gMock is Google‘s framework for creating and using C++ mock classes. It helps you design better systems and write better tests. A mock object is an object that you use in a test instead of a real object. A mock object implements the same interface as a real object but lets you specify at run time how the object will be used. When you write tests that use a mock, you define expectations about how the mock’s methods will be called. Your test then verifies how your real code behaves when interacting with the mock. See the Mock Objects Best Practices Guide for a comparison of mocks with stubs, fakes, and other kinds of test doubles.
For example, gMock provides a simple syntax for declaring “I expect the RetryQuery method on this mock object to be called three times in the course of this test”. Your test will fail if the expectation isn't met.
The gMock library provides a mock framework for C++ similar to jMock or EasyMock? for Java. In gMock you use macros to define methods for your mock objects and set expectations for those methods. gMock runs on Linux, Windows, and Mac OS X.
Mocks in general are good for:
gMock in particular is good for writing quality C++ mocks. Without the help of a mocking framework like gMock, good C++ mocks are hard to create.
gMock is not good for testing the behavior of dependencies. The point of testing with mocks is to test the classes that use the mocks, not the mocks themselves. Objects that have working toy implementations are called fakes instead of mocks. For example, you could use an in-memory file system to fake disk operations.
Mocks aren‘t useful for very simple classes like Dumb Data Objects. If it’s more trouble to use a mock than the real class, just use the real class.
There are over 30K tests using gmock. Virtually every C++ test at Google that needs a mock object uses gMock.
gMock is bundled with gUnit. To use gMock, include a dependency on //testing/base/public:gunit in the BUILD rule for your mocks, and use the following include statement in the file that defines your mock class:
#include "gmock/gmock.h"
| Implementation language | C++ |
| Code location | google3/third_party/googletest/googlemock/ |
| Build target | //testing/base/public:gunit |
Use dependency injection to enable easy mocking. If you define dependencies as interfaces rather than concrete classes, you can swap out the production version of a class for a mock during testing.
You can also use gMock during the design phase for your system. By sketching your architecture using mocks rather than full implementations, you can evolve your design more quickly.
In January 2007 Zhanyong Wan and the Testing Technology team met with experienced C++ engineers to find out about C++ testing needs. The team learned that creating mocks in C++ was a major pain point. They looked around for existing frameworks but didn't find anything satisfactory. So Zhanyong Wan tackled the problem of creating a usable C++ mocking framework.
C++ posed a unique problem for mocking: while reflection in Java and Python make it easy to generate a mock implementation of any interface, C++ does not have reflection. Wan hit on macros as a way to simplify mock writing in C++, and gMock was born.