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Piotr's Less Obvious Advice on Google Mock: Returning new objects from a mock

Google Mock provides a way to return newly created objects from a mock method. Suppose we have a 
Generator class that is supposed to generate new objects when createNewRecord method is called:

class Generator
    virtual ~Generator() {}
    virtual Record * createNewRecord() = 0;

...and suppose we want to mock this class:

class MockGenerator : public Generator
    MOCK_METHOD0(createNewRecord, Record * ());

Suppose the caller class Client has run method defined as follows:

void Client::run()
    for(int i = 0; i < 3; i++)
        rec_tab[i] = gen.createNewRecord();

We want the mock to return a pointer to a new object each time createNewRecord is called. This is how we can express this in the test code:

TEST(ClientTest, CanRun)
    MockGenerator gen;
    Client c(gen);

    EXPECT_CALL(gen, createNewRecord())
                 //this is equivalent of returning new Record(1,2,3)

Creating and passing new objects through a method parameter
But what if we want to return a new object through a method parameter? Sometimes caller passes a pointer-to-pointer to a method that is supposed to create a new object and assign its address to the method parameter. Let's modify our example above so that:
  • the caller (class Client) has a private member:
Record * rec;
  • the caller (class Client) has the following method:
void Client::getNewRec()
        // we are passing Record ** (pointer to pointer) to createNewRecord
        // and Generator is supposed to allocate memory
  • our Generator.h has the following declaration:
virtual void createNewRecord(Record ** rec) = 0;
  • which results in the following declaration in MockGenerator.h:
MOCK_METHOD1(createNewRecord, void (Record ** rec));

How can we now emulate the desired behavior in a test code? A straightforward way of doing this would be:

TEST(ClientTest, GetsNewRec)
    MockGenerator gen;
    Client c(gen);

    EXPECT_CALL(gen, createNewRecord(_))


Where my_create is a small function that does the trick:

void my_create(Record ** rec)
         *rec = new Record(1,2,3);
But isn't it awkward? The need to define this small function, the hard-coded constructor's parameters... It would  be much more convenient if we had a Google Mock action that could actually create the new object and assign its address to the pointer. Then, in our test, we could simply write:


The CreateAndPass action must be smart enough to create an object of a certain type (Record), create it with the parameters that we specify (1,2,3) and do the proper assignment to the pointer. For a constructor that takes three parameters, as in the example above, our new action would look like this:

                HAS_1_TEMPLATE_PARAMS(typename, T), // the type of object to be created
                AND_3_VALUE_PARAMS(p0, p1, p2))         // constructor parameters
    // we are using TR1; assign the pointer of the newly created object to dereferenced call parameter
    // this is the only parameter of the call in this case, so it has the index of 0

  *(::std::tr1::get<0>(args)) = new T(p0, p1, p2);

That's it. This example can be easily extended for different constructors that take less or more parameters. Since the action is templatized, we can use it to construct objects of any class.


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Piotr's Less Obvious Advice on Google Mock: State maintenance

Google Mock provides several ways to maintain state inside mock objects. One way of implementing state maintenance is with SaveArg. Consider the following example.

We have a class Configurator, which allows a caller to set and get values of a parameter:

class Configurator

    virtual ~Configurator() {}

    virtual void setParamX(int n) = 0;
    virtual int getParamX() = 0;

And we have a class Client that calls Configurator's methods and it also has a method incParamXBy, that can be used to increase the current value of paramX by a certain value.

class Client

    Client(Configurator & cfg);
    virtual ~Client() {}

    void setParamX(int n);
    void incParamXBy(int n);
    int getParamX();


    Configurator & _cfg;

incParamXBy internally calls setParamX and getParamX on Configurator:

void Client::incParamXBy(int n)
    _cfg.setParamX(_cfg.getParamX() + n);

Let's assume that the initial value of paramX is A and that we want to increase paramX by…