Mocking C++ classes with dependency injection

Question

Say you\'re testing class A and it has a dependency injection of B which has a dependency injection of C.
So you mock B

Answer 1

Emile has the right idea, you should depend on interfaces not concrete classes. So in your example it would be something like:

#include 
using namespace std;

class C {
public:
    int x;
};

class B {
public:
    ~B(){};
    virtual void doSomething() = 0;
};

class ConcreteB : public B{
public:
    ConcreteB(C c) : m_c(c) {}
    void doSomething(){
        std::cout << "HelloWorld" << std::endl;
    }
private:
    C m_c;
};
class A{
public:
    A(B *b): m_b(b){}

    void functionToTestWithSideEffect(){
        m_b->doSomething();
    }
private:
    B *m_b;

};

//#include 

int main() {
    C c;
    c.x = 42;
    ConcreteB b(c);
    A a(&b);
    a.functionToTestWithSideEffect();
    return 0;
}

In your tests you create a mock B which does not rely on any class C. Then you are only testing the interface with B. In this way you break A's dependency on C. Creating a mock B that doesn't depend on C is pretty simple:

class MockB : public B {
 public:
  MOCK_METHOD0(doSomething, void());
};