Question

I want to use one pointer ( _ref ) to point to different class types. In order to use it, I must cast it to the type that is addressed. I cannot do that because of the incomplete type that is at line 5. If i move the definition of B to line 5, it requires class A to be defined.

#include <iostream>
#include <vector>
#include <string>

class B;

class A{
    void *_ref;
    std::string _reft;
public:
    void setref(A &a){
        _ref=&a;
        _reft=typeid(a).name();
    }
    void setref(B &b){
        _ref=&b;
        _reft=typeid(b).name();
    }
    void test(){
        if(_ref && _reft==std::string(typeid(B).name())){
            std::cout<<"Ref to B: ";
            static_cast<B*>(_ref)->test(); //error here
        }
    }
};

class B{
    std::vector<A> a;
public:
    A A(int i){
        return a[i];
    }
    void test(){
        std::cout<<"IT WORKS!";
    }
};

int main(){
    A a;
    B b;
    a.setref(b);
    a.test();
    return 0;
}

Answer 1

Move the implementation of the function that requires B to be complete out of the class; put it either in a