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gmock-actions: simplify Return and add better documentation.

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Better document requirements, API decisions, and historical accidents. Make an
implicit conversion easier and in a more appropriate place, and ease the burden
of some assertions in the conversion operator. Stop using the legacy
ActionInterface style for defining the action.

PiperOrigin-RevId: 447894892
Change-Id: I179e23ec2abdd9bf05c204ab18dbb492f1372e8e
This commit is contained in:
Aaron Jacobs 2022-05-10 20:40:26 -07:00 committed by Copybara-Service
parent 6386897feb
commit 4224c770a3
3 changed files with 190 additions and 91 deletions
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232
googlemock/include/gmock/gmock-actions.h
View File

@ -866,93 +866,155 @@ struct ByMoveWrapper {
T payload;
};
// Implements the polymorphic Return(x) action, which can be used in
// any function that returns the type of x, regardless of the argument
// types.
//
// Note: The value passed into Return must be converted into
// Function<F>::Result when this action is cast to Action<F> rather than
// when that action is performed. This is important in scenarios like
//
// MOCK_METHOD1(Method, T(U));
// ...
// {
// Foo foo;
// X x(&foo);
// EXPECT_CALL(mock, Method(_)).WillOnce(Return(x));
// }
//
// In the example above the variable x holds reference to foo which leaves
// scope and gets destroyed. If copying X just copies a reference to foo,
// that copy will be left with a hanging reference. If conversion to T
// makes a copy of foo, the above code is safe. To support that scenario, we
// need to make sure that the type conversion happens inside the EXPECT_CALL
// statement, and conversion of the result of Return to Action<T(U)> is a
// good place for that.
//
// The real life example of the above scenario happens when an invocation
// of gtl::Container() is passed into Return.
//
// The general implementation of Return(R). Specializations follow below.
template <typename R>
class ReturnAction final {
public:
// Constructs a ReturnAction object from the value to be returned.
// 'value' is passed by value instead of by const reference in order
// to allow Return("string literal") to compile.
explicit ReturnAction(R value) : value_(std::move(value)) {}
// This template type conversion operator allows Return(x) to be
// used in ANY function that returns x's type.
template <typename F>
operator Action<F>() const { // NOLINT
// Assert statement belongs here because this is the best place to verify
// conditions on F. It produces the clearest error messages
// in most compilers.
// Impl really belongs in this scope as a local class but can't
// because MSVC produces duplicate symbols in different translation units
// in this case. Until MS fixes that bug we put Impl into the class scope
// and put the typedef both here (for use in assert statement) and
// in the Impl class. But both definitions must be the same.
typedef typename Function<F>::Result Result;
static_assert(!std::is_reference<Result>::value,
// Support conversion to function types with compatible return types. See the
// documentation on Return for the definition of compatible.
template <typename U, typename... Args>
operator Action<U(Args...)>() const { // NOLINT
// Check our requirements on the return type.
static_assert(!std::is_reference<U>::value,
"use ReturnRef instead of Return to return a reference");
static_assert(!std::is_void<Result>::value,
static_assert(!std::is_void<U>::value,
"Can't use Return() on an action expected to return `void`.");
return Action<F>(new Impl<F>(value_));
return Impl<U>(value_);
}
private:
// Implements the Return(x) action for a particular function type F.
template <typename F>
class Impl : public ActionInterface<F> {
// Implements the Return(x) action for a mock function that returns type U.
template <typename U>
class Impl final {
public:
typedef typename Function<F>::Result Result;
typedef typename Function<F>::ArgumentTuple ArgumentTuple;
explicit Impl(const R& input_value) : state_(new State(input_value)) {}
explicit Impl(const R& value)
: value_before_cast_(value),
// Make an implicit conversion to Result before initializing the
// Result object we store, avoiding calling any explicit constructor
// of Result from R.
//
// This simulates the language rules: a function with return type
// Result that does `return R()` requires R to be implicitly
// convertible to Result, and uses that path for the conversion, even
// if Result has an explicit constructor from R.
value_(ImplicitCast_<Result>(value_before_cast_)) {}
Result Perform(const ArgumentTuple&) override { return value_; }
U operator()() const { return state_->value; }
private:
static_assert(!std::is_reference<Result>::value,
"Result cannot be a reference type");
// We save the value before casting just in case it is being cast to a
// wrapper type.
R value_before_cast_;
Result value_;
// We put our state on the heap so that the compiler-generated copy/move
// constructors work correctly even when U is a reference-like type. This is
// necessary only because we eagerly create State::value (see the note on
// that symbol for details). If we instead had only the input value as a
// member then the default constructors would work fine.
//
// For example, when R is std::string and U is std::string_view, value is a
// reference to the string backed by input_value. The copy constructor would
// copy both, so that we wind up with a new input_value object (with the
// same contents) and a reference to the *old* input_value object rather
// than the new one.
struct State {
explicit State(const R& input_value_in)
: input_value(input_value_in),
