostream.
  virtual void DescribeTo(std::ostream* os) const = 0;

  // Describes the negation of this matcher to an ostream.
  virtual void DescribeNegationTo(std::ostream* os) const;
};

Fortunately, most of the time you can define a polymorphic matcher easily with the help of MakePolymorphicMatcher(). Here's how you can define NotNull() as an example:

using ::testing::MakePolymorphicMatcher;
using ::testing::MatchResultListener;
using ::testing::PolymorphicMatcher;

class NotNullMatcher {
 public:
  // To implement a polymorphic matcher, first define a COPYABLE class
  // that has three members MatchAndExplain(), DescribeTo(), and
  // DescribeNegationTo(), like the following.

  // In this example, we want to use NotNull() with any pointer, so
  // MatchAndExplain() accepts a pointer of any type as its first argument.
  // In general, you can define MatchAndExplain() as an ordinary method or
  // a method template, or even overload it.
  template <typename T>
  bool MatchAndExplain(T* p,
                       MatchResultListener* /* listener */) const {
    return p != NULL;
  }

  // Describes the property of a value matching this matcher.
  void DescribeTo(std::ostream* os) const { *os << "is not NULL"; }

  // Describes the property of a value NOT matching this matcher.
  void DescribeNegationTo(std::ostream* os) const { *os << "is NULL"; }
};

// To construct a polymorphic matcher, pass an instance of the class
// to MakePolymorphicMatcher().  Note the return type.
PolymorphicMatcher<NotNullMatcher> NotNull() {
  return MakePolymorphicMatcher(NotNullMatcher());
}

...

  EXPECT_CALL(foo, Bar(NotNull()));  // The argument must be a non-NULL pointer.

{: .callout .note} Note: Your polymorphic matcher class does not need to inherit from MatcherInterface or any other class, and its methods do not need to be virtual.

Like in a monomorphic matcher, you may explain the match result by streaming additional information to the listener argument in MatchAndExplain().

Writing New Cardinalities

A cardinality is used in Times() to tell gMock how many times you expect a call to occur. It doesn't have to be exact. For example, you can say AtLeast(5) or Between(2, 4).

If the built-in set of cardinalities doesn't suit you, you are free to define your own by implementing the following interface (in namespace testing):

class CardinalityInterface {
 public:
  virtual ~CardinalityInterface();

  // Returns true if and only if call_count calls will satisfy this cardinality.
  virtual bool IsSatisfiedByCallCount(int call_count) const = 0;

  // Returns true if and only if call_count calls will saturate this
  // cardinality.
  virtual bool IsSaturatedByCallCount(int call_count) const = 0;

  // Describes self to an ostream.
  virtual void DescribeTo(std::ostream* os) const = 0;
};

For example, to specify that a call must occur even number of times, you can write

using ::testing::Cardinality;
using ::testing::CardinalityInterface;
using ::testing::MakeCardinality;

class EvenNumberCardinality : public CardinalityInterface {
 public:
  bool IsSatisfiedByCallCount(int call_count) const override {
    return (call_count % 2) == 0;
  }

  bool IsSaturatedByCallCount(int call_count) const override {
    return false;
  }

  void DescribeTo(std::ostream* os) const {
    *os << "called even number of times";
  }
};

Cardinality EvenNumber() {
  return MakeCardinality(new EvenNumberCardinality);
}

...
  EXPECT_CALL(foo, Bar(3))
      .Times(EvenNumber());

Writing New Actions

If the built-in actions don't work for you, you can easily define your own one. All you need is a call operator with a signature compatible with the mocked function. So you can use a lambda:

MockFunction<int(int)> mock;
EXPECT_CALL(mock, Call).WillOnce([](const int input) { return input * 7; });
EXPECT_EQ(mock.AsStdFunction()(2), 14);

Or a struct with a call operator (even a templated one):

struct MultiplyBy {
  template <typename T>
  T operator()(T arg) { return arg * multiplier; }

  int multiplier;
};

// Then use:
// EXPECT_CALL(...).WillOnce(MultiplyBy{7});

It's also fine for the callable to take no arguments, ignoring the arguments supplied to the mock function:

MockFunction<int(int)> mock;
EXPECT_CALL(mock, Call).WillOnce([] { return 17; });
EXPECT_EQ(mock.AsStdFunction()(0), 17);

When used with WillOnce, the callable can assume it will be called at most once and is allowed to be a move-only type:

// An action that contains move-only types and has an &&-qualified operator,
// demanding in the type system that it be called at most once. This can be
// used with WillOnce, but the compiler will reject it if handed to
// WillRepeatedly.
struct MoveOnlyAction {
  std::unique_ptr<int> move_only_state;
  std::unique_ptr<int> operator()() && { return std::move(move_only_state); }
};

MockFunction<std::unique_ptr<int>()> mock;
EXPECT_CALL(mock, Call).WillOnce(MoveOnlyAction{std::make_unique<int>(17)});
EXPECT_THAT(mock.AsStdFunction()(), Pointee(Eq(17)));

More generally, to use with a mock function whose signature is R(Args...) the object can be anything convertible to OnceAction<R(Args...)> or Action<R(Args...)>. The difference between the two is that OnceAction has weaker requirements (Action requires a copy-constructible input that can be called repeatedly whereas OnceAction requires only move-constructible and supports &&-qualified call operators), but can be used only with WillOnce. OnceAction is typically relevant only when supporting move-only types or actions that want a type-system guarantee that they will be called at most once.

