proxygen/wangle/_build/gtest/src/gtest/googletest/docs/Primer.md

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# Introduction: Why Google C++ Testing Framework? #

_Google C++ Testing Framework_ helps you write better C++ tests.

No matter whether you work on Linux, Windows, or a Mac, if you write C++ code,
Google Test can help you.

So what makes a good test, and how does Google C++ Testing Framework 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.  Google C++ Testing Framework isolates the tests by running each of them on a different object. When a test fails, Google C++ Testing Framework allows you to run it in isolation for quick debugging.
  1. Tests should be well _organized_ and reflect the structure of the tested code.  Google C++ Testing Framework groups related tests into test cases 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.
  1. Tests should be _portable_ and _reusable_. The open-source community has a lot of code that is platform-neutral, its tests should also be platform-neutral.  Google C++ Testing Framework works on different OSes, with different compilers (gcc, MSVC, and others), with or without exceptions, so Google C++ Testing Framework tests can easily work with a variety of configurations.  (Note that the current release only contains build scripts for Linux - we are actively working on scripts for other platforms.)
  1. When tests fail, they should provide as much _information_ about the problem as possible. Google C++ Testing Framework 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.
  1. The testing framework should liberate test writers from housekeeping chores and let them focus on the test _content_.  Google C++ Testing Framework automatically keeps track of all tests defined, and doesn't require the user to enumerate them in order to run them.
  1. Tests should be _fast_. With Google C++ Testing Framework, 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 Google C++ Testing Framework 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!

_Note:_ We sometimes refer to Google C++ Testing Framework informally
as _Google Test_.

# Setting up a New Test Project #

To write a test program using Google Test, you need to compile Google
Test into a library and link your test with it.  We provide build
files for some popular build systems: `msvc/` for Visual Studio,
`xcode/` for Mac Xcode, `make/` for GNU make, `codegear/` for Borland
C++ Builder, and the autotools script (deprecated) and
`CMakeLists.txt` for CMake (recommended) in the Google Test root
directory.  If your build system is not on this list, you can take a
look at `make/Makefile` to learn how Google Test should be compiled
(basically you want to compile `src/gtest-all.cc` with `GTEST_ROOT`
and `GTEST_ROOT/include` in the header search path, where `GTEST_ROOT`
is the Google Test root directory).

Once you are able to compile the Google Test library, you should
create a project or build target for your test program.  Make sure you
have `GTEST_ROOT/include` in the header search path so that the
compiler can find `"gtest/gtest.h"` when compiling your test.  Set up
your test project to link with the Google Test library (for example,
in Visual Studio, this is done by adding a dependency on
`gtest.vcproj`).

If you still have questions, take a look at how Google Test's own
tests are built and use them as examples.

# Basic Concepts #

When using Google Test, 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 case_ contains one or many tests. You should group your tests into test
cases that reflect the structure of the tested code. When multiple tests in a
test case need to share common objects and subroutines, you can put them into a
_test fixture_ class.

A _test program_ can contain multiple test cases.

