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.