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author | Abseil Team <absl-team@google.com> | 2021-01-05 21:46:37 (GMT) |
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committer | Derek Mauro <dmauro@google.com> | 2021-01-14 01:59:12 (GMT) |
commit | 489283524e3726b7adb9692763c2fb61b235d41a (patch) | |
tree | 0c21fdc64b8978bcac9cee62f14584ce4f024bc9 /docs/gmock_for_dummies.md | |
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diff --git a/docs/gmock_for_dummies.md b/docs/gmock_for_dummies.md new file mode 100644 index 0000000..7bec29a --- /dev/null +++ b/docs/gmock_for_dummies.md @@ -0,0 +1,702 @@ +# gMock for Dummies {#GMockForDummies} + +<!-- GOOGLETEST_CM0013 DO NOT DELETE --> + +<!-- GOOGLETEST_CM0035 DO NOT DELETE --> + +## What Is gMock? + +When you write a prototype or test, often it's not feasible or wise to rely on +real objects entirely. 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? etc). + +**Note:** It is easy to confuse the term *fake objects* with mock objects. Fakes +and mocks actually mean very different things in the Test-Driven Development +(TDD) community: + +* **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. + +If all this seems too abstract for you, don't worry - the most important thing +to remember is that a mock allows you to check the *interaction* between itself +and code that uses it. The difference between fakes and mocks shall become much +clearer once you start to use mocks. + +**gMock** is a library (sometimes we also call it a "framework" to make it sound +cool) for creating mock classes and using them. It does to C++ what +jMock/EasyMock does to Java (well, more or less). + +When using gMock, + +1. first, you use some simple macros to describe the interface you want to + mock, and they will expand to the implementation of your mock class; +2. next, you create some mock objects and specify its expectations and behavior + using an intuitive syntax; +3. then you exercise code that uses the mock objects. gMock will catch any + violation to the expectations as soon as it arises. + +## Why gMock? + +While mock objects help you remove unnecessary dependencies in tests and make +them fast and reliable, using mocks manually in C++ is *hard*: + +* Someone has to implement the mocks. The job is usually tedious and + error-prone. No wonder people go great distance to avoid it. +* The quality of those manually written mocks is a bit, uh, unpredictable. You + may see some really polished ones, but you may also see some that were + hacked 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 next + one. + +In contrast, Java and Python programmers have some fine mock frameworks (jMock, +EasyMock, [Mox](http://wtf/mox), etc), which automate the creation of mocks. As +a result, mocking is a proven 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 the +following problems is bothering you: + +* You are stuck with a sub-optimal design and wish you had done more + prototyping 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 expensive + resources (e.g. a database). +* Your tests are brittle as some resources they use are unreliable (e.g. the + network). +* You want to test how your code handles a failure (e.g. a file checksum + error), but it's not easy to cause one. +* You need to make sure that your module interacts with other modules in the + right way, but it's hard to observe the interaction; therefore you resort to + observing the side effects at the end of the action, but it's awkward at + best. +* You want to "mock out" your dependencies, except that they don't have mock + implementations yet; and, frankly, you aren't thrilled by some of those + hand-written mocks. + +We encourage you to use gMock as + +* a *design* tool, for it lets you experiment with your interface design early + and often. More iterations lead to better designs! +* a *testing* tool to cut your tests' outbound dependencies and probe the + interaction 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 that +relies on a [LOGO](http://en.wikipedia.org/wiki/Logo_programming_language)-like +API for drawing. How would you test that it does the right thing? Well, you can +run 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 a +shiny new graphics card that has better anti-aliasing? Suddenly you have to +update all your golden images.). It would be too painful if all your tests are +like this. Fortunately, you learned about +[Dependency Injection](http://en.wikipedia.org/wiki/Dependency_injection) and know the right thing +to do: instead of having your application talk to the system API directly, wrap +the API in an interface (say, `Turtle`) and code to that interface: + +```cpp +class Turtle { + ... + virtual ~Turtle() {} + virtual void PenUp() = 0; + virtual void PenDown() = 0; + virtual void Forward(int distance) = 0; + virtual void