Typically the OnceAction and Action templates need not be referenced directly in your actions: a struct or class with a call operator is sufficient, as in the examples above. But fancier polymorphic actions that need to know the specific return type of the mock function can define templated conversion operators to make that possible. See gmock-actions.h for examples.

Legacy macro-based Actions

Before C++11, the functor-based actions were not supported; the old way of writing actions was through a set of ACTION* macros. We suggest to avoid them in new code; they hide a lot of logic behind the macro, potentially leading to harder-to-understand compiler errors. Nevertheless, we cover them here for completeness.

By writing

ACTION(name) { statements; }

in a namespace scope (i.e. not inside a class or function), you will define an action with the given name that executes the statements. The value returned by statements will be used as the return value of the action. Inside the statements, you can refer to the K-th (0-based) argument of the mock function as argK. For example:

ACTION(IncrementArg1) { return ++(*arg1); }

allows you to write

... WillOnce(IncrementArg1());

Note that you don't need to specify the types of the mock function arguments. Rest assured that your code is type-safe though: you'll get a compiler error if *arg1 doesn't support the ++ operator, or if the type of ++(*arg1) isn't compatible with the mock function's return type.

Another example:

ACTION(Foo) {
  (*arg2)(5);
  Blah();
  *arg1 = 0;
  return arg0;
}

defines an action Foo() that invokes argument #2 (a function pointer) with 5, calls function Blah(), sets the value pointed to by argument #1 to 0, and returns argument #0.

For more convenience and flexibility, you can also use the following pre-defined symbols in the body of ACTION:

argK_type	The type of the K-th (0-based) argument of the mock function
args	All arguments of the mock function as a tuple
args_type	The type of all arguments of the mock function as a tuple
return_type	The return type of the mock function
function_type	The type of the mock function

For example, when using an ACTION as a stub action for mock function:

int DoSomething(bool flag, int* ptr);

we have:

Pre-defined Symbol	Is Bound To
arg0	the value of flag
arg0_type	the type bool
arg1	the value of ptr
arg1_type	the type int*
args	the tuple (flag, ptr)
args_type	the type std::tuple<bool, int*>
return_type	the type int
function_type	the type int(bool, int*)

Legacy macro-based parameterized Actions

Sometimes you'll want to parameterize an action you define. For that we have another macro

ACTION_P(name, param) { statements; }

For example,

ACTION_P(Add, n) { return arg0 + n; }

will allow you to write

// Returns argument #0 + 5.
... WillOnce(Add(5));

For convenience, we use the term arguments for the values used to invoke the mock function, and the term parameters for the values used to instantiate an action.

Note that you don't need to provide the type of the parameter either. Suppose the parameter is named param, you can also use the gMock-defined symbol param_type to refer to the type of the parameter as inferred by the compiler. For example, in the body of ACTION_P(Add, n) above, you can write n_type for the type of n.

gMock also provides ACTION_P2, ACTION_P3, and etc to support multi-parameter actions. For example,

ACTION_P2(ReturnDistanceTo, x, y) {
  double dx = arg0 - x;
  double dy = arg1 - y;
  return sqrt(dx*dx + dy*dy);
}

lets you write

... WillOnce(ReturnDistanceTo(5.0, 26.5));

You can view ACTION as a degenerated parameterized action where the number of parameters is 0.

You can also easily define actions overloaded on the number of parameters:

ACTION_P(Plus, a) { ... }
ACTION_P2(Plus, a, b) { ... }

Restricting the Type of an Argument or Parameter in an ACTION

For maximum brevity and reusability, the ACTION* macros don't ask you to provide the types of the mock function arguments and the action parameters. Instead, we let the compiler infer the types for us.

Sometimes, however, we may want to be more explicit about the types. There are several tricks to do that. For example:

ACTION(Foo) {
  // Makes sure arg0 can be converted to int.
  int n = arg0;
  ... use n instead of arg0 here ...
}

ACTION_P(Bar, param) {
  // Makes sure the type of arg1 is const char*.
  ::testing::StaticAssertTypeEq<const char*, arg1_type>();

  // Makes sure param can be converted to bool.
  bool flag = param;
}

where StaticAssertTypeEq is a compile-time assertion in googletest that verifies two types are the same.

