Google Mock Cheat Sheet

Posted : admin On 1/29/2022

I am new to google test and google mock so I am still a little bit confused. I just tried to implement a simple calculator with integer addition, multiplication and division and create a mock for it. About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features Press Copyright Contact us Creators. Mocking Cheat Sheet - A handy reference for matchers, actions, invariants, and more. Mocking FAQ - Contains answers to some mocking-specific questions. From our free mock Draft Simulator – which allows you to mock draft against realistic opponents – to our Cheat Sheet Creator – which allows you to combine rankings from 100+ experts into one. I am new to google test and google mock so I am still a little bit confused. I just tried to implement a simple calculator with integer addition, multiplication and division and create a mock for it which follows real behaviours.

Legacy gMock FAQ {#GMockFaq}¶

When I call a method on my mock object, the method for the real object is invoked instead. What's the problem?¶

In order for a method to be mocked, it must be virtual, unless you use thehigh-perf dependency injection technique.

Can I mock a variadic function?¶

You cannot mock a variadic function (i.e. a function taking ellipsis (...)arguments) directly in gMock.

The problem is that in general, there is no way for a mock object to know howmany arguments are passed to the variadic method, and what the arguments' typesare. Only the author of the base class knows the protocol, and we cannot lookinto his or her head.


Therefore, to mock such a function, the user must teach the mock object how tofigure out the number of arguments and their types. One way to do it is toprovide overloaded versions of the function.

Ellipsis arguments are inherited from C and not really a C++ feature. They areunsafe to use and don't work with arguments that have constructors ordestructors. Therefore we recommend to avoid them in C++ as much as possible.

MSVC gives me warning C4301 or C4373 when I define a mock method with a const parameter. Why?¶

If you compile this using Microsoft Visual C++ 2005 SP1:

You may get the following warning:

This is a MSVC bug. The same code compiles fine with gcc, for example. If youuse Visual C++ 2008 SP1, you would get the warning:


In C++, if you declare a function with a const parameter, the constmodifier is ignored. Therefore, the Foo base class above is equivalent to:

In fact, you can declareBar() with an int parameter, and define it with aconst int parameter. The compiler will still match them up.

Since making a parameter const is meaningless in the method declaration, werecommend to remove it in both Foo and MockFoo. That should workaround theVC bug.

Note that we are talking about the top-levelconst modifier here. If thefunction parameter is passed by pointer or reference, declaring the pointee orreferee as const is still meaningful. For example, the following twodeclarations are not equivalent:


I can't figure out why gMock thinks my expectations are not satisfied. What should I do?¶

You might want to run your test with --gmock_verbose=info. This flag letsgMock print a trace of every mock function call it receives. By studying thetrace, you'll gain insights on why the expectations you set are not met.

If you see the message 'The mock function has no default action set, and itsreturn type has no default value set.', then tryadding a default action. Due to a known issue,unexpected calls on mocks without default actions don't print out a detailedcomparison between the actual arguments and the expected arguments.

My program crashed and ScopedMockLog spit out tons of messages. Is it a gMock bug?¶

gMock and ScopedMockLog are likely doing the right thing here.

When a test crashes, the failure signal handler will try to log a lot ofinformation (the stack trace, and the address map, for example). The messagesare compounded if you have many threads with depth stacks. When ScopedMockLogintercepts these messages and finds that they don't match any expectations, itprints an error for each of them.

You can learn to ignore the errors, or you can rewrite your expectations to makeyour test more robust, for example, by adding something like:

How can I assert that a function is NEVER called?¶

I have a failed test where gMock tells me TWICE that a particular expectation is not satisfied. Isn't this redundant?¶

When gMock detects a failure, it prints relevant information (the mock functionarguments, the state of relevant expectations, and etc) to help the user debug.If another failure is detected, gMock will do the same, including printing thestate of relevant expectations.

Sometimes an expectation's state didn't change between two failures, and you'llsee the same description of the state twice. They are however not redundant,as they refer to different points in time. The fact they are the same isinteresting information.

I get a heapcheck failure when using a mock object, but using a real object is fine. What can be wrong?¶

Does the class (hopefully a pure interface) you are mocking have a virtualdestructor?

Whenever you derive from a base class, make sure its destructor is virtual.Otherwise Bad Things will happen. Consider the following code:

By changing ~Base() to virtual, ~Derived() will be correctly called whendelete p is executed, and the heap checker will be happy.

The 'newer expectations override older ones' rule makes writing expectations awkward. Why does gMock do that?¶

When people complain about this, often they are referring to code like:

The problem, is that they didn't pick the best way to express the test'sintent.

By default, expectations don't have to be matched in any particular order. Ifyou want them to match in a certain order, you need to be explicit. This isgMock's (and jMock's) fundamental philosophy: it's easy to accidentallyover-specify your tests, and we want to make it harder to do so.

