Monday, February 25, 2013

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.

See also