Writing New Action Templates Quickly

Sometimes you want to give an action explicit template parameters that cannot be inferred from its value parameters. ACTION_TEMPLATE() supports that and can be viewed as an extension to ACTION() and ACTION_P*().

The syntax:

ACTION_TEMPLATE(ActionName,
                HAS_m_TEMPLATE_PARAMS(kind1, name1, ..., kind_m, name_m),
                AND_n_VALUE_PARAMS(p1, ..., p_n)) { statements; }

defines an action template that takes m explicit template parameters and n value parameters, where m is in [1, 10] and n is in [0, 10]. name_i is the name of the i-th template parameter, and kind_i specifies whether it's a typename, an integral constant, or a template. p_i is the name of the i-th value parameter.

Example:

// DuplicateArg<k, T>(output) converts the k-th argument of the mock
// function to type T and copies it to *output.
ACTION_TEMPLATE(DuplicateArg,
                // Note the comma between int and k:
                HAS_2_TEMPLATE_PARAMS(int, k, typename, T),
                AND_1_VALUE_PARAMS(output)) {
  *output = T(std::get<k>(args));
}

To create an instance of an action template, write:

ActionName<t1, ..., t_m>(v1, ..., v_n)

where the ts are the template arguments and the vs are the value arguments. The value argument types are inferred by the compiler. For example:

using ::testing::_;
...
  int n;
  EXPECT_CALL(mock, Foo).WillOnce(DuplicateArg<1, unsigned char>(&n));

If you want to explicitly specify the value argument types, you can provide additional template arguments:

ActionName<t1, ..., t_m, u1, ..., u_k>(v1, ..., v_n)

where u_i is the desired type of v_i.

ACTION_TEMPLATE and ACTION/ACTION_P* can be overloaded on the number of value parameters, but not on the number of template parameters. Without the restriction, the meaning of the following is unclear:

  OverloadedAction<int, bool>(x);

Are we using a single-template-parameter action where bool refers to the type of x, or a two-template-parameter action where the compiler is asked to infer the type of x?

Using the ACTION Object's Type

If you are writing a function that returns an ACTION object, you'll need to know its type. The type depends on the macro used to define the action and the parameter types. The rule is relatively simple:

Given Definition	Expression	Has Type
ACTION(Foo)	Foo()	FooAction
ACTION_TEMPLATE(Foo, HAS_m_TEMPLATE_PARAMS(...), AND_0_VALUE_PARAMS())	Foo<t1, ..., t_m>()	FooAction<t1, ..., t_m>
ACTION_P(Bar, param)	Bar(int_value)	BarActionP<int>
ACTION_TEMPLATE(Bar, HAS_m_TEMPLATE_PARAMS(...), AND_1_VALUE_PARAMS(p1))	Bar<t1, ..., t_m>(int_value)	BarActionP<t1, ..., t_m, int>
ACTION_P2(Baz, p1, p2)	Baz(bool_value, int_value)	BazActionP2<bool, int>
ACTION_TEMPLATE(Baz, HAS_m_TEMPLATE_PARAMS(...), AND_2_VALUE_PARAMS(p1, p2))	Baz<t1, ..., t_m>(bool_value, int_value)	BazActionP2<t1, ..., t_m, bool, int>
...	...	...

Note that we have to pick different suffixes (Action, ActionP, ActionP2, and etc) for actions with different numbers of value parameters, or the action definitions cannot be overloaded on the number of them.

Writing New Monomorphic Actions

While the ACTION* macros are very convenient, sometimes they are inappropriate. For example, despite the tricks shown in the previous recipes, they don't let you directly specify the types of the mock function arguments and the action parameters, which in general leads to unoptimized compiler error messages that can baffle unfamiliar users. They also don't allow overloading actions based on parameter types without jumping through some hoops.

An alternative to the ACTION* macros is to implement ::testing::ActionInterface<F>, where F is the type of the mock function in which the action will be used. For example:

template <typename F>
class ActionInterface {
 public:
  virtual ~ActionInterface();

  // Performs the action.  Result is the return type of function type
  // F, and ArgumentTuple is the tuple of arguments of F.
  //

  // For example, if F is int(bool, const string&), then Result would
  // be int, and ArgumentTuple would be std::tuple<bool, const string&>.
  virtual Result Perform(const ArgumentTuple& args) = 0;
};

using ::testing::_;
using ::testing::Action;
using ::testing::ActionInterface;
using ::testing::MakeAction;

typedef int IncrementMethod(int*);

class IncrementArgumentAction : public ActionInterface<IncrementMethod> {
 public:
  int Perform(const std::tuple<int*>& args) override {
    int* p = std::get<0>(args);  // Grabs the first argument.
    return *p++;
  }
};

Action<IncrementMethod> IncrementArgument() {
  return MakeAction(new IncrementArgumentAction);
}

...
  EXPECT_CALL(foo, Baz(_))
      .WillOnce(IncrementArgument());

  int n = 5;
  foo.Baz(&n);  // Should return 5 and change n to 6.

Writing New Polymorphic Actions

The previous recipe showed you how to define your own action. This is all good, except that you need to know the type of the function in which the action will be used. Sometimes that can be a problem. For example, if you want to use the action in functions with different types (e.g. like Return() and SetArgPointee()).

If an action can be used in several types of mock functions, we say it's polymorphic. The MakePolymorphicAction() function template makes it easy to define such an action:

namespace testing {
template <typename Impl>
PolymorphicAction<Impl> MakePolymorphicAction(const Impl& impl);
}  // namespace testing

As an example, let's define an action that returns the second argument in the mock function's argument list. The first step is to define an implementation class:

class ReturnSecondArgumentAction {
 public:
  template <typename Result, typename ArgumentTuple>
  Result Perform(const ArgumentTuple& args) const {
    // To get the i-th (0-based) argument, use std::get(args).
    return std::get<1>(args);
  }
};

This implementation class does not need to inherit from any particular class. What matters is that it must have a Perform() method template. This method template takes the mock function's arguments as a tuple in a single argument, and returns the result of the action. It can be either const or not, but must be invocable with exactly one template argument, which is the result type. In other words, you must be able to call Perform<R>(args) where R is the mock function's return type and args is its arguments in a tuple.

Next, we use MakePolymorphicAction() to turn an instance of the implementation class into the polymorphic action we need. It will be convenient to have a wrapper for this:

using ::testing::MakePolymorphicAction;
using ::testing::PolymorphicAction;

PolymorphicAction<ReturnSecondArgumentAction> ReturnSecondArgument() {
  return MakePolymorphicAction(ReturnSecondArgumentAction());
}

Now, you can use this polymorphic action the same way you use the built-in ones:

using ::testing::_;

class MockFoo : public Foo {
 public:
  MOCK_METHOD(int, DoThis, (bool flag, int n), (override));
  MOCK_METHOD(string, DoThat, (int x, const char* str1, const char* str2),
              (override));
};

  ...
  MockFoo foo;
  EXPECT_CALL(foo, DoThis).WillOnce(ReturnSecondArgument());
  EXPECT_CALL(foo, DoThat).WillOnce(ReturnSecondArgument());
  ...
  foo.DoThis(true, 5);  // Will return 5.
  foo.DoThat(1, "Hi", "Bye");  // Will return "Hi".

Teaching gMock How to Print Your Values

When an uninteresting or unexpected call occurs, gMock prints the argument values and the stack trace to help you debug. Assertion macros like EXPECT_THAT and EXPECT_EQ also print the values in question when the assertion fails. gMock and googletest do this using googletest's user-extensible value printer.

This printer knows how to print built-in C++ types, native arrays, STL containers, and any type that supports the << operator. For other types, it prints the raw bytes in the value and hopes that you the user can figure it out. The GoogleTest advanced guide explains how to extend the printer to do a better job at printing your particular type than to dump the bytes.

Useful Mocks Created Using gMock

Mock std::function

std::function is a general function type introduced in C++11. It is a preferred way of passing callbacks to new interfaces. Functions are copyable, and are not usually passed around by pointer, which makes them tricky to mock. But fear not - MockFunction can help you with that.

MockFunction<R(T1, ..., Tn)> has a mock method Call() with the signature:

  R Call(T1, ..., Tn);

It also has a AsStdFunction() method, which creates a std::function proxy forwarding to Call:

  std::function<R(T1, ..., Tn)> AsStdFunction();

To use MockFunction, first create MockFunction object and set up expectations on its Call method. Then pass proxy obtained from AsStdFunction() to the code you are testing. For example:

TEST(FooTest, RunsCallbackWithBarArgument) {
  // 1. Create a mock object.
  MockFunction<int(string)> mock_function;

  // 2. Set expectations on Call() method.
  EXPECT_CALL(mock_function, Call("bar")).WillOnce(Return(1));

  // 3. Exercise code that uses std::function.
  Foo(mock_function.AsStdFunction());
  // Foo's signature can be either of:
  // void Foo(const std::function<int(string)>& fun);
  // void Foo(std::function<int(string)> fun);

  // 4. All expectations will be verified when mock_function
  //     goes out of scope and is destroyed.
}

Remember that function objects created with AsStdFunction() are just forwarders. If you create multiple of them, they will share the same set of expectations.

Although std::function supports unlimited number of arguments, MockFunction implementation is limited to ten. If you ever hit that limit... well, your callback has bigger problems than being mockable. :-)