* Subsystem *	* Free capable *	* Constant time *	* Safe *	* From IRQ *	* Notes *
* Static Objects *	N/A	N/A	YES	YES	* Preferred solution for safety applications. *
* Core Memory Manager *	NO	YES	YES	YES	* Fast and safe but unable to free allocated memory. *
* Heap Allocator *	YES	NO	NO	NO	* Unsafe because fragmentation and not constant time, cannot be used * from IRQ handlers. *
* Memory Pools *	YES	YES	YES	YES	* Fast and safe but it can handle fixed size objects only, you may have * multiple memory pools however. *
* C-Runtime *	YES	NO	NO	NO	* Unsafe because fragmentation, not constant time, cannot be used * from IRQ handlers and not thread safe. The C runtime must also be * modified in order to work with the other allocators. *
You can find recipes for using Google Mock here. If you haven't yet,
please read the [ForDummies](V1_7_ForDummies.md) document first to make sure you understand
the basics.

**Note:** Google Mock 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 Google Mock. We omit
such `using` statements in this page for brevity, but you should do it
in your own code.

# Creating Mock Classes #

## 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_METHOD1(Transform, bool(Gadget* g));

  // The following must be in the public section, even though the
  // methods are protected or private in the base class.
  MOCK_METHOD0(Resume, void());
  MOCK_METHOD0(GetTimeOut, int());
};
```

## 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_METHOD1(Add, int(Element x));
  MOCK_METHOD2(Add, int(int times, Element x);

  MOCK_METHOD0(GetBar, Bar&());
  MOCK_CONST_METHOD0(GetBar, const Bar&());
};
```

**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_METHOD1(Add, int(Element x));
  // We don't want to mock int Add(int times, Element x);
  ...
};
```

## Mocking Class Templates ##

To mock a class template, append `_T` to the `MOCK_*` macros:

```
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_CONST_METHOD0_T(GetSize, int());
  MOCK_METHOD1_T(Push, void(const Elem& x));
};
```

## Mocking Nonvirtual Methods ##

Google Mock can mock non-virtual functions to be used in what we call _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:

```
// 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_CONST_METHOD1(GetPacket, const Packet*(size_t packet_number));
  MOCK_CONST_METHOD0(NumberOfPackets, size_t());
  ...
};
```

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's possible to use Google Mock to mock a free function (i.e. a
C-style function or a static method).  You just need to 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:
  ...
  virtual bool Open(const char* path, const char* mode) {
    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 much 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](#Mocking_Nonvirtual_Methods.md).

## The Nice, the Strict, and the Naggy ##

If a mock method has no `EXPECT_CALL` spec but is called, Google Mock
will print a warning about the "uninteresting call". The rationale is:

  * New methods may be added to an interface after a test is written. We shouldn't fail a test just because a method it doesn't know about is called.
  * However, this may also mean there's a bug in the test, so Google Mock shouldn't be silent either. If the user believes these calls are harmless, he can add an `EXPECT_CALL()` to suppress the warning.

However, sometimes you may want to suppress all "uninteresting call"
warnings, while sometimes you may want the opposite, i.e. to treat all
of them as errors. Google Mock 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, it will be
reported by Google Mock as a warning. However, if you rewrite your
test to use `NiceMock<MockFoo>` instead, the warning will be gone,
resulting in a cleaner test output:

```
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.
}
```

There are some caveats though (I don't like them just as much as the
next guy, but 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*` family of macros **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.
  1. The constructors of the base mock (`MockFoo`) cannot have arguments passed by non-const reference, which happens to be banned by the [Google C++ style guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml).
  1. During the constructor or destructor of `MockFoo`, the mock object is _not_ nice or strict.  This may cause surprises if the constructor or destructor calls a mock method on `this` object. (This behavior, however, is consistent with C++'s general rule: if a constructor or destructor calls a virtual method of `this` object, that method is treated as non-virtual.  In other words, to the base class's constructor or destructor, `this` object behaves like an instance of the base class, not the derived class.  This rule is required for safety.  Otherwise a base constructor may use members of a derived class before they are initialized, or a base destructor may use members of a derived class after they have been destroyed.)

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 should'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 (let's
say that 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 re-dispatch the method in the mock class:

```
class ScopedMockLog : public LogSink {
 public:
  ...
  virtual 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) {
    // 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_METHOD3(Log, void(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 much more user-friendly.