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

# Assertions #

Google Test assertions are macros that resemble function calls. You test a
class or function by making assertions about its behavior. When an assertion
fails, Google Test 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 Google Test'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 failures 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. An example:
```
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.

## Basic Assertions ##

These assertions do basic true/false condition testing.

| **Fatal assertion** | **Nonfatal assertion** | **Verifies** |
|:--------------------|:-----------------------|:-------------|
| `ASSERT_TRUE(`_condition_`)`;  | `EXPECT_TRUE(`_condition_`)`;   | _condition_ is true |
| `ASSERT_FALSE(`_condition_`)`; | `EXPECT_FALSE(`_condition_`)`;  | _condition_ is false |

Remember, when they fail, `ASSERT_*` yields a fatal failure and
returns from the current function, while `EXPECT_*` yields a nonfatal
failure, allowing the function to continue running. In either case, an
assertion failure means its containing test fails.

_Availability_: Linux, Windows, Mac.

## Binary Comparison ##

This section describes assertions that compare two values.

| **Fatal assertion** | **Nonfatal assertion** | **Verifies** |
|:--------------------|:-----------------------|:-------------|
|`ASSERT_EQ(`_val1_`, `_val2_`);`|`EXPECT_EQ(`_val1_`, `_val2_`);`| _val1_ `==` _val2_ |
|`ASSERT_NE(`_val1_`, `_val2_`);`|`EXPECT_NE(`_val1_`, `_val2_`);`| _val1_ `!=` _val2_ |
|`ASSERT_LT(`_val1_`, `_val2_`);`|`EXPECT_LT(`_val1_`, `_val2_`);`| _val1_ `<` _val2_ |
|`ASSERT_LE(`_val1_`, `_val2_`);`|`EXPECT_LE(`_val1_`, `_val2_`);`| _val1_ `<=` _val2_ |
|`ASSERT_GT(`_val1_`, `_val2_`);`|`EXPECT_GT(`_val1_`, `_val2_`);`| _val1_ `>` _val2_ |
|`ASSERT_GE(`_val1_`, `_val2_`);`|`EXPECT_GE(`_val1_`, `_val2_`);`| _val1_ `>=` _val2_ |

In the event of a failure, Google Test prints both _val1_ and _val2_.

Value arguments must be comparable by the assertion's comparison
operator or you'll get a compiler error.  We used to require the
arguments to support the `<<` operator for streaming to an `ostream`,
but it's no longer necessary since v1.6.0 (if `<<` is supported, it
will be called to print the arguments when the assertion fails;
otherwise Google Test will attempt to print them in the best way it
can. For more details and how to customize the printing of the
arguments, see this Google Mock [recipe](../../googlemock/docs/CookBook.md#teaching-google-mock-how-to-print-your-values).).

These assertions can work with a user-defined type, but only if you define the
corresponding comparison operator (e.g. `==`, `<`, etc).  If the corresponding
operator is defined, prefer using the `ASSERT_*()` macros because they will
print out not only the result of the comparison, but the two operands as well.

Arguments are always evaluated exactly once. Therefore, it's OK for the
arguments to have side effects. However, as with any ordinary C/C++ function,
the arguments' evaluation order is undefined (i.e. the compiler is free to
choose any order) and your code should not depend on any particular argument
evaluation order.

`ASSERT_EQ()` does pointer equality on pointers. If used on two C strings, it
tests if they are in the same memory location, not if they have the same value.
Therefore, if you want to compare C strings (e.g. `const char*`) by value, use
`ASSERT_STREQ()` , which will be described later on. In particular, to assert
that a C string is `NULL`, use `ASSERT_STREQ(NULL, c_string)` . However, to
compare two `string` objects, you should use `ASSERT_EQ`.

Macros in this section work with both narrow and wide string objects (`string`
and `wstring`).

_Availability_: Linux, Windows, Mac.

_Historical note_: Before February 2016 `*_EQ` had a convention of calling it as
`ASSERT_EQ(expected, actual)`, so lots of existing code uses this order.
Now `*_EQ` treats both parameters in the same way.

## String Comparison ##

The assertions in this group compare two **C strings**. If you want to compare
two `string` objects, use `EXPECT_EQ`, `EXPECT_NE`, and etc instead.

| **Fatal assertion** | **Nonfatal assertion** | **Verifies** |
|:--------------------|:-----------------------|:-------------|
| `ASSERT_STREQ(`_str1_`, `_str2_`);`    | `EXPECT_STREQ(`_str1_`, `_str_2`);`     | the two C strings have the same content |
| `ASSERT_STRNE(`_str1_`, `_str2_`);`    | `EXPECT_STRNE(`_str1_`, `_str2_`);`     | the two C strings have different content |
| `ASSERT_STRCASEEQ(`_str1_`, `_str2_`);`| `EXPECT_STRCASEEQ(`_str1_`, `_str2_`);` | the two C strings have the same content, ignoring case |
| `ASSERT_STRCASENE(`_str1_`, `_str2_`);`| `EXPECT_STRCASENE(`_str1_`, `_str2_`);` | the two C strings have different content, ignoring case |

Note that "CASE" in an assertion name means that case is ignored.

`*STREQ*` and `*STRNE*` also accept wide C strings (`wchar_t*`). If a
comparison of two wide strings fails, their values will be printed as UTF-8
narrow strings.

A `NULL` pointer and an empty string are considered _different_.

_Availability_: Linux, Windows, Mac.

See also: For more string comparison tricks (substring, prefix, suffix, and
regular expression matching, for example), see the [Advanced Google Test Guide](AdvancedGuide.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.
  1. In this function, along with any valid C++ statements you want to include, use the various Google Test assertions to check values.
  1. 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.

```
TEST(test_case_name, test_name) {
 ... test body ...
}
```


`TEST()` arguments go from general to specific. The _first_ argument is the
name of the test case, and the _second_ argument is the test's name within the
test case. Both names must be valid C++ identifiers, and they should not contain underscore (`_`). A test's _full name_ consists of its containing test case and its
individual name. Tests from different test cases can have the same individual
name.

For example, let's take a simple integer function:
```
int Factorial(int n); // Returns the factorial of n
```

A test case for this function might look like:
```
// Tests factorial of 0.
TEST(FactorialTest, HandlesZeroInput) {
  EXPECT_EQ(1, Factorial(0));
}

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

Google Test groups the test results by test cases, so logically-related tests
should be in the same test case; 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
case `FactorialTest`.

_Availability_: Linux, Windows, Mac.