Turn(int degrees) = 0; + virtual void GoTo(int x, int y) = 0; + virtual int GetX() const = 0; + virtual int GetY() const = 0; +}; +``` + +(Note that the destructor of `Turtle` **must** be virtual, as is the case for +**all** classes you intend to inherit from - otherwise the destructor of the +derived class will not be called when you delete an object through a base +pointer, 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()`, and +`GoTo()`. Finally, `GetX()` and `GetY()` tell you the current position of the +turtle. + +Your program will normally use a real implementation of this interface. In +tests, you can use a mock implementation instead. This allows you to easily +check what drawing primitives your program is calling, with what arguments, and +in which order. Tests written this way are much more robust (they won't break +because your new machine does anti-aliasing differently), easier to read and +maintain (the intent of a test is expressed in the code, not in some binary +images), and run *much, much faster*. + +## Writing the Mock Class + +If you are lucky, the mocks you need to use have already been implemented by +some nice people. If, however, you find yourself in the position to write a mock +class, 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 to +follow: + +* Derive a class `MockTurtle` from `Turtle`. +* Take a *virtual* function of `Turtle` (while it's possible to + [mock non-virtual methods using templates](gmock_cook_book.md#MockingNonVirtualMethods), + 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 it + into the macro, and add two commas - one between the return type and the + name, 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 `override` + keyword. 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 goes + without saying that *all* pure virtual methods in your abstract class must + be either mocked or overridden.) + +After the process, you should have something like: + +```cpp +#include "gmock/gmock.h" // Brings in gMock. + +class MockTurtle : public Turtle { + public: + ... + MOCK_METHOD(void, PenUp, (), (override)); + MOCK_METHOD(void, PenDown, (), (override)); + MOCK_METHOD(void, Forward, (int distance), (override)); + MOCK_METHOD(void, Turn, (int degrees), (override)); + MOCK_METHOD(void, GoTo, (int x, int y), (override)); + MOCK_METHOD(int, GetX, (), (const, override)); + MOCK_METHOD(int, GetY, (), (const, override)); +}; +``` + +You don't need to define these mock methods somewhere else - the `MOCK_METHOD` +macro 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 of +`Foo` changes it, your test could break. (You can't really expect `Foo`'s +maintainer 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-package +such that you can clearly separate production code and testing utilities), put +it in a `.h` and a `cc_library`. Then everyone can reference them from their +tests. If `Foo` ever changes, there is only one copy of `MockFoo` to change, and +only 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 of +`Foo` and code to this new interface. Since you own `FooAdaptor`, you can absorb +changes in `Foo` much more easily. While this is more work initially, carefully +choosing the adaptor interface can make your code easier to write and more +readable (a net win in the long run), as you can choose `FooAdaptor` to fit your +specific domain much better than `Foo` does. + +<!-- GOOGLETEST_CM0029 DO NOT DELETE --> + +## 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 use + them unqualified (You only have to do it once per file). Remember that + namespaces 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 using + googletest 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, gMock will automatically check whether all + expectations on it have been satisfied. + +Here's an example: + +```cpp +#include "path/to/mock-turtle.h" +#include "gmock/gmock.h" +#include "gtest/gtest.h" + +using ::testing::AtLeast; // #1 + +TEST(PainterTest, CanDrawSomething) { + MockTurtle turtle; // #2 + EXPECT_CALL(turtle, PenDown()) // #3 + .Times(AtLeast(1)); + + Painter painter(&turtle); // #4 + + EXPECT_TRUE(painter.DrawCircle(0, 0, 10)); // #5 +} +``` + +As you might have guessed, this test checks that `PenDown()` is called at least +once. If the `painter` object didn't call this method, your test will fail with +a message like this: + +```text +path/to/my_test.cc:119: Failure +Actual function call count doesn't match this expectation: +Actually: never called; +Expected: called at least once. +Stack trace: +... +``` + +**Tip 1:** If you run the test from an Emacs buffer, you can hit `<Enter>` on +the line number to jump right to the failed expectation. + +**Tip 2:** If your mock objects are never deleted, the final verification won't +happen. Therefore it's a good idea to turn on the heap checker in your tests +when you