tile/third_party/googletest/docs/primer.md

# Googletest Primer

## Introduction: Why googletest?

*googletest* helps you write better C++ tests.

googletest is a testing framework developed by the Testing Technology team with
Google's specific requirements and constraints in mind. Whether you work on
Linux, Windows, or a Mac, if you write C++ code, googletest can help you. And it
supports *any* kind of tests, not just unit tests.

So what makes a good test, and how does googletest fit in? We believe:

1.  Tests should be *independent* and *repeatable*. It's a pain to debug a test
    that succeeds or fails as a result of other tests. googletest isolates the
    tests by running each of them on a different object. When a test fails,
    googletest allows you to run it in isolation for quick debugging.
2.  Tests should be well *organized* and reflect the structure of the tested
    code. googletest groups related tests into test suites that can share data
    and subroutines. This common pattern is easy to recognize and makes tests
    easy to maintain. Such consistency is especially helpful when people switch
    projects and start to work on a new code base.
3.  Tests should be *portable* and *reusable*. Google has a lot of code that is
    platform-neutral; its tests should also be platform-neutral. googletest
    works on different OSes, with different compilers, with or without
    exceptions, so googletest tests can work with a variety of configurations.
4.  When tests fail, they should provide as much *information* about the problem
    as possible. googletest doesn't stop at the first test failure. Instead, it
    only stops the current test and continues with the next. You can also set up
    tests that report non-fatal failures after which the current test continues.
    Thus, you can detect and fix multiple bugs in a single run-edit-compile
    cycle.
5.  The testing framework should liberate test writers from housekeeping chores
    and let them focus on the test *content*. googletest automatically keeps
    track of all tests defined, and doesn't require the user to enumerate them
    in order to run them.
6.  Tests should be *fast*. With googletest, you can reuse shared resources
    across tests and pay for the set-up/tear-down only once, without making
    tests depend on each other.

Since googletest is based on the popular xUnit architecture, you'll feel right
at home if you've used JUnit or PyUnit before. If not, it will take you about 10
minutes to learn the basics and get started. So let's go!

## Beware of the nomenclature

{: .callout .note}
_Note:_ There might be some confusion arising from different definitions of the
terms _Test_, _Test Case_ and _Test Suite_, so beware of misunderstanding these.

Historically, googletest started to use the term _Test Case_ for grouping
related tests, whereas current publications, including International Software
Testing Qualifications Board ([ISTQB](http://www.istqb.org/)) materials and
various textbooks on software quality, use the term
_[Test Suite][istqb test suite]_ for this.

The related term _Test_, as it is used in googletest, corresponds to the term
_[Test Case][istqb test case]_ of ISTQB and others.

The term _Test_ is commonly of broad enough sense, including ISTQB's definition
of _Test Case_, so it's not much of a problem here. But the term _Test Case_ as
was used in Google Test is of contradictory sense and thus confusing.

googletest recently started replacing the term _Test Case_ with _Test Suite_.
The preferred API is *TestSuite*. The older TestCase API is being slowly
deprecated and refactored away.

So please be aware of the different definitions of the terms:


Meaning                                                                              | googletest Term         | [ISTQB](http://www.istqb.org/) Term
:----------------------------------------------------------------------------------- | :---------------------- | :----------------------------------
Exercise a particular program path with specific input values and verify the results | [TEST()](#simple-tests) | [Test Case][istqb test case]


[istqb test case]: http://glossary.istqb.org/en/search/test%20case
[istqb test suite]: http://glossary.istqb.org/en/search/test%20suite

## Basic Concepts

When using googletest, you start by writing *assertions*, which are statements
that check whether a condition is true. An assertion's result can be *success*,
*nonfatal failure*, or *fatal failure*. If a fatal failure occurs, it aborts the
current function; otherwise the program continues normally.

*Tests* use assertions to verify the tested code's behavior. If a test crashes
or has a failed assertion, then it *fails*; otherwise it *succeeds*.

A *test suite* contains one or many tests. You should group your tests into test
suites that reflect the structure of the tested code. When multiple tests in a
test suite need to share common objects and subroutines, you can put them into a
*test fixture* class.

A *test program* can contain multiple test suites.

We'll now explain how to write a test program, starting at the individual
assertion level and building up to tests and test suites.

