This document provides an overview of GoogleMock, a mocking framework for C++. It defines what a mock object is and why they are useful for testing. GoogleMock allows specifying expectations for mock method calls, including how many times a method will be called and what it will return. It provides matchers to check arguments and actions to define return values or behaviors. The document demonstrates how to write mock classes, set expectations, and use mocks in tests to verify code interactions and behavior.

1. GoogleMock for Dummies

2. Outline
• What is Mock?
• What Is Google C++ Mocking Framework?
• Getting Started
• A Case for Mock Turtles
• Writing the Mock Class
• Using Mocks in Tests
• Setting Expectations
• Appendix
• Reference

3. What is Mock?
• We creates a mock object to test the behavior
of some other object
• An object that we want to mock has the
following characteristics:
 Supplies non-deterministic results (e.g. the current time).
 Has states that are difficult to create or reproduce (e.g. a network error).
 Is slow (e.g. a complete database, which would have to be initialized before
the test).
 Does not yet exist or may change behavior.
 Would have to include information and methods exclusively for testing
purposes (and not for its actual task).

4. What is Mock?
• For example, crash test dummy is a mock
object used to simulate the dynamic behavior
of a human in vehicle impacts

5. What Is Google C++ Mocking
Framework?
• A mock object implements the same interface
as a real object (so it can be used as one), but
lets you specify at run time how it will be used
and what it should do
Which methods will be called?
In which order?
How many times?
With what arguments?
What will they return?

6. What Is Google C++ Mocking
Framework?
• A mock allows you to check the interaction
between itself and code that uses it
• Google C++ Mocking Framework is a library
for creating mock classes and using them

7. What Is Google C++ Mocking
Framework?
• Using Google Mock involves three basic steps:
1. Use some simple macros to describe the
interface you want to mock, and they will expand
to the implementation of your mock class
2. Create some mock objects and specify its
expectations and behavior using an intuitive
syntax
3. Exercise code that uses the mock objects. Google
Mock will catch any violation of the expectations
as soon as it arises

8. Getting Started
• #include <gtest/gtest.h>
• #include <gmock/gmock.h>

9. A Case for Mock Turtles
• Suppose you are developing a graphics program
that relies on a API (say, Turtle) for drawing
• Run your program and compare the screen with a
golden screen snapshot to see if it does the right
thing
• What if the API has been upgraded? Re-run your
program and compare the screen with a up-to-
date golden screen snapshot?
• Instead of having your application talk to the
drawing API directly, wrap the API in an interface
and code to that interface

10. A Case for Mock Turtles
void foo(int i, char c)
{
……
// bar object is not maintained by
// your team
bar.method(i);
……
}
void foo(int i, char c)
{
……
// we can see how our function
// interact with bar object
mock_bar.method(i);
……
}

11. Getting Started
• This is the API we want to mock

12. Writing the Mock Class
- How to Define It
• Here are the simple steps you need to follow:
1. Derive a class MockTurtle from Turtle.
2. Take a virtual function of Turtle. Count how many
arguments it has.
3. Inside the child class, write
MOCK_METHODn()/MOCK_CONST_METHODn();,
where n is the number of the arguments.
4. Cut-and-paste the function name as the first
argument to the macro, and leave what's left as the
second argument.
5. Repeat until all virtual functions you want to mock
are done.
How about non-virtual function then? Overload function? Class template?
See http://code.google.com/p/googlemock/wiki/CookBook

13. Writing the Mock Class
- How to Define It
• After the process, you should have something
like:

14. Writing the Mock Class
- How to Define It

15. Writing the Mock Class
- Where to Put It
• Put the mock’s definition in mock_xxx.h so
that anyone want to use the mock only need
to include mock_xxx.h

16. Using Mocks in Tests
• The typical work flow is:
1. Import the Google Mock names from the testing namespace such
that you can use them unqualified.
2. Create some mock objects.
3. Specify your expectations on them (How many times will a
method be called? With what arguments? What should it do?
etc.).
4. Exercise some code that uses the mocks; optionally, check the
result using Google Test assertions. If a mock method is called
more than expected or with wrong arguments, you'll get an error
immediately.
5. When a mock is destructed, Google Mock will automatically
check whether all expectations on it have been satisfied.

17. Using Mocks in Tests
We expect that PenDown() is called at least
once.
Our painter object
will use Turtle object
to do the drawing.
googletest
assertion

18. Setting Expectations
• The key to using a mock object successfully is
to set the right expectations on it
• Too strict or too loose expectation is not good

19. Setting Expectations
- General Syntax
• use the EXPECT_CALL() macro to set an
expectation on a mock method
• Two arguments: first the mock object, and
then the method and its arguments
• Note that the two are separated by a comma
It’s a comma
Optional clauses that provide more
information about the expectation.

20. Setting Expectations
- General Syntax
• The turtle object's GetX() method will be
called five times, it will return 100 the first
time, 150 the second time, and then 200
every time

21. Matchers
- What Arguments Do We Expect?
• When a mock function takes arguments, we
must specify what arguments we are
expecting
• Matchers are more flexible
• A matcher is like a predicate and can test
whether an argument is what we'd expect
Maybe too strict?
A list of built-in matchers can be found on
http://code.google.com/p/googlemock/wiki/CheatSheet

22. Cardinalities
- How Many Times Will It Be Called?
• Times() is a clause can be specified follows an
EXPECT_CALL()
• We call its argument a cardinality as it tells
how many times the call should occur
• Times(cardinality) can:
 Repeat an expectation many times.
 A cardinality can be "fuzzy", just like a matcher can be.