allocate mocks on the heap. You get that automatically if you use the +`gtest_main` library already. + +**Important note:** gMock requires expectations to be set **before** the mock +functions 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 occur +*in the future*, not that a call has occurred. Why does gMock work like that? +Well, specifying the expectation beforehand allows gMock to report a violation +as 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 achieve +the same effect without using gMock. However, as we shall reveal soon, gMock +allows you to do *so much more* with the mocks. + +## Setting Expectations + +The key to using a mock object successfully is to set the *right expectations* +on it. If you set the expectations too strict, your test will fail as the result +of unrelated changes. If you set them too loose, bugs can slip through. You want +to do it just right such that your test can catch exactly the kind of bugs you +intend it to catch. gMock provides the necessary means for you to do it "just +right." + +### General Syntax + +In gMock we use the `EXPECT_CALL()` macro to set an expectation on a mock +method. The general syntax is: + +```cpp +EXPECT_CALL(mock_object, method(matchers)) + .Times(cardinality) + .WillOnce(action) + .WillRepeatedly(action); +``` + +The macro has two arguments: first the mock object, and then the method and its +arguments. 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: + +```cpp +EXPECT_CALL(mock_object, non-overloaded-method) + .Times(cardinality) + .WillOnce(action) + .WillRepeatedly(action); +``` + +This syntax allows the test writer to specify "called with any arguments" +without explicitly specifying the number or types of arguments. To avoid +unintended ambiguity, this syntax may only be used for methods which are not +overloaded + +Either form of the macro can be followed by some optional *clauses* that provide +more information about the expectation. We'll discuss how each clause works in +the coming sections. + +This syntax is designed to make an expectation read like English. For example, +you can probably guess that + +```cpp +using ::testing::Return; +... +EXPECT_CALL(turtle, GetX()) + .Times(5) + .WillOnce(Return(100)) + .WillOnce(Return(150)) + .WillRepeatedly(Return(200)); +``` + +says that 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. +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: first +it makes expectations easily identifiable (either by `gsearch` or by a human +reader), and second it allows gMock to include the source file location of a +failed expectation in messages, making debugging easier. + +### Matchers: What Arguments Do We Expect? + +When a mock function takes arguments, we may specify what arguments we are +expecting, for example: + +```cpp +// Expects the turtle to move forward by 100 units. +EXPECT_CALL(turtle, Forward(100)); +``` + +Oftentimes you do not want to be too specific. Remember that talk about tests +being too rigid? Over specification leads to brittle tests and obscures the +intent of tests. Therefore we encourage you to specify only what's necessary—no +more, no less. If you aren't interested in the value of an argument, write `_` +as the argument, which means "anything goes": + +```cpp +using ::testing::_; +... +// Expects that the turtle jumps to somewhere on the x=50 line. +EXPECT_CALL(turtle, GoTo(50, _)); +``` + +`_` is an instance of what we call **matchers**. A matcher is like a predicate +and can test whether an argument is what we'd expect. You can use a matcher +inside `EXPECT_CALL()` wherever a function argument is expected. `_` is a +convenient way of saying "any value". + +In the above examples, `100` and `50` are also matchers; implicitly, they are +the same as `Eq(100)` and `Eq(50)`, which specify that the argument must be +equal (using `operator==`) to the matcher argument. There are many +[built-in matchers](gmock_cheat_sheet.md#MatcherList) for common types (as well +as [custom matchers](gmock_cook_book.md#NewMatchers)); for example: + +```cpp +using ::testing::Ge; +... +// Expects the turtle moves forward by at least 100. +EXPECT_CALL(turtle, Forward(Ge(100))); +``` + +If you don't care about *any* arguments, rather than specify `_` for each of +them you may instead omit the parameter list: + +```cpp +// Expects the turtle to move forward. +EXPECT_CALL(turtle, Forward); +// Expects the turtle to jump somewhere. +EXPECT_CALL(turtle, GoTo); +``` + +This works for all non-overloaded methods; if a method is overloaded, you need +to help gMock resolve which overload is expected by specifying the number of +arguments and possibly also the +[types of the arguments](gmock_cook_book.md#SelectOverload). + +### Cardinalities: How Many Times Will It Be Called? + +The first clause we can specify following an `EXPECT_CALL()` is `Times()`. We +call