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

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

class MockGenerator : public Generator
{
    public:
    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())
        .Times(3)
                 //this is equivalent of returning new Record(1,2,3)
        .WillOnce(ReturnNew<Record>(1,2,3))
        .WillOnce(ReturnNew<Record>(2,3,4))
        .WillOnce(ReturnNew<Record>(3,4,5));

    c.run();
}


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    gen.createNewRecord(&rec);
}

• 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(_))
        .Times(1)
        .WillOnce(Invoke(my_create));

    c.getNewRec();
}

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:

        .WillOnce(CreateAndPass<Record>(1,2,3));

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:

ACTION_TEMPLATE(CreateAndPass,
                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.