@grain/libbinaryen/binaryen/third_party/googletest/docs/gmock_cook_book.md

Libbinaryen packaged for OCaml.

# gMock Cookbook

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

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

## Creating Mock Classes

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

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

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

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

### Dealing with unprotected commas

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

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

Solution 1 - wrap with parentheses:

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

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

Solution 2 - define an alias:

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

### Mocking Private or Protected Methods

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

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

 protected:
  virtual void Resume();

 private:
  virtual int GetTimeOut();
};

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

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

### Mocking Overloaded Methods

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

```cpp
class Foo {
  ...

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

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

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

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

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

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

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

### Mocking Class Templates

You can mock class templates just like any class.

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

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

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

### Mocking Non-virtual Methods {#MockingNonVirtualMethods}

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

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

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

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

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

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

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

```cpp
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.

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

### Mocking Free Functions

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

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

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

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

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

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

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

### Old-Style `MOCK_METHODn` Macros

Before the generic `MOCK_METHOD` macro
[was introduced in 2018](https://github.com/google/googletest/commit/c5f08bf91944ce1b19bcf414fa1760e69d20afc2),
mocks where created using a family of macros collectively called `MOCK_METHODn`.
These macros are still supported, though migration to the new `MOCK_METHOD` is
recommended.

The macros in the `MOCK_METHODn` family differ from `MOCK_METHOD`:

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

Old macros and their new equivalents:

<table>
  <tr><th colspan=2>Simple</th></tr>
  <tr>
    <td>Old</td>
    <td><code>MOCK_METHOD1(Foo, bool(int))</code></td>
  </tr>
  <tr>
    <td>New</td>
    <td><code>MOCK_METHOD(bool, Foo, (int))</code></td>
  </tr>

  <tr><th colspan=2>Const Method</th></tr>
  <tr>
    <td>Old</td>
    <td><code>MOCK_CONST_METHOD1(Foo, bool(int))</code></td>
  </tr>
  <tr>
    <td>New</td>
    <td><code>MOCK_METHOD(bool, Foo, (int), (const))</code></td>
  </tr>

  <tr><th colspan=2>Method in a Class Template</th></tr>
  <tr>
    <td>Old</td>
    <td><code>MOCK_METHOD1_T(Foo, bool(int))</code></td>
  </tr>
  <tr>
    <td>New</td>
    <td><code>MOCK_METHOD(bool, Foo, (int))</code></td>
  </tr>

  <tr><th colspan=2>Const Method in a Class Template</th></tr>
  <tr>
    <td>Old</td>
    <td><code>MOCK_CONST_METHOD1_T(Foo, bool(int))</code></td>
  </tr>
  <tr>
    <td>New</td>
    <td><code>MOCK_METHOD(bool, Foo, (int), (const))</code></td>
  </tr>

  <tr><th colspan=2>Method with Call Type</th></tr>
  <tr>
    <td>Old</td>
    <td><code>MOCK_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))</code></td>
  </tr>
  <tr>
    <td>New</td>
    <td><code>MOCK_METHOD(bool, Foo, (int), (Calltype(STDMETHODCALLTYPE)))</code></td>
  </tr>

  <tr><th colspan=2>Const Method with Call Type</th></tr>
  <tr>
    <td>Old</td>
    <td><code>MOCK_CONST_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))</code></td>
  </tr>
  <tr>
    <td>New</td>
    <td><code>MOCK_METHOD(bool, Foo, (int), (const, Calltype(STDMETHODCALLTYPE)))</code></td>
  </tr>

  <tr><th colspan=2>Method with Call Type in a Class Template</th></tr>
  <tr>
    <td>Old</td>
    <td><code>MOCK_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))</code></td>
  </tr>
  <tr>
    <td>New</td>
    <td><code>MOCK_METHOD(bool, Foo, (int), (Calltype(STDMETHODCALLTYPE)))</code></td>
  </tr>

  <tr><th colspan=2>Const Method with Call Type in a Class Template</th></tr>
  <tr>
    <td>Old</td>
    <td><code>MOCK_CONST_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))</code></td>
  </tr>
  <tr>
    <td>New</td>
    <td><code>MOCK_METHOD(bool, Foo, (int), (const, Calltype(STDMETHODCALLTYPE)))</code></td>
  </tr>
</table>