## 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:
  virtual char DoThis(int n) {
    return (n > 0) ? '+' :
        (n < 0) ? '-' : '0';
  }

  virtual void DoThat(const char* s, int* p) {
    *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 Google Mock, you can have it
delegate its default action to a fake class you already have, using
this pattern:

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

class MockFoo : public Foo {
 public:
  // Normal mock method definitions using Google Mock.
  MOCK_METHOD1(DoThis, char(int n));
  MOCK_METHOD2(DoThat, void(const char* s, int* p));

  // 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(Invoke(&fake_, &FakeFoo::DoThis));
    ON_CALL(*this, DoThat(_, _))
        .WillByDefault(Invoke(&fake_, &FakeFoo::DoThat));
  }
 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(2, n);
}
```

**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()`), see the "Selecting Between Overloaded Functions" section on this page; 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::_;
using ::testing::AtLeast;
using ::testing::Invoke;

class MockFoo : public Foo {
 public:
  MockFoo() {
    // By default, all calls are delegated to the real object.
    ON_CALL(*this, DoThis())
        .WillByDefault(Invoke(&real_, &Foo::DoThis));
    ON_CALL(*this, DoThat(_))
        .WillByDefault(Invoke(&real_, &Foo::DoThat));
    ...
  }
  MOCK_METHOD0(DoThis, ...);
  MOCK_METHOD1(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, Google Mock 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_METHOD1(Pure, void(int n));
  // Mocking a concrete method.  Foo::Concrete() is shadowed.
  MOCK_METHOD1(Concrete, int(const char* str));
};
```

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).

The trick is to leave a back door in your mock class for accessing the
real methods in the base class:

```
class MockFoo : public Foo {
 public:
  // Mocking a pure method.
  MOCK_METHOD1(Pure, void(int n));
  // Mocking a concrete method.  Foo::Concrete() is shadowed.
  MOCK_METHOD1(Concrete, int(const char* str));

  // Use this to call Concrete() defined in Foo.
  int FooConcrete(const char* str) { return Foo::Concrete(str); }
};
```

Now, you can call `Foo::Concrete()` inside an action by:

```
using ::testing::_;
using ::testing::Invoke;
...
  EXPECT_CALL(foo, Concrete(_))
      .WillOnce(Invoke(&foo, &MockFoo::FooConcrete));
```

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

```
using ::testing::Invoke;
...
  ON_CALL(foo, Concrete(_))
      .WillByDefault(Invoke(&foo, &MockFoo::FooConcrete));
```

(Why don't we just write `Invoke(&foo, &Foo::Concrete)`? 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::Ge;
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:

```
using ::testing::_;
using ::testing::NotNull;
...
  EXPECT_CALL(foo, DoThat(_, NotNull()));
```

## Combining Matchers ##

You can build complex matchers from existing ones using `AllOf()`,
`AnyOf()`, 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));
```

## Casting Matchers ##

Google Mock 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` before giving it to the
matcher.

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

  1. Type `T` can be implicitly cast to type `U`;
  1. 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
  1. 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_METHOD1(DoThis, void(Derived* derived));
};
...

  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_METHOD0(GetBar, Bar&());
  MOCK_CONST_METHOD0(GetBar, const Bar&());
};
...

  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 Google Mock 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::Lt;
using ::testing::Matcher;
using ::testing::TypedEq;

class MockPrinter : public Printer {
 public:
  MOCK_METHOD1(Print, void(int n));
  MOCK_METHOD1(Print, void(char c));
};

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::Lt;
using ::testing::Ne;
...
  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<tr1::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`.

As a convenience and example, Google Mock provides some matchers for
2-tuples, including the `Lt()` matcher above. See the [CheatSheet](V1_7_CheatSheet.md) 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 `tr1::tuple` as its argument; Google Mock 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 Google Mock matchers are 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>

std::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
Google Mock, 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:

```
Matches(AllOf(Ge(0), Le(100), Ne(50)))
```

## Using Matchers in Google Test Assertions ##

Since matchers are basically predicates that also know how to describe
themselves, there is a way to take advantage of them in
[Google Test](http://code.google.com/p/googletest/) assertions. It's
called `ASSERT_THAT` and `EXPECT_THAT`:

```
  ASSERT_THAT(value, matcher);  // Asserts that value matches matcher.
  EXPECT_THAT(value, matcher);  // The non-fatal version.
```

For example, in a Google Test test you can write:

```
#include "gmock/gmock.h"

using ::testing::AllOf;
using ::testing::Ge;
using ::testing::Le;
using ::testing::MatchesRegex;
using ::testing::StartsWith;
...