23. Cardinalities
- How Many Times Will It Be Called?
• Times(0)
 Google Mock will report a Google Test failure whenever the function is
(wrongfully) called
• AtLeast(n)
 An example of fuzzy cardinalities. The call is expected at least n times.
• If Times() ommitted, Google Mock will infer the
cardinality
 If neither WillOnce() nor WillRepeatedly() is in the EXPECT_CALL(), the
inferred cardinality is Times(1).
 If there are n WillOnce()'s but no WillRepeatedly(), where n >= 1, the
cardinality is Times(n).
 If there are n WillOnce()'s and one WillRepeatedly(), where n >= 0, the
cardinality is Times(AtLeast(n)).

24. Cardinalities
- How Many Times Will It Be Called?
• How do Google Mock infer the cardinality?
WillRepeatedly()
WillOnce()
None one
None Times(1)
n ≥ 1 Times(n)
n ≥ 0 Times(AtLeast(n))

25. Actions
- What Should It Do?
• If the return type of a mock function is a built-
in type or a pointer, the function has a default
action. If you don't say anything, this behavior
will be used
 void function will just return.
 bool function will return false.
 Other functions will return 0.
Say something

26. Actions
- What Should It Do?
• If a mock function doesn't have a default
action, or the default action doesn't suit you,
you can specify the action to be taken each
time the expectation matches using a series of
WillOnce() clauses followed by an optional
WillRepeatedly()

27. Actions
- What Should It Do?

28. Actions
- What Should It Do?

29. Actions
- What Should It Do?
• What can we do inside WillOnce() besides
Return()? You can return a reference using
ReturnRef(variable), or invoke a pre-defined
function
See http://code.google.com/p/googlemock/wiki/CheatSheet#Actions
Invoke pre-defined function

30. Actions
- What Should It Do?
• EXPECT_CALL() statement evaluates the action
clause only once, even though the action may
be performed many times

31. Actions
- What Should It Do?
• Instead of returning 100, 101, 102, ..., consecutively,
this mock function will always return 100 as n++ is only
evaluated once, which means it will return 100, 0, 0, 0
• turtle.GetY() will return 100 the first time, but return 0
from the second time on, as returning 0 is the default
action for int functions
Postfix increment

32. Using Multiple Expectations
• Specify expectations on multiple mock
methods, which may be from multiple mock
objects
• Google Mock will search the expectations in
the reverse order they are defined, and stop
when an active expectation that matches the
arguments is found
• Newer rules override older ones!

33. Using Multiple Expectations
• If the matching expectation cannot take any
more calls, you will get an upper-bound-
violated failure
• If Forward(10) is called three times in a row,
the third time it will be an error, as the last
matching expectation (#2) has been saturated

34. Using Multiple Expectations
• Match in the reverse order?
• This allows a user to set up the default
expectations in a mock object's constructor or
the test fixture's set-up phase and then
customize the mock by writing more specific
expectations in the test body
• Put the one with more specific matchers after
the other

35. Ordered vs Unordered Calls
• The calls don't have to occur in the order the
expectations are specified
• Want all the expected calls to occur in a strict
order?
• Creating an object of type InSequence
Want to specify partial order?
See http://code.google.com/p/googlemock/wiki/CookBook

36. All Expectations Are Sticky
• Test that the turtle is asked to go to the origin
exactly twice. What if GOTO(0, 0) called 3
times?
• Expectations in Google Mock are "sticky" by
default, in the sense that they remain active
even after we have reached their invocation
upper bounds

37. All Expectations Are Sticky
• turtle.GetX() will be called n times and will
return 10, 20, 30, ..., consecutively?
• The second time turtle.GetX() is called, the
last (latest) EXPECT_CALL() statement will
match. It’s a "upper bound exceeded" error!

38. All Expectations Are Sticky
• How to make expectation not sticky?
• Use RetiresOnSaturation()
• turtle.GetX() will return 10, 20, 30, ...
It should retire as soon as it is
saturated.

39. All Expectations Are Sticky
• The other situation where an expectation may
not be sticky is when it's in a sequence
• As soon as another expectation that comes
after it in the sequence has been used, it
automatically retires

40. Uninteresting Calls
• In Google Mock, if you are not interested in a
method, just don't say anything about it
• If a call to this method occurs, you'll see a
warning in the test output, but it won't be a
failure

41. Appendix
- Mocks and fakes
• Fakes implement the same interface as the
object that they represent and returning pre-
arranged responses
• Mocks do a little more. They will examine the
context of each call
 Checking the order in which its methods are called.
 Performing tests on the data passed into the method calls as arguments.

42. Appendix
- Mocks and fakes
• Fake objects have working implementations,
but usually take some shortcut (perhaps to
make the operations less expensive), which
makes them not suitable for production. An
in-memory file system would be an example
of a fake.
• Mocks are objects pre-programmed with
expectations, which form a specification of the
calls they are expected to receive

43. Reference
• http://code.google.com/p/googlemock/wiki/ForDummies
• http://en.wikipedia.org/wiki/Mock_object