## Assertions

googletest assertions are macros that resemble function calls. You test a class
or function by making assertions about its behavior. When an assertion fails,
googletest prints the assertion's source file and line number location, along
with a failure message. You may also supply a custom failure message which will
be appended to googletest's message.

The assertions come in pairs that test the same thing but have different effects
on the current function. `ASSERT_*` versions generate fatal failures when they
fail, and **abort the current function**. `EXPECT_*` versions generate nonfatal
failures, which don't abort the current function. Usually `EXPECT_*` are
preferred, as they allow more than one failure to be reported in a test.
However, you should use `ASSERT_*` if it doesn't make sense to continue when the
assertion in question fails.

Since a failed `ASSERT_*` returns from the current function immediately,
possibly skipping clean-up code that comes after it, it may cause a space leak.
Depending on the nature of the leak, it may or may not be worth fixing - so keep
this in mind if you get a heap checker error in addition to assertion errors.

To provide a custom failure message, simply stream it into the macro using the
`<<` operator or a sequence of such operators. See the following example, using
the [`ASSERT_EQ` and `EXPECT_EQ`](reference/assertions.md#EXPECT_EQ) macros to
verify value equality:

```c++
ASSERT_EQ(x.size(), y.size()) << "Vectors x and y are of unequal length";

for (int i = 0; i < x.size(); ++i) {
  EXPECT_EQ(x[i], y[i]) << "Vectors x and y differ at index " << i;
}
```

Anything that can be streamed to an `ostream` can be streamed to an assertion
macro--in particular, C strings and `string` objects. If a wide string
(`wchar_t*`, `TCHAR*` in `UNICODE` mode on Windows, or `std::wstring`) is
streamed to an assertion, it will be translated to UTF-8 when printed.

GoogleTest provides a collection of assertions for verifying the behavior of
your code in various ways. You can check Boolean conditions, compare values
based on relational operators, verify string values, floating-point values, and
much more. There are even assertions that enable you to verify more complex
states by providing custom predicates. For the complete list of assertions
provided by GoogleTest, see the [Assertions Reference](reference/assertions.md).

## Simple Tests

To create a test:

1.  Use the `TEST()` macro to define and name a test function. These are
    ordinary C++ functions that don't return a value.
2.  In this function, along with any valid C++ statements you want to include,
    use the various googletest assertions to check values.
3.  The test's result is determined by the assertions; if any assertion in the
    test fails (either fatally or non-fatally), or if the test crashes, the
    entire test fails. Otherwise, it succeeds.

```c++
TEST(TestSuiteName, TestName) {
  ... test body ...
}
```

`TEST()` arguments go from general to specific. The *first* argument is the name
of the test suite, and the *second* argument is the test's name within the test
suite. Both names must be valid C++ identifiers, and they should not contain any
underscores (`_`). A test's *full name* consists of its containing test suite
and its individual name. Tests from different test suites can have the same
individual name.

For example, let's take a simple integer function:

```c++
int Factorial(int n);  // Returns the factorial of n
```

A test suite for this function might look like:

```c++
// Tests factorial of 0.
TEST(FactorialTest, HandlesZeroInput) {
  EXPECT_EQ(Factorial(0), 1);
}

// Tests factorial of positive numbers.
TEST(FactorialTest, HandlesPositiveInput) {
  EXPECT_EQ(Factorial(1), 1);
  EXPECT_EQ(Factorial(2), 2);
  EXPECT_EQ(Factorial(3), 6);
  EXPECT_EQ(Factorial(8), 40320);
}
```

googletest groups the test results by test suites, so logically related tests
should be in the same test suite; in other words, the first argument to their
`TEST()` should be the same. In the above example, we have two tests,
`HandlesZeroInput` and `HandlesPositiveInput`, that belong to the same test
suite `FactorialTest`.

When naming your test suites and tests, you should follow the same convention as
for
[naming functions and classes](https://google.github.io/styleguide/cppguide.html#Function_Names).

**Availability**: Linux, Windows, Mac.

## Test Fixtures: Using the Same Data Configuration for Multiple Tests {#same-data-multiple-tests}

If you find yourself writing two or more tests that operate on similar data, you
can use a *test fixture*. This allows you to reuse the same configuration of
objects for several different tests.

