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Google's framework for writing and using C++ mock classes. It can help you derive better designs of your system and write better tests.

It is inspired by:

It is designed with C++'s specifics in mind.

gMock:

• Provides a declarative syntax for defining mocks.
• Can define partial (hybrid) mocks, which are a cross of real and mock objects.
• Handles functions of arbitrary types and overloaded functions.
• Comes with a rich set of matchers for validating function arguments.
• Uses an intuitive syntax for controlling the behavior of a mock.
• Does automatic verification of expectations (no record-and-replay needed).
• Allows arbitrary (partial) ordering constraints on function calls to be expressed.
• Lets a user extend it by defining new matchers and actions.
• Does not use exceptions.
• Is easy to learn and use.

Details and examples can be found here:

Please note that code under scripts/generator/ is from the cppclean project and under the Apache License, which is different from GoogleMock's license.

GoogleMock is a part of GoogleTest C++ testing framework and a subject to the same requirements.

What Is gMock?

When you write a prototype or test, often it’s not feasible or wise to rely onreal objects entirely. A mock object implements the same interface as a realobject (so it can be used as one), but lets you specify at run time how it willbe used and what it should do (which methods will be called? in which order? howmany times? with what arguments? what will they return? etc).

It is easy to confuse the term fake objects with mock objects. Fakes and mocksactually mean very different things in the Test-Driven Development (TDD)community:

• Fake objects have working implementations, but usually take someshortcut (perhaps to make the operations less expensive), which makes themnot suitable for production. An in-memory file system would be an example ofa fake.
• Mocks are objects pre-programmed with expectations, which form aspecification of the calls they are expected to receive.

If all this seems too abstract for you, don’t worry - the most important thingto remember is that a mock allows you to check the interaction between itselfand code that uses it. The difference between fakes and mocks shall become muchclearer once you start to use mocks.

gMock is a library (sometimes we also call it a “framework” to make it soundcool) for creating mock classes and using them. It does to C++ whatjMock/EasyMock does to Java (well, more or less).

When using gMock,

1. first, you use some simple macros to describe the interface you want tomock, and they will expand to the implementation of your mock class;
2. next, you create some mock objects and specify its expectations and behaviorusing an intuitive syntax;
3. then you exercise code that uses the mock objects. gMock will catch anyviolation to the expectations as soon as it arises.

Why gMock?

While mock objects help you remove unnecessary dependencies in tests and makethem fast and reliable, using mocks manually in C++ is hard:

• Someone has to implement the mocks. The job is usually tedious anderror-prone. No wonder people go great distance to avoid it.
• The quality of those manually written mocks is a bit, uh, unpredictable. Youmay see some really polished ones, but you may also see some that werehacked up in a hurry and have all sorts of ad hoc restrictions.
• The knowledge you gained from using one mock doesn’t transfer to the nextone.

In contrast, Java and Python programmers have some fine mock frameworks (jMock,EasyMock, etc), which automate the creation of mocks. As a result, mocking is aproven effective technique and widely adopted practice in those communities.Having the right tool absolutely makes the difference.

gMock was built to help C++ programmers. It was inspired by jMock and EasyMock,but designed with C++’s specifics in mind. It is your friend if any of thefollowing problems is bothering you:

• You are stuck with a sub-optimal design and wish you had done moreprototyping before it was too late, but prototyping in C++ is by no means“rapid”.
• Your tests are slow as they depend on too many libraries or use expensiveresources (e.g. a database).
• Your tests are brittle as some resources they use are unreliable (e.g. thenetwork).
• You want to test how your code handles a failure (e.g. a file checksumerror), but it’s not easy to cause one.
• You need to make sure that your module interacts with other modules in theright way, but it’s hard to observe the interaction; therefore you resort toobserving the side effects at the end of the action, but it’s awkward atbest.
• You want to “mock out” your dependencies, except that they don’t have mockimplementations yet; and, frankly, you aren’t thrilled by some of thosehand-written mocks.

We encourage you to use gMock as Fl studio plugins zip.

