Mock in the Middle with C++/Gmock

I have a class A that instantiates an object of class B in its constructor. I want to test A with a mock object of B.

And no, I can not pass object of B as a parameter. Is there any other way?

I saw an article http://www.ibm.com/developerworks/library/j-mocktest/index.html, with "Mock in the Middle" as the interesting topic, but that is in Java. Is it possible in C++?

class B {...};

class A {
  private:
    B* b;

  public:
    A() {
        b = new B();
    }
    ~A() {..}
};

EDIT:

In general, the object may be created in some other method, on demand. For example,

class A {
    ...
    int doSomething() {
        // Create an object of class B
        b = new B();
    }
};

Solution

You can use factory pattern

given this code

class B {
  public:
    virtual std::string name() { return "B::name"; }
    virtual ~B() {}
};

class A {
  private:
    std::unique_ptr<B> b;

  public:
    A() {}
    void createB() {
        b.reset(new B); // you want to replace `new B` with something else right?
    }
    void print() {
        std::cout << (b ? b->name() : std::string()) << std::endl;
    }
    ~A() {}
};

with factory function

class A {
  private:
    std::unique_ptr<B> b;

  public:
    std::function<std::unique_ptr<B>()> b_maker;

    A() {
        // default maker
        b_maker = []{ return std::unique_ptr<B>(new B); };
    }

    A(std::function<std::unique_ptr<B>()> func) {
        b_maker = func;
    }

    void createB() {
        b = b_maker();
    }

    void print() {
        std::cout << (b ? b->name() : std::string()) << std::endl;
    }
    ~A() {}
};

create A with default B is same

A();

and now you can supply mocked B with

A([]{return std::unique_ptr<B>{new MockedB};});

live demo

you can also make b_maker to be a global variable so you don't need to pass it around (but I don't recommended to to it)

you can do it with the complicated way with AbstractBFactory, BFactory, MockBFactory, but it is too much overhead and looks like Java...