为什么在前向声明后出现未定义的类型错误？

Question

I'm using this code in C++ CLI. However this shouldn't make any difference from C++.
I'm looking for a solution to get rid of that error.

Code :

ref class B;
ref class A;

public ref class A
{
public:
    A() {}
    B^ b;
    void HelloFromA(){
        b->HelloFromB();
    }
};

public ref class B
{
public :
    A^ a;
    B() {}
    void HelloFromB(){
        a->HelloFromA();
    }
};

Answer 1

You need to move the bodies of the functions that invoke member functions on the forward-declared classes outside of the headers, to places where definitions are available:

void A::HelloFromA(){
    b->HelloFromB();
}

Otherwise, the compiler knows that B is available, but it does not know that B has the HelloFromB member function that takes no arguments.