• a design tool, for it lets you experiment with your interface design earlyand often. More iterations lead to better designs!
• a testing tool to cut your tests’ outbound dependencies and probe theinteraction between your module and its collaborators.

Getting Started

gMock is bundled with googletest.

A Case for Mock Turtles

Let’s look at an example. Suppose you are developing a graphics program thatrelies on a LOGO-likeAPI for drawing. How would you test that it does the right thing? Well, you canrun it and compare the screen with a golden screen snapshot, but let’s admit it:tests like this are expensive to run and fragile (What if you just upgraded to ashiny new graphics card that has better anti-aliasing? Suddenly you have toupdate all your golden images.). It would be too painful if all your tests arelike this. Fortunately, you learned aboutDependency Injection and know the right thingto do: instead of having your application talk to the system API directly, wrapthe API in an interface (say, Turtle) and code to that interface:

(Note that the destructor of Turtlemust be virtual, as is the case forall classes you intend to inherit from - otherwise the destructor of thederived class will not be called when you delete an object through a basepointer, and you’ll get corrupted program states like memory leaks.)

You can control whether the turtle’s movement will leave a trace using PenUp()and PenDown(), and control its movement using Forward(), Turn(), andGoTo(). Finally, GetX() and GetY() tell you the current position of theturtle.

Your program will normally use a real implementation of this interface. Intests, you can use a mock implementation instead. Cnet asia download. This allows you to easilycheck what drawing primitives your program is calling, with what arguments, andin which order. Tests written this way are much more robust (they won’t breakbecause your new machine does anti-aliasing differently), easier to read andmaintain (the intent of a test is expressed in the code, not in some binaryimages), and run much, much faster.

Writing the Mock Class

If you are lucky, the mocks you need to use have already been implemented bysome nice people. If, however, you find yourself in the position to write a mockclass, relax - gMock turns this task into a fun game! (Well, almost.)

How to Define It

Using the Turtle interface as example, here are the simple steps you need tofollow:

• Derive a class MockTurtle from Turtle.
• Take a virtual function of Turtle (while it’s possible tomock non-virtual methods using templates,it’s much more involved).
• In the public: section of the child class, write MOCK_METHOD();
• Now comes the fun part: you take the function signature, cut-and-paste itinto the macro, and add two commas - one between the return type and thename, another between the name and the argument list.
• If you’re mocking a const method, add a 4th parameter containing (const)(the parentheses are required).
• Since you’re overriding a virtual method, we suggest adding the overridekeyword. For const methods the 4th parameter becomes (const, override),for non-const methods just (override). This isn’t mandatory.
• Repeat until all virtual functions you want to mock are done. (It goeswithout saying that all pure virtual methods in your abstract class mustbe either mocked or overridden.)

After the process, you should have something like:

You don’t need to define these mock methods somewhere else - the MOCK_METHODmacro will generate the definitions for you. It’s that simple!

Where to Put It

When you define a mock class, you need to decide where to put its definition.Some people put it in a _test.cc. This is fine when the interface being mocked(say, Foo) is owned by the same person or team. Otherwise, when the owner ofFoo changes it, your test could break. (You can’t really expect Foo’smaintainer to fix every test that uses Foo, can you?)

So, the rule of thumb is: if you need to mock Foo and it’s owned by others,define the mock class in Foo’s package (better, in a testing sub-packagesuch that you can clearly separate production code and testing utilities), putit in a .h and a cc_library. Then everyone can reference them from theirtests. If Foo ever changes, there is only one copy of MockFoo to change, andonly tests that depend on the changed methods need to be fixed.

Another way to do it: you can introduce a thin layer FooAdaptor on top ofFoo and code to this new interface. Since you own FooAdaptor, you can absorbchanges in Foo much more easily. While this is more work initially, carefullychoosing the adaptor interface can make your code easier to write and morereadable (a net win in the long run), as you can choose FooAdaptor to fit yourspecific domain much better than Foo does.