its argument a **cardinality** as it tells *how many times* the call should +occur. It allows us to repeat an expectation many times without actually writing +it as many times. More importantly, a cardinality can be "fuzzy", just like a +matcher 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 - it +means that the function shouldn't be called with the given arguments at all, and +gMock will report a googletest failure whenever the function is (wrongfully) +called. + +We've seen `AtLeast(n)` as an example of fuzzy cardinalities earlier. For the +list of built-in cardinalities you can use, see +[here](gmock_cheat_sheet.md#CardinalityList). + +The `Times()` clause can be omitted. **If you omit `Times()`, gMock will infer +the cardinality for you.** The rules are easy to remember: + +* 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))`. + +**Quick quiz:** what do you think will happen if a function is expected to be +called twice but actually called four times? + +### Actions: What Should It Do? + +Remember that a mock object doesn't really have a working implementation? We as +users have to tell it what to do when a method is invoked. This is easy in +gMock. + +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, a +`bool` function will return `false`, and other functions will return 0). In +addition, in C++ 11 and above, a mock function whose return type is +default-constructible (i.e. has a default constructor) has a default action of +returning a default-constructed value. If you don't say anything, this behavior +will be used. + +Second, 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()`. For example, + +```cpp +using ::testing::Return; +... +EXPECT_CALL(turtle, GetX()) + .WillOnce(Return(100)) + .WillOnce(Return(200)) + .WillOnce(Return(300)); +``` + +says that `turtle.GetX()` will be called *exactly three times* (gMock inferred +this from how many `WillOnce()` clauses we've written, since we didn't +explicitly write `Times()`), and will return 100, 200, and 300 respectively. + +```cpp +using ::testing::Return; +... +EXPECT_CALL(turtle, GetY()) + .WillOnce(Return(100)) + .WillOnce(Return(200)) + .WillRepeatedly(Return(300)); +``` + +says that `turtle.GetY()` will be called *at least twice* (gMock knows this as +we've written two `WillOnce()` clauses and a `WillRepeatedly()` while having no +explicit `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 the +cardinality itself. What if the number you specified is larger than there are +`WillOnce()` clauses? Well, after all `WillOnce()`s are used up, gMock will do +the *default* action for the function every time (unless, of course, you have a +`WillRepeatedly()`.). + +What can we do inside `WillOnce()` besides `Return()`? You can return a +reference using `ReturnRef(*variable*)`, or invoke a pre-defined function, among +[others](gmock_cook_book.md#using-actions). + +**Important note:** The `EXPECT_CALL()` statement evaluates the action clause +only once, even though the action may be performed many times. Therefore you +must be careful about side effects. The following may not do what you want: + +```cpp +using ::testing::Return; +... +int n = 100; +EXPECT_CALL(turtle, GetX()) + .Times(4) + .WillRepeatedly(Return(n++)); +``` + +Instead of returning 100, 101, 102, ..., consecutively, this mock function will +always 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 will +return the same pointer every time. If you want the side effect to happen every +time, you need to define a custom action, which we'll teach in the +[cook book](http://<!-- GOOGLETEST_CM0012 DO NOT DELETE -->). + +Time for another quiz! What do you think the following means? + +```cpp +using ::testing::Return; +... +EXPECT_CALL(turtle, GetY()) + .Times(4) + .WillOnce(Return(100)); +``` + +Obviously `turtle.GetY()` is expected to be called four times. But if you think +it will return 100 every time, think twice! Remember that one `WillOnce()` +clause will be consumed each time the function is invoked and the default action +will be taken afterwards. So the right answer is that `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. + +### Using Multiple Expectations {#MultiExpectations} + +So far we've only shown examples where you have a single expectation. More +realistically, you'll specify expectations on multiple mock methods which may be +from multiple mock objects. + +By default, when a mock method is invoked, gMock will search the expectations in +the **reverse order** they are defined, and stop when an active expectation that +matches the arguments is found (you can think of it as "newer rules override +older ones."). If the matching expectation cannot take any more calls, you will +get an upper-bound-violated failure. Here's an example: + +```cpp +using ::testing::_; +... +EXPECT_CALL(turtle, Forward(_)); // #1 +EXPECT_CALL(turtle, Forward(10)) // #2 + .Times(2); +``` + +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. 