Specify a class member function as a friend of another class?

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Try putting the B definition before A’s:

class A; // forward declaration of A needed by B

class B
{
public:
    // if these require full definition of A, then put body in implementation file
    void fB(A& a); // Note: no body, unlike original.
    void fB2(A& a); // no body.
};

class A
{
public:
    friend void B::fB(A& a);
    void fA(){}
};

A needs the full definition of B. However, B needs to know about A, but does not need the full definition, so you need the forward declaration of A.

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