Using Mocks in Tests

Once you have a mock class, using it is easy. The typical work flow is:

1. Import the gMock names from the testing namespace such that you can usethem unqualified (You only have to do it once per file). Remember thatnamespaces are a good idea.
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 usinggoogletest assertions. If a mock method is called more than expected or withwrong arguments, you’ll get an error immediately.
5. When a mock is destructed, gMock will automatically check whether allexpectations on it have been satisfied.

Here’s an example:

As you might have guessed, this test checks that PenDown() is called at leastonce. If the painter object didn’t call this method, your test will fail witha message like this:

Tip 1: If you run the test from an Emacs buffer, you can hit <Enter> onthe line number to jump right to the failed expectation.

Tip 2: If your mock objects are never deleted, the final verification won’thappen. Therefore it’s a good idea to turn on the heap checker in your testswhen you allocate mocks on the heap. You get that automatically if you use thegtest_main library already.

Important note: gMock requires expectations to be set before the mockfunctions are called, otherwise the behavior is undefined. In particular,you mustn’t interleave EXPECT_CALL()s and calls to the mock functions.

This means EXPECT_CALL() should be read as expecting that a call will occurin the future, not that a call has occurred. Why does gMock work like that?Well, specifying the expectation beforehand allows gMock to report a violationas soon as it rises, when the context (stack trace, etc) is still available.This makes debugging much easier.

Admittedly, this test is contrived and doesn’t do much. You can easily achievethe same effect without using gMock. However, as we shall reveal soon, gMockallows you to do so much more with the mocks.

Setting Expectations

The key to using a mock object successfully is to set the right expectationson it. If you set the expectations too strict, your test will fail as the resultof unrelated changes. If you set them too loose, bugs can slip through. You wantto do it just right such that your test can catch exactly the kind of bugs youintend it to catch. gMock provides the necessary means for you to do it “justright.”

General Syntax

In gMock we use the EXPECT_CALL() macro to set an expectation on a mockmethod. The general syntax is:

The macro has two arguments: first the mock object, and then the method and itsarguments. Note that the two are separated by a comma (,), not a period (.).(Why using a comma? The answer is that it was necessary for technical reasons.)If the method is not overloaded, the macro can also be called without matchers:

This syntax allows the test writer to specify “called with any arguments”without explicitly specifying the number or types of arguments. To avoidunintended ambiguity, this syntax may only be used for methods that are notoverloaded.

Either form of the macro can be followed by some optional clauses that providemore information about the expectation. We’ll discuss how each clause works inthe coming sections.

Google Mock Cheat Sheets

This syntax is designed to make an expectation read like English. For example,you can probably guess that

says that the turtle object’s GetX() method will be called five times, itwill return 100 the first time, 150 the second time, and then 200 every time.Some people like to call this style of syntax a Domain-Specific Language (DSL).

Note: Why do we use a macro to do this? Well it serves two purposes: firstit makes expectations easily identifiable (either by grep or by a humanreader), and second it allows gMock to include the source file location of afailed expectation in messages, making debugging easier.

Matchers: What Arguments Do We Expect?

When a mock function takes arguments, we may specify what arguments we areexpecting, for example:

Oftentimes you do not want to be too specific. Remember that talk about testsbeing too rigid? Over specification leads to brittle tests and obscures theintent of tests. Therefore we encourage you to specify only what’s necessary—nomore, no less. If you aren’t interested in the value of an argument, write _as the argument, which means “anything goes”:

_ is an instance of what we call matchers. A matcher is like a predicateand can test whether an argument is what we’d expect. You can use a matcherinside EXPECT_CALL() wherever a function argument is expected. _ is aconvenient way of saying “any value”.

In the above examples, 100 and 50 are also matchers; implicitly, they arethe same as Eq(100) and Eq(50), which specify that the argument must beequal (using operator) to the matcher argument. There are manybuilt-in matchers for common types (as wellas custom matchers); for example:

If you don’t care about any arguments, rather than specify _ for each ofthem you may instead omit the parameter list:

This works for all non-overloaded methods; if a method is overloaded, you needto help gMock resolve which overload is expected by specifying the number ofarguments and possibly also thetypes of the arguments.

Cardinalities: How Many Times Will It Be Called?