To create a fixture:

1.  Derive a class from `::testing::Test` . Start its body with `protected:`, as
    we'll want to access fixture members from sub-classes.
2.  Inside the class, declare any objects you plan to use.
3.  If necessary, write a default constructor or `SetUp()` function to prepare
    the objects for each test. A common mistake is to spell `SetUp()` as
    **`Setup()`** with a small `u` - Use `override` in C++11 to make sure you
    spelled it correctly.
4.  If necessary, write a destructor or `TearDown()` function to release any
    resources you allocated in `SetUp()` . To learn when you should use the
    constructor/destructor and when you should use `SetUp()/TearDown()`, read
    the [FAQ](faq.md#CtorVsSetUp).
5.  If needed, define subroutines for your tests to share.

When using a fixture, use `TEST_F()` instead of `TEST()` as it allows you to
access objects and subroutines in the test fixture:

```c++
TEST_F(TestFixtureName, TestName) {
  ... test body ...
}
```

Like `TEST()`, the first argument is the test suite name, but for `TEST_F()`
this must be the name of the test fixture class. You've probably guessed: `_F`
is for fixture.

Unfortunately, the C++ macro system does not allow us to create a single macro
that can handle both types of tests. Using the wrong macro causes a compiler
error.

Also, you must first define a test fixture class before using it in a
`TEST_F()`, or you'll get the compiler error "`virtual outside class
declaration`".

For each test defined with `TEST_F()`, googletest will create a *fresh* test
fixture at runtime, immediately initialize it via `SetUp()`, run the test, clean
up by calling `TearDown()`, and then delete the test fixture. Note that
different tests in the same test suite have different test fixture objects, and
googletest always deletes a test fixture before it creates the next one.
googletest does **not** reuse the same test fixture for multiple tests. Any
changes one test makes to the fixture do not affect other tests.

As an example, let's write tests for a FIFO queue class named `Queue`, which has
the following interface:

```c++
template <typename E>  // E is the element type.
class Queue {
 public:
  Queue();
  void Enqueue(const E& element);
  E* Dequeue();  // Returns NULL if the queue is empty.
  size_t size() const;
  ...
};
```

First, define a fixture class. By convention, you should give it the name
`FooTest` where `Foo` is the class being tested.

```c++
class QueueTest : public ::testing::Test {
 protected:
  void SetUp() override {
     q1_.Enqueue(1);
     q2_.Enqueue(2);
     q2_.Enqueue(3);
  }

  // void TearDown() override {}

  Queue<int> q0_;
  Queue<int> q1_;
  Queue<int> q2_;
};
```

In this case, `TearDown()` is not needed since we don't have to clean up after
each test, other than what's already done by the destructor.

Now we'll write tests using `TEST_F()` and this fixture.

```c++
TEST_F(QueueTest, IsEmptyInitially) {
  EXPECT_EQ(q0_.size(), 0);
}

TEST_F(QueueTest, DequeueWorks) {
  int* n = q0_.Dequeue();
  EXPECT_EQ(n, nullptr);

  n = q1_.Dequeue();
  ASSERT_NE(n, nullptr);
  EXPECT_EQ(*n, 1);
  EXPECT_EQ(q1_.size(), 0);
  delete n;

  n = q2_.Dequeue();
  ASSERT_NE(n, nullptr);
  EXPECT_EQ(*n, 2);
  EXPECT_EQ(q2_.size(), 1);
  delete n;
}
```

The above uses both `ASSERT_*` and `EXPECT_*` assertions. The rule of thumb is
to use `EXPECT_*` when you want the test to continue to reveal more errors after
the assertion failure, and use `ASSERT_*` when continuing after failure doesn't
make sense. For example, the second assertion in the `Dequeue` test is
`ASSERT_NE(n, nullptr)`, as we need to dereference the pointer `n` later, which
would lead to a segfault when `n` is `NULL`.

When these tests run, the following happens:

1.  googletest constructs a `QueueTest` object (let's call it `t1`).
2.  `t1.SetUp()` initializes `t1`.
3.  The first test (`IsEmptyInitially`) runs on `t1`.
4.  `t1.TearDown()` cleans up after the test finishes.
5.  `t1` is destructed.
6.  The above steps are repeated on another `QueueTest` object, this time
    running the `DequeueWorks` test.

**Availability**: Linux, Windows, Mac.

## Invoking the Tests

`TEST()` and `TEST_F()` implicitly register their tests with googletest. So,
unlike with many other C++ testing frameworks, you don't have to re-list all
your defined tests in order to run them.

