Allow class access to single private member

Question

I have a class A which has a private method called a(). I have also a class B which needs to access a() (but just B should have access to a(), thats why a() is private). I could now use a friend specifier but that would make other private methods of A (lets call them b() and c()) also available to B and I dont want that behaviour.

Is there a way to make just a() of A accessable to B?

Answer 1

There is a way -- if your class has a public template function:

class A {
    // apparently private
    void priv () { std::cout << "got you A::a()" << std::endl ; }
public:
    template <class T> 
    void abuse() {}
    };


struct Thief {};

template <>
void A::abuse<Thief>() {
    this->priv();
    }

int main() {
    A a;
    // obviously do not compile :   a.priv();
    // this i OK
    a.abuse<Thief>();

    return 0;
    }

I must confess I stole this from GotW...

Answer 2

No there's not, but as you specify the precise class, just B could access A's private members. You just have to take care of what method are called.

As friend relationship are not inherited, you don't have to worry about B's possible subclasses.