The first clause we can specify following an EXPECT_CALL() is Times(). Wecall its argument a cardinality as it tells how many times the call shouldoccur. It allows us to repeat an expectation many times without actually writingit as many times. More importantly, a cardinality can be “fuzzy”, just like amatcher can be. This allows a user to express the intent of a test exactly.

An interesting special case is when we say Times(0). You may have guessed - itmeans that the function shouldn’t be called with the given arguments at all, andgMock will report a googletest failure whenever the function is (wrongfully)called.

We’ve seen AtLeast(n) as an example of fuzzy cardinalities earlier. For thelist of built-in cardinalities you can use, seehere.

The Times() clause can be omitted. If you omit Times(), gMock will inferthe cardinality for you. The rules are easy to remember:

• If neitherWillOnce()norWillRepeatedly() is in theEXPECT_CALL(), the inferred cardinality is Times(1).
• If there are nWillOnce()’s but noWillRepeatedly(), where n >=1, the cardinality is Times(n).
• If there are nWillOnce()’s and oneWillRepeatedly(), where n >=0, the cardinality is Times(AtLeast(n)).

Quick quiz: what do you think will happen if a function is expected to becalled twice but actually called four times?

Actions: What Should It Do?

Remember that a mock object doesn’t really have a working implementation? We asusers have to tell it what to do when a method is invoked. This is easy ingMock.

First, if the return type of a mock function is a built-in type or a pointer,the function has a default action (a void function will just return, abool function will return false, and other functions will return 0). Inaddition, in C++ 11 and above, a mock function whose return type isdefault-constructible (i.e. has a default constructor) has a default action ofreturning a default-constructed value. If you don’t say anything, this behaviorwill be used.

Second, if a mock function doesn’t have a default action, or the default actiondoesn’t suit you, you can specify the action to be taken each time theexpectation matches using a series of WillOnce() clauses followed by anoptional WillRepeatedly(). For example,

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says that turtle.GetX() will be called exactly three times (gMock inferredthis from how many WillOnce() clauses we’ve written, since we didn’texplicitly write Times()), and will return 100, 200, and 300 respectively.

says that turtle.GetY() will be called at least twice (gMock knows this aswe’ve written two WillOnce() clauses and a WillRepeatedly() while having noexplicit Times()), will return 100 and 200 respectively the first two times,and 300 from the third time on.

Of course, if you explicitly write a Times(), gMock will not try to infer thecardinality itself. What if the number you specified is larger than there areWillOnce() clauses? Well, after all WillOnce()s are used up, gMock will dothe default action for the function every time (unless, of course, you have aWillRepeatedly().).

What can we do inside WillOnce() besides Return()? You can return areference using ReturnRef(*variable*), or invoke a pre-defined function, amongothers.

Important note: The EXPECT_CALL() statement evaluates the action clauseonly once, even though the action may be performed many times. Therefore youmust be careful about side effects. The following may not do what you want:

Instead of returning 100, 101, 102, …, consecutively, this mock function willalways return 100 as n++ is only evaluated once. Similarly, Return(new Foo)will create a new Foo object when the EXPECT_CALL() is executed, and willreturn the same pointer every time. If you want the side effect to happen everytime, you need to define a custom action, which we’ll teach in thecook book.

Time for another quiz! What do you think the following means?

Obviously turtle.GetY() is expected to be called four times. But if you thinkit will return 100 every time, think twice! Remember that one WillOnce()clause will be consumed each time the function is invoked and the default actionwill be taken afterwards. So the right answer is that turtle.GetY() willreturn 100 the first time, but return 0 from the second time on, asreturning 0 is the default action for int functions.

Google Mock Tutorial

Using Multiple Expectations

So far we’ve only shown examples where you have a single expectation. Morerealistically, you’ll specify expectations on multiple mock methods which may befrom multiple mock objects.

By default, when a mock method is invoked, gMock will search the expectations inthe reverse order they are defined, and stop when an active expectation thatmatches the arguments is found (you can think of it as “newer rules overrideolder ones.”). If the matching expectation cannot take any more calls, you willget an upper-bound-violated failure. Here’s an example:

If Forward(10) is called three times in a row, the third time it will be anerror, as the last matching expectation (#2) has been saturated. If, however,the third Forward(10) call is replaced by Forward(20), then it would be OK,as now #1 will be the matching expectation.