### The Nice, the Strict, and the Naggy {#NiceStrictNaggy}

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

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

Suppose your test uses a mock class `MockFoo`:

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

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

```cpp
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:

```cpp
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:

```cpp
using ::testing::StrictMock;

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

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

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

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

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

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

### Simplifying the Interface without Breaking Existing Code {#SimplerInterfaces}

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

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

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

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

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

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

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

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

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

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

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

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

### Alternative to Mocking Concrete Classes

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

Try not to do that.

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

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

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

This technique incurs some overhead:

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

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

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

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

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

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

### Delegating Calls to a Fake {#DelegatingToFake}

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

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

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

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

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

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

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

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

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

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

```cpp
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()`),
    use [this technique](#SelectOverload); 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](#DelegatingToFake)
technique, the difference being that we use a real object instead of a fake.
Here's an example:

```cpp
using ::testing::AtLeast;

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

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

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

### Delegating Calls to a Parent Class

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

```cpp
class Foo {
 public:
  virtual ~Foo();

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

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

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

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

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

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

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

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

## Using Matchers

### Matching Argument Values Exactly

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

```cpp
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:

```cpp
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:

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

### Combining Matchers {#CombiningMatchers}

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

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

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

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

```cpp
using ::testing::Contains;
using ::testing::Property;

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

### Casting Matchers {#SafeMatcherCast}

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

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

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

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

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

Here's one example:

```cpp
using ::testing::SafeMatcherCast;

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

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

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

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

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

### Selecting Between Overloaded Functions {#SelectOverload}

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.

```cpp
using ::testing::ReturnRef;

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

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

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

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

```cpp
using ::testing::An;
using ::testing::Matcher;
using ::testing::TypedEq;

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

TEST(PrinterTest, Print) {
  MockPrinter printer;

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

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

### Performing Different Actions Based on the Arguments

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

```cpp
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,

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

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

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

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

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

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

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

As a convenience and example, gMock provides some matchers for 2-tuples,
including the `Lt()` matcher above. See
[Multi-argument Matchers](reference/matchers.md#MultiArgMatchers) for the
complete list.

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

### Using Matchers as Predicates

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

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

```cpp
#include <algorithm>
#include <vector>

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

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

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

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

### Using Matchers in googletest Assertions

See [`EXPECT_THAT`](reference/assertions.md#EXPECT_THAT) in the Assertions
Reference.

### Using Predicates as Matchers

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

```cpp
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 in statement
`if (condition) ...`.

### Matching Arguments that Are Not Copyable

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

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

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

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

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

### Validating a Member of an Object

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

```cpp
Field(&Foo::bar, m)
```

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

```cpp
Property(&Foo::baz, m)
```

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

For example:

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

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

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

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

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

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

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

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

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

### Validating the Value Pointed to by a Pointer Argument

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

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

```cpp
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

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

without worrying that a `NULL` pointer will crash your test.

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

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

### 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](#NewMatchers). 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:

```cpp
using ::testing::Matcher;

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

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

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

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

Matcher<const Foo&> BarPlusBazEq(int expected_sum) {
  return 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:

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

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

If you instead write:

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

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

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

```cpp
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:

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

Use `Pair` when comparing maps or other associative containers.

{% raw %}

```cpp
using testing::ElementsAre;
using testing::Pair;
...
  std::map<string, int> m = {{"a", 1}, {"b", 2}, {"c", 3}};
  EXPECT_THAT(m, ElementsAre(Pair("a", 1), Pair("b", 2), Pair("c", 3)));
```

{% endraw %}

**Tips:**

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

### Sharing Matchers

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

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

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

### Matchers must have no side-effects {#PureMatchers}

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

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

## Setting Expectations

### Knowing When to Expect {#UseOnCall}

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

There are basically two constructs for defining the behavior of a mock object:
`ON_CALL` and `EXPECT_CALL`. The difference? `ON_CALL` defines what happens when
a mock method is called, but <em>doesn't imply any expectation on the method
being called</em>. `EXPECT_CALL` not only defines the behavio