// Make an implicit conversion to Result before initializing the U
// object we store, avoiding calling any explicit constructor of U
// from R.
//
// This simulates the language rules: a function with return type U
// that does `return R()` requires R to be implicitly convertible to
// U, and uses that path for the conversion, even U Result has an
// explicit constructor from R.
//
// We provide non-const access to input_value to the conversion
// code. It's not clear whether this makes semantic sense -- what
// would it mean for the conversion to modify the input value? This
// appears to be an accident of history:
//
// 1. Before the first public commit the input value was simply an
// object of type R embedded directly in the Impl object. The
// result value wasn't yet eagerly created, and the Impl class's
// Perform method was const, so the implicit conversion when it
// returned the value was from const R&.
//
// 2. Google changelist 6490411 changed ActionInterface::Perform to
// be non-const, citing the fact that an action can have side
// effects and be stateful. Impl::Perform was updated like all
// other actions, probably without consideration of the fact
// that side effects and statefulness don't make sense for
// Return. From this point on the conversion had non-const
// access to the input value.
//
value(ImplicitCast_<U>(input_value)) {}
Impl(const Impl&) = delete;
Impl& operator=(const Impl&) = delete;
// A copy of the value originally provided by the user. We retain this in
// addition to the value of the mock function's result type below in case
// the latter is a reference-like type. See the std::string_view example
// in the documentation on Return.
R input_value;
// The value we actually return, as the type returned by the mock function
// itself.
//
// We eagerly initialize this here, rather than lazily doing the implicit
// conversion automatically each time Perform is called, for historical
// reasons: in 2009-11, commit a070cbd91c (Google changelist 13540126)
// made the Action<U()> conversion operator eagerly convert the R value to
// U, but without keeping the R alive. This broke the use case discussed
// in the documentation for Return, making reference-like types such as
// std::string_view not safe to use as U where the input type R is a
// value-like type such as std::string.
//
// The example the commit gave was not very clear, nor was the issue
// thread (https://github.com/google/googlemock/issues/86), but it seems
// the worry was about reference-like input types R that flatten to a
// value-like type U when being implicitly converted. An example of this
// is std::vector<bool>::reference, which is often a proxy type with an
// reference to the underlying vector:
//
// // Helper method: have the mock function return bools according
// // to the supplied script.
// void SetActions(MockFunction<bool(size_t)>& mock,
// const std::vector<bool>& script) {
// for (size_t i = 0; i < script.size(); ++i) {
// EXPECT_CALL(mock, Call(i)).WillOnce(Return(script[i]));
// }
// }
//
// TEST(Foo, Bar) {
// // Set actions using a temporary vector, whose operator[]
// // returns proxy objects that references that will be
// // dangling once the call to SetActions finishes and the
// // vector is destroyed.
// MockFunction<bool(size_t)> mock;
// SetActions(mock, {false, true});
//
// EXPECT_FALSE(mock.AsStdFunction()(0));
// EXPECT_TRUE(mock.AsStdFunction()(1));
// }
//
// This eager conversion helps with a simple case like this, but doesn't
// fully make these types work in general. For example the following still
// uses a dangling reference:
//
// TEST(Foo, Baz) {
// MockFunction<std::vector<std::string>()> mock;
//
// // Return the same vector twice, and then the empty vector
// // thereafter.
// auto action = Return(std::initializer_list<std::string>{
// "taco", "burrito",
// });
//
// EXPECT_CALL(mock, Call)
// .WillOnce(action)
// .WillOnce(action)
// .WillRepeatedly(Return(std::vector<std::string>{}));
//
// EXPECT_THAT(mock.AsStdFunction()(),
// ElementsAre("taco", "burrito"));
// EXPECT_THAT(mock.AsStdFunction()(),
// ElementsAre("taco", "burrito"));
// EXPECT_THAT(mock.AsStdFunction()(), IsEmpty());
// }
//
U value;
};
const std::shared_ptr<State> state_;
};
R value_;
@ -1693,9 +1755,29 @@ internal::WithArgsAction<typename std::decay<InnerAction>::type> WithoutArgs(
return {std::forward<InnerAction>(action)};
}
// Creates an action that returns 'value'. 'value' is passed by value
// instead of const reference - otherwise Return("string literal")
// will trigger a compiler error about using array as initializer.
// Creates an action that returns a value.
//
// R must be copy-constructible. The returned type can be used as an
// Action<U(Args...)> for any type U where all of the following are true:
//
// * U is not void.
// * U is not a reference type. (Use ReturnRef instead.)
// * U is copy-constructible.
// * R& is convertible to U.
//
// The Action<U(Args)...> object contains the R value from which the U return
// value is constructed (a copy of the argument to Return). This means that the
// R value will survive at least until the mock object's expectations are
// cleared or the mock object is destroyed, meaning that U can be a
// reference-like type such as std::string_view:
//
// // The mock function returns a view of a copy of the string fed to
// // Return. The view is valid even after the action is performed.
// MockFunction<std::string_view()> mock;
// EXPECT_CALL(mock, Call).WillOnce(Return(std::string("taco")));
// const std::string_view result = mock.AsStdFunction()();
// EXPECT_EQ("taco", result);
//
template <typename R>
internal::ReturnAction<R> Return(R value) {
return internal::ReturnAction<R>(std::move(value));