# Test Fixtures: Using the Same Data Configuration for Multiple Tests #

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

To create a fixture, just:
  1. Derive a class from `::testing::Test` . Start its body with `protected:` or `public:` as we'll want to access fixture members from sub-classes.
  1. Inside the class, declare any objects you plan to use.
  1. 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` - don't let that happen to you.
  1. 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 this [FAQ entry](FAQ.md#should-i-use-the-constructordestructor-of-the-test-fixture-or-the-set-uptear-down-function).
  1. 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:
```
TEST_F(test_case_name, test_name) {
 ... test body ...
}
```

Like `TEST()`, the first argument is the test case 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()`, Google Test will:
  1. Create a _fresh_ test fixture at runtime
  1. Immediately initialize it via `SetUp()` ,
  1. Run the test
  1. Clean up by calling `TearDown()`
  1. Delete the test fixture.  Note that different tests in the same test case have different test fixture objects, and Google Test always deletes a test fixture before it creates the next one. Google Test 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:
```
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.
```
class QueueTest : public ::testing::Test {
 protected:
  virtual void SetUp() {
    q1_.Enqueue(1);
    q2_.Enqueue(2);
    q2_.Enqueue(3);
  }

  // virtual void TearDown() {}

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

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

  n = q1_.Dequeue();
  ASSERT_TRUE(n != NULL);
  EXPECT_EQ(1, *n);
  EXPECT_EQ(0, q1_.size());
  delete n;

  n = q2_.Dequeue();
  ASSERT_TRUE(n != NULL);
  EXPECT_EQ(2, *n);
  EXPECT_EQ(1, q2_.size());
  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_TRUE(n != NULL)`, 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. Google Test constructs a `QueueTest` object (let's call it `t1` ).
  1. `t1.SetUp()` initializes `t1` .
  1. The first test ( `IsEmptyInitially` ) runs on `t1` .
  1. `t1.TearDown()` cleans up after the test finishes.
  1. `t1` is destructed.
  1. The above steps are repeated on another `QueueTest` object, this time running the `DequeueWorks` test.

_Availability_: Linux, Windows, Mac.

_Note_: Google Test automatically saves all _Google Test_ flags when a test
object is constructed, and restores them when it is destructed.

# Invoking the Tests #

`TEST()` and `TEST_F()` implicitly register their tests with Google Test. 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 cases, or even different source files.

When invoked, the `RUN_ALL_TESTS()` macro:
  1. Saves the state of all  Google Test flags.
  1. Creates a test fixture object for the first test.
  1. Initializes it via `SetUp()`.
  1. Runs the test on the fixture object.
  1. Cleans up the fixture via `TearDown()`.
  1. Deletes the fixture.
  1. Restores the state of all Google Test flags.
  1. Repeats the above steps for the next test, until all tests have run.

In addition, if the text fixture's constructor generates a fatal failure in
step 2, there is no point for step 3 - 5 and they are thus skipped. Similarly,
if step 3 generates a fatal failure, step 4 will be skipped.

_Important_: You must not ignore the return value of `RUN_ALL_TESTS()`, or `gcc`
will give you 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 Google Test features (e.g. thread-safe death
tests) and thus is not supported.

_Availability_: Linux, Windows, Mac.

# Writing the main() Function #

You can start from this boilerplate:
```
#include "this/package/foo.h"
#include "gtest/gtest.h"

namespace {

// The fixture for testing class Foo.
class FooTest : public ::testing::Test {
 protected:
  // You can remove any or all of the following functions if its body
  // is empty.

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

  virtual ~FooTest() {
    // 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:

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

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

  // Objects declared here can be used by all tests in the test case for Foo.
};

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

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

}  // namespace

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

The `::testing::InitGoogleTest()` function parses the command line for Google
Test 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 [AdvancedGuide](AdvancedGuide.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 gtest\_main library and you are good to go.

## Important note for Visual C++ users ##
If you put your tests into a library and your `main()` function is in a different library or in your .exe file, those tests will not run. The reason is a [bug](https://connect.microsoft.com/feedback/viewfeedback.aspx?FeedbackID=244410&siteid=210) in Visual C++. When you define your tests, Google Test creates certain static objects to register them. These objects are not referenced from elsewhere but their constructors are still supposed to run. When Visual C++ linker sees that nothing in the library is referenced from other places it throws the library out. You have to reference your library with tests from your main program to keep the linker from discarding it. Here is how to do it. Somewhere in your library code declare a function:
```
__declspec(dllexport) int PullInMyLibrary() { return 0; }
```
If you put your tests in a static library (not DLL) then `__declspec(dllexport)` is not required. Now, in your main program, write a code that invokes that function:
```
int PullInMyLibrary();
static int dummy = PullInMyLibrary();
```
This will keep your tests referenced and will make them register themselves at startup.

In addition, if you define your tests in a static library, add `/OPT:NOREF` to your main program linker options. If you use MSVC++ IDE, go to your .exe project properties/Configuration Properties/Linker/Optimization and set References setting to `Keep Unreferenced Data (/OPT:NOREF)`. This will keep Visual C++ linker from discarding individual symbols generated by your tests from the final executable.

There is one more pitfall, though. If you use Google Test as a static library (that's how it is defined in gtest.vcproj) your tests must also reside in a static library. If you have to have them in a DLL, you _must_ change Google Test to build into a DLL as well. Otherwise your tests will not run correctly or will not run at all. The general conclusion here is: make your life easier - do not write your tests in libraries!

# Where to Go from Here #

Congratulations! You've learned the Google Test basics. You can start writing
and running Google Test tests, read some [samples](Samples.md), or continue with
[AdvancedGuide](AdvancedGuide.md), which describes many more useful Google Test features.

# 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.