  EXPECT_THAT(Foo(), StartsWith("Hello"));
  EXPECT_THAT(Bar(), MatchesRegex("Line \\d+"));
  ASSERT_THAT(Baz(), AllOf(Ge(5), Le(10)));
```

which (as you can probably guess) executes `Foo()`, `Bar()`, and
`Baz()`, and verifies that:

  * `Foo()` returns a string that starts with `"Hello"`.
  * `Bar()` returns a string that matches regular expression `"Line \\d+"`.
  * `Baz()` returns a number in the range [5, 10].

The nice thing about these macros is that _they read like
English_. They generate informative messages too. For example, if the
first `EXPECT_THAT()` above fails, the message will be something like:

```
Value of: Foo()
  Actual: "Hi, world!"
Expected: starts with "Hello"
```

**Credit:** The idea of `(ASSERT|EXPECT)_THAT` was stolen from the
[Hamcrest](http://code.google.com/p/hamcrest/) project, which adds
`assertThat()` to JUnit.

## Using Predicates as Matchers ##

Google Mock provides a built-in set of matchers. In case you find them
lacking, you can use an arbitray 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 statement `if (condition) ...`.

## Matching Arguments that Are Not Copyable ##

When you do an `EXPECT_CALL(mock_obj, Foo(bar))`, Google Mock saves
away a copy of `bar`. When `Foo()` is called later, Google Mock
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 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 Google Mock that it should
save a reference to `bar`, instead of a copy of it. Here's how:

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

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

> | `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`.

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

```
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 is `AllOf()`.

## 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 iff `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

```
  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 (`linked_ptr`, `shared_ptr`, `scoped_ptr`, and
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...).

## Testing a Certain Property of an Object ##

Sometimes you want to specify that an object argument has a certain
property, but there is no existing matcher that does this. If you want
good error messages, you should define a matcher. If you want to do it
quick and dirty, you could get away with writing an ordinary function.

Let's say you have a mock function that takes an object of type `Foo`,
which has an `int bar()` method and an `int baz()` method, and you
want to constrain that the argument's `bar()` value plus its `baz()`
value is a given number. Here's how you can define a matcher to do it:

```
using ::testing::MatcherInterface;
using ::testing::MatchResultListener;

class BarPlusBazEqMatcher : public MatcherInterface<const Foo&> {
 public:
  explicit BarPlusBazEqMatcher(int expected_sum)
      : expected_sum_(expected_sum) {}

  virtual bool MatchAndExplain(const Foo& foo,
                               MatchResultListener* listener) const {
    return (foo.bar() + foo.baz()) == expected_sum_;
  }

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

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

inline Matcher<const Foo&> BarPlusBazEq(int expected_sum) {
  return MakeMatcher(new BarPlusBazEqMatcher(expected_sum));
}

...

  EXPECT_CALL(..., DoThis(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_METHOD1(Foo, void(const vector<int>& numbers));
...

  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_METHOD1(Foo, void(const vector<int>& numbers));
...

  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.

`ElementsAre()` and `UnorderedElementsAre()` are overloaded to take 0
to 10 arguments. If more are needed, you can place them 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)));
```

**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*()`. Therefore don't use it with containers whose element order is undefined (e.g. `hash_map`).

## Sharing Matchers ##

Under the hood, a Google Mock 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 everytime. Just assign it to a
matcher variable and use that variable repeatedly! For example,

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

# Setting Expectations #

## Knowing When to Expect ##

`ON_CALL` is likely the single most under-utilized construct in Google Mock.

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_CALL`s in your test fixture to set the common mock behavior shared by all tests in the same group, and write (scarcely) different `EXPECT_CALL`s in different `TEST_F`s to verify different aspects of the code's behavior. Compared with the style where each `TEST` has many `EXPECT_CALL`s, 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).

## 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,
Google Mock will perform its default action to allow the test program
to continue. If you are not happy with the default action taken by
Google Mock, you can override it using `DefaultValue<T>::Set()`
(described later in this document) 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.

## Expecting Ordered Calls ##

Although an `EXPECT_CALL()` statement defined earlier takes precedence
when Google Mock 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 third `EXPECT_CALL()`, but not those in the first two,
then the third 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, Google Mock 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.

Google Mock allows you to impose an arbitrary DAG (directed acyclic
graph) on the calls. One way to express the DAG is to use the
[After](http://code.google.com/p/googlemock/wiki/V1_7_CheatSheet#The_After_Clause) 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 orginal 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, Google Mock only consider 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_METHOD0(GetBar, Bar&());
};
...

  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_.).

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, Google Mock 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?

You may be tempted to try `ByRef()`:

```
using testing::ByRef;
using testing::Return;

class MockFoo : public Foo {
 public:
  MOCK_METHOD0(GetValue, int());
};
...
  int x = 0;
  MockFoo foo;
  EXPECT_CALL(foo, GetValue())
      .WillRepeatedly(Return(ByRef(x)));
  x = 42;
  EXPECT_EQ(42, foo.GetValue());
```

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

```
Value of: foo.GetValue()
  Actual: 0
The three subsystems