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 the +expectations? The reason is that 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. So, if you have two expectations on the same method, you want to put the +one with more specific matchers **after** the other, or the more specific rule +would 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 method +and `Times(AnyNumber())` (omitting arguments, or with `_` for all arguments, if +overloaded). This makes any calls to the method expected. This is not necessary +for methods that are not mentioned at all (these are "uninteresting"), but is +useful for methods that have some expectations, but for which other calls are +ok. See +[Understanding Uninteresting vs Unexpected Calls](gmock_cook_book.md#uninteresting-vs-unexpected). + +### Ordered vs Unordered Calls {#OrderedCalls} + +By default, an expectation can match a call even though an earlier expectation +hasn't been satisfied. In other words, the calls don't have to occur in the +order the expectations are specified. + +Sometimes, you may want all the expected calls to occur in a strict order. To +say this in gMock is easy: + +```cpp +using ::testing::InSequence; +... +TEST(FooTest, DrawsLineSegment) { + ... + { + InSequence seq; + + EXPECT_CALL(turtle, PenDown()); + EXPECT_CALL(turtle, Forward(100)); + EXPECT_CALL(turtle, PenUp()); + } + Foo(); +} +``` + +By creating an object of type `InSequence`, all expectations in its scope are +put into a *sequence* and have to occur *sequentially*. Since we are just +relying 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 the +order 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 of +them? Can you specify an arbitrary partial order? The answer is ... yes! The +details can be found [here](gmock_cook_book.md#OrderedCalls).) + +### All Expectations Are Sticky (Unless Said Otherwise) {#StickyExpectations} + +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!): + +```cpp +using ::testing::_; +using ::testing::AnyNumber; +... +EXPECT_CALL(turtle, GoTo(_, _)) // #1 + .Times(AnyNumber()); +EXPECT_CALL(turtle, GoTo(0, 0)) // #2 + .Times(2); +``` + +Suppose `turtle.GoTo(0, 0)` is called three times. In the third time, gMock will +see that the arguments match expectation #2 (remember that we always pick the +last matching expectation). Now, since we said that there should be only two +such calls, gMock will report an error immediately. This is basically what we've +told you in the [Using Multiple Expectations](#MultiExpectations) section above. + +This example shows that **expectations in gMock are "sticky" by default**, in +the sense that they remain active even after we have reached their invocation +upper bounds. This is an important rule to remember, as it affects the meaning +of the spec, and is **different** to how it's done in many other mocking +frameworks (Why'd we do that? Because we think our rule makes the common cases +easier to express and understand.). + +Simple? Let's see if you've really understood it: what does the following code +say? + +```cpp +using ::testing::Return; +... +for (int i = n; i > 0; i--) { + EXPECT_CALL(turtle, GetX()) + .WillOnce(Return(10*i)); +} +``` + +If you think it says that `turtle.GetX()` will be called `n` times and will +return 10, 20, 30, ..., consecutively, think twice! The problem is that, as we +said, expectations are sticky. So, the second time `turtle.GetX()` is called, +the last (latest) `EXPECT_CALL()` statement will match, and will immediately +lead 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, ..., is +to explicitly say that the expectations are *not* sticky. In other words, they +should *retire* as soon as they are saturated: + +```cpp +using ::testing::Return; +... +for (int i = n; i > 0; i--) { + EXPECT_CALL(turtle, GetX()) + .WillOnce(Return(10*i)) + .RetiresOnSaturation(); +} +``` + +And, there's a better way to do it: in this case, we expect the calls to occur +in a specific order, and we line up the actions to match the order. Since the +order is important here, we should make it explicit using a sequence: + +```cpp +using ::testing::InSequence; +using ::testing::Return; +... +{ + InSequence s; + + for (int i = 1; i <= n; i++) { + EXPECT_CALL(turtle, GetX()) + .WillOnce(Return(10*i)) + .RetiresOnSaturation(); + } +} +``` + +By the way, 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 (and will never be used to +match any call). + +### Uninteresting Calls + +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()` and +`GetY()` get called. + +In gMock, 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. This is called "naggy" behavior; to change, see +[The Nice, the Strict, and the Naggy](gmock_cook_book.md#NiceStrictNaggy). |