Note: Why does gMock search for a match in the reverse order of theexpectations? The reason is that this allows a user to set up the defaultexpectations in a mock object’s constructor or the test fixture’s set-up phaseand then customize the mock by writing more specific expectations in the testbody. So, if you have two expectations on the same method, you want to put theone with more specific matchers after the other, or the more specific rulewould be shadowed by the more general one that comes after it.

Tip: It is very common to start with a catch-all expectation for a methodand Times(AnyNumber()) (omitting arguments, or with _ for all arguments, ifoverloaded). This makes any calls to the method expected. This is not necessaryfor methods that are not mentioned at all (these are “uninteresting”), but isuseful for methods that have some expectations, but for which other calls areok. SeeUnderstanding Uninteresting vs Unexpected Calls.

Google Test Cheat Sheet

Ordered vs Unordered Calls

By default, an expectation can match a call even though an earlier expectationhasn’t been satisfied. In other words, the calls don’t have to occur in theorder the expectations are specified.

Sometimes, you may want all the expected calls to occur in a strict order. Tosay this in gMock is easy:

By creating an object of type InSequence, all expectations in its scope areput into a sequence and have to occur sequentially. Since we are justrelying on the constructor and destructor of this object to do the actual work,its name is really irrelevant.

In this example, we test that Foo() calls the three expected functions in theorder as written. If a call is made out-of-order, it will be an error.

(What if you care about the relative order of some of the calls, but not all ofthem? Can you specify an arbitrary partial order? The answer is … yes! Thedetails can be found here.)

All Expectations Are Sticky (Unless Said Otherwise)

Now let’s do a quick quiz to see how well you can use this mock stuff already.How would you test that the turtle is asked to go to the origin exactly twice(you want to ignore any other instructions it receives)?

After you’ve come up with your answer, take a look at ours and compare notes(solve it yourself first - don’t cheat!):

Suppose turtle.GoTo(0, 0) is called three times. In the third time, gMock willsee that the arguments match expectation #2 (remember that we always pick thelast matching expectation). Now, since we said that there should be only twosuch calls, gMock will report an error immediately. This is basically what we’vetold you in the Using Multiple Expectations section above.

This example shows that expectations in gMock are “sticky” by default, inthe sense that they remain active even after we have reached their invocationupper bounds. This is an important rule to remember, as it affects the meaningof the spec, and is different to how it’s done in many other mockingframeworks (Why’d we do that? Because we think our rule makes the common caseseasier to express and understand.).

Simple? Let’s see if you’ve really understood it: what does the following codesay?

If you think it says that turtle.GetX() will be called n times and willreturn 10, 20, 30, …, consecutively, think twice! The problem is that, as wesaid, expectations are sticky. So, the second time turtle.GetX() is called,the last (latest) EXPECT_CALL() statement will match, and will immediatelylead to an “upper bound violated” error - this piece of code is not very useful!

One correct way of saying that turtle.GetX() will return 10, 20, 30, …, isto explicitly say that the expectations are not sticky. In other words, theyshould retire as soon as they are saturated:

And, there’s a better way to do it: in this case, we expect the calls to occurin a specific order, and we line up the actions to match the order. Since theorder is important here, we should make it explicit using a sequence:

Google Mock Cheat Sheet

By the way, the other situation where an expectation may not be sticky is whenit’s in a sequence - as soon as another expectation that comes after it in thesequence has been used, it automatically retires (and will never be used tomatch any call).

Uninteresting Calls

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A mock object may have many methods, and not all of them are that interesting.For example, in some tests we may not care about how many times GetX() andGetY() get called.

Google Mock Test

In gMock, if you are not interested in a method, just don’t say anything aboutit. If a call to this method occurs, you’ll see a warning in the test output,but it won’t be a failure. This is called “naggy” behavior; to change, seeThe Nice, the Strict, and the Naggy.