There are two better ways to write the test spec. You could either put theexpectations in sequence:

or you can put the sequence of actions in the same expectation:

Back to the original questions: why does gMock search the expectations (andON_CALLs) from back to front? Because this allows a user to set up a mock'sbehavior for the common case early (e.g. in the mock's constructor or the testfixture's set-up phase) and customize it with more specific rules later. IfgMock searches from front to back, this very useful pattern won't be possible.

gMock prints a warning when a function without EXPECT_CALL is called, even if I have set its behavior using ON_CALL. Would it be reasonable not to show the warning in this case?¶

When choosing between being neat and being safe, we lean toward the latter. Sothe answer is that we think it's better to show the warning.

Often people write ON_CALLs in the mock object's constructor or SetUp(), asthe default behavior rarely changes from test to test. Then in the test bodythey set the expectations, which are often different for each test. Having anON_CALL in the set-up part of a test doesn't mean that the calls are expected.If there's no EXPECT_CALL and the method is called, it's possibly an error. Ifwe quietly let the call go through without notifying the user, bugs may creep inunnoticed.

If, however, you are sure that the calls are OK, you can write

instead of

This tells gMock that you do expect the calls and no warning should be printed.

Also, you can control the verbosity by specifying --gmock_verbose=error. Othervalues are info and warning. If you find the output too noisy whendebugging, just choose a less verbose level.

How can I delete the mock function's argument in an action?¶

If your mock function takes a pointer argument and you want to delete thatargument, you can use testing::DeleteArg() to delete the N'th (zero-indexed)argument:

How can I perform an arbitrary action on a mock function's argument?¶

If you find yourself needing to perform some action that's not supported bygMock directly, remember that you can define your own actions usingMakeAction() orMakePolymorphicAction(), or you can write a stub functionand invoke it using Invoke().

My code calls a static/global function. Can I mock it?¶

You can, but you need to make some changes.

In general, if you find yourself needing to mock a static function, it's a signthat your modules are too tightly coupled (and less flexible, less reusable,less testable, etc). You are probably better off defining a small interface andcall the function through that interface, which then can be easily mocked. It'sa bit of work initially, but usually pays for itself quickly.

This Google Testing Blogpost says itexcellently. Check it out.

My mock object needs to do complex stuff. It's a lot of pain to specify the actions. gMock sucks!¶

I know it's not a question, but you get an answer for free any way. :-)

With gMock, you can create mocks in C++ easily. And people might be tempted touse them everywhere. Sometimes they work great, and sometimes you may find them,well, a pain to use. So, what's wrong in the latter case?

When you write a test without using mocks, you exercise the code and assert thatit returns the correct value or that the system is in an expected state. This issometimes called 'state-based testing'.

Mocks are great for what some call 'interaction-based' testing: instead ofchecking the system state at the very end, mock objects verify that they areinvoked the right way and report an error as soon as it arises, giving you ahandle on the precise context in which the error was triggered. This is oftenmore effective and economical to do than state-based testing.

If you are doing state-based testing and using a test double just to simulatethe real object, you are probably better off using a fake. Using a mock in thiscase causes pain, as it's not a strong point for mocks to perform complexactions. If you experience this and think that mocks suck, you are just notusing the right tool for your problem. Or, you might be trying to solve thewrong problem. :-)

I got a warning 'Uninteresting function call encountered - default action taken..' Should I panic?¶

By all means, NO! It's just an FYI. :-)

What it means is that you have a mock function, you haven't set any expectationson it (by gMock's rule this means that you are not interested in calls to thisfunction and therefore it can be called any number of times), and it is called.That's OK - you didn't say it's not OK to call the function!

What if you actually meant to disallow this function to be called, but forgot towrite EXPECT_CALL(foo, Bar()).Times(0)? While one can argue that it's theuser's fault, gMock tries to be nice and prints you a note.

So, when you see the message and believe that there shouldn't be anyuninteresting calls, you should investigate what's going on. To make your lifeeasier, gMock dumps the stack trace when an uninteresting call is encountered.From that you can figure out which mock function it is, and how it is called.

I want to define a custom action. Should I use Invoke() or implement the ActionInterface interface?¶

Either way is fine - you want to choose the one that's more convenient for yourcircumstance.

Usually, if your action is for a particular function type, defining it usingInvoke() should be easier; if your action can be used in functions ofdifferent types (e.g. if you are defining Return(*value*)),MakePolymorphicAction() is easiest. Sometimes you want precise control on whattypes of functions the action can be used in, and implementing ActionInterfaceis the way to go here. See the implementation of Return() intesting/base/public/gmock-actions.h for an example.

Gmock Cheat Sheet

I use SetArgPointee() in WillOnce(), but gcc complains about 'conflicting return type specified'. What does it mean?¶

Google Mock Matcher

You got this error as gMock has no idea what value it should return when themock method is called. SetArgPointee() says what the side effect is, butdoesn't say what the return value should be. You need DoAll() to chain aSetArgPointee() with a Return() that provides a value appropriate to the APIbeing mocked.

See this recipe for more details and anexample.

I have a huge mock class, and Microsoft Visual C++ runs out of memory when compiling it. What can I do?¶

We've noticed that when the /clr compiler flag is used, Visual C++ uses 5~6times as much memory when compiling a mock class. We suggest to avoid /clrwhen compiling native C++ mocks.

Google Mock Tutorial

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