After defining your tests, you can run them with `RUN_ALL_TESTS()`, which
returns `0` if all the tests are successful, or `1` otherwise. Note that
`RUN_ALL_TESTS()` runs *all tests* in your link unit--they can be from different
test suites, or even different source files.

When invoked, the `RUN_ALL_TESTS()` macro:

*   Saves the state of all googletest flags.

*   Creates a test fixture object for the first test.

*   Initializes it via `SetUp()`.

*   Runs the test on the fixture object.

*   Cleans up the fixture via `TearDown()`.

*   Deletes the fixture.

*   Restores the state of all googletest flags.

*   Repeats the above steps for the next test, until all tests have run.

If a fatal failure happens the subsequent steps will be skipped.

{: .callout .important}
> IMPORTANT: You must **not** ignore the return value of `RUN_ALL_TESTS()`, or
> you will get a compiler error. The rationale for this design is that the
> automated testing service determines whether a test has passed based on its
> exit code, not on its stdout/stderr output; thus your `main()` function must
> return the value of `RUN_ALL_TESTS()`.
>
> Also, you should call `RUN_ALL_TESTS()` only **once**. Calling it more than
> once conflicts with some advanced googletest features (e.g., thread-safe
> [death tests](advanced.md#death-tests)) and thus is not supported.

**Availability**: Linux, Windows, Mac.

## Writing the main() Function

Most users should _not_ need to write their own `main` function and instead link
with `gtest_main` (as opposed to with `gtest`), which defines a suitable entry
point. See the end of this section for details. The remainder of this section
should only apply when you need to do something custom before the tests run that
cannot be expressed within the framework of fixtures and test suites.

If you write your own `main` function, it should return the value of
`RUN_ALL_TESTS()`.

You can start from this boilerplate:

```c++
#include "this/package/foo.h"

#include "gtest/gtest.h"

namespace my {
namespace project {
namespace {

// The fixture for testing class Foo.
class FooTest : public ::testing::Test {
 protected:
  // You can remove any or all of the following functions if their bodies would
  // be empty.

  FooTest() {
     // You can do set-up work for each test here.
  }

  ~FooTest() override {
     // You can do clean-up work that doesn't throw exceptions here.
  }

  // If the constructor and destructor are not enough for setting up
  // and cleaning up each test, you can define the following methods:

  void SetUp() override {
     // Code here will be called immediately after the constructor (right
     // before each test).
  }

  void TearDown() override {
     // Code here will be called immediately after each test (right
     // before the destructor).
  }

  // Class members declared here can be used by all tests in the test suite
  // for Foo.
};

// Tests that the Foo::Bar() method does Abc.
TEST_F(FooTest, MethodBarDoesAbc) {
  const std::string input_filepath = "this/package/testdata/myinputfile.dat";
  const std::string output_filepath = "this/package/testdata/myoutputfile.dat";
  Foo f;
  EXPECT_EQ(f.Bar(input_filepath, output_filepath), 0);
}

// Tests that Foo does Xyz.
TEST_F(FooTest, DoesXyz) {
  // Exercises the Xyz feature of Foo.
}

}  // namespace
}  // namespace project
}  // namespace my

int main(int argc, char **argv) {
  ::testing::InitGoogleTest(&argc, argv);
  return RUN_ALL_TESTS();
}
```

The `::testing::InitGoogleTest()` function parses the command line for
googletest flags, and removes all recognized flags. This allows the user to
control a test program's behavior via various flags, which we'll cover in the
[AdvancedGuide](advanced.md). You **must** call this function before calling
`RUN_ALL_TESTS()`, or the flags won't be properly initialized.

On Windows, `InitGoogleTest()` also works with wide strings, so it can be used
in programs compiled in `UNICODE` mode as well.

But maybe you think that writing all those `main` functions is too much work? We
agree with you completely, and that's why Google Test provides a basic
implementation of main(). If it fits your needs, then just link your test with
the `gtest_main` library and you are good to go.

{: .callout .note}
NOTE: `ParseGUnitFlags()` is deprecated in favor of `InitGoogleTest()`.

## Known Limitations

*   Google Test is designed to be thread-safe. The implementation is thread-safe
    on systems where the `pthreads` library is available. It is currently
    _unsafe_ to use Google Test assertions from two threads concurrently on
    other systems (e.g. Windows). In most tests this is not an issue as usually
    the assertions are done in the main thread. If you want to help, you can
    volunteer to implement the necessary synchronization primitives in
    `gtest-port.h` for your platform.