43
googlemock/test/gmock-actions_test.cc
View File

@ -51,6 +51,7 @@
#include <memory>
#include <string>
#include <type_traits>
#include <vector>
#include "gmock/gmock.h"
#include "gmock/internal/gmock-port.h"
@ -647,24 +648,34 @@ TEST(ReturnTest, AcceptsStringLiteral) {
EXPECT_EQ("world", a2.Perform(std::make_tuple()));
}
// Test struct which wraps a vector of integers. Used in
// 'SupportsWrapperReturnType' test.
struct IntegerVectorWrapper {
std::vector<int>* v;
IntegerVectorWrapper(std::vector<int>& _v) : v(&_v) {} // NOLINT
};
// Return(x) should work fine when the mock function's return type is a
// reference-like wrapper for decltype(x), as when x is a std::string and the
// mock function returns std::string_view.
TEST(ReturnTest, SupportsReferenceLikeReturnType) {
// A reference wrapper for std::vector<int>, implicitly convertible from it.
struct Result {
std::vector<int>* v;
Result(std::vector<int>& v) : v(&v) {} // NOLINT
};
// Tests that Return() works when return type is a wrapper type.
TEST(ReturnTest, SupportsWrapperReturnType) {
// Initialize vector of integers.
std::vector<int> v;
for (int i = 0; i < 5; ++i) v.push_back(i);
// Set up an action for a mock function that returns the reference wrapper
// type, initializing it with an actual vector.
//
// The returned wrapper should be initialized with a copy of that vector
// that's embedded within the action itself (which should stay alive as long
// as the mock object is alive), rather than e.g. a reference to the temporary
// we feed to Return. This should work fine both for WillOnce and
// WillRepeatedly.
MockFunction<Result()> mock;
EXPECT_CALL(mock, Call)
.WillOnce(Return(std::vector<int>{17, 19, 23}))
.WillRepeatedly(Return(std::vector<int>{29, 31, 37}));
// Return() called with 'v' as argument. The Action will return the same data
// as 'v' (copy) but it will be wrapped in an IntegerVectorWrapper.
Action<IntegerVectorWrapper()> a = Return(v);
const std::vector<int>& result = *(a.Perform(std::make_tuple()).v);
EXPECT_THAT(result, ::testing::ElementsAre(0, 1, 2, 3, 4));
EXPECT_THAT(mock.AsStdFunction()(),
Field(&Result::v, Pointee(ElementsAre(17, 19, 23))));
EXPECT_THAT(mock.AsStdFunction()(),
Field(&Result::v, Pointee(ElementsAre(29, 31, 37))));
}
TEST(ReturnTest, PrefersConversionOperator) {

6
googlemock/test/gmock-function-mocker_test.cc
View File

@ -27,6 +27,12 @@
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
// Silence C4503 (decorated name length exceeded) for MSVC.
#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable : 4503)
#endif
// Google Mock - a framework for writing C++ mock classes.
//
// This file tests the function mocker classes.