我应该如何在嵌套类中正确使用友元声明？

Question

For example, suppose I made a code like :

class A
{
private:
    class B
    {
    private:
        int a;
        friend int A::foo(B &b);
    };
    int foo(B &b)
    {
        return b.a;
    }
};

Since a in B is private, to use a in function foo of A , I would use a friend so that foo can actually access a .

However this code gives error that it cannot access a . What is the problem of the code, and how should I change the code while keeping a private and A not being friend of B ? Or is there a better way?

Answer 1

If you want to get only the a of B class you need a getter function. This should be the simplest way to go.

class B
{
private:
    int a;
public:
    // provide getter function
    const int& getMember_a()const { return a; }
};

and in the foo function

const int& foo(const B &b)const 
{
    return b.getMember_a(); // call the getter to get the a
}

---

Regarding the issue of your code; at the line friend int A::foo(B &b); in B class, it does not know that the function A::foo . Therefore we need to forward declare int foo(B &); before the class B . Then the question; whether A::foo(B &) knows about B . Also no. But fortunately, C++ allows having an incomplete type by forward declaring the classes as well. That means, following way-way, you can achieve the goal you want.

class A
{
private:
    class B;      // forward declare class B for A::foo(B &)
    int foo(B &); // forward declare the member function of A
    class B
    {
    private:
        int a;
    public:
        friend int A::foo(B &b);
    };
};
// define, as a non-member friend function
int A::foo(B &b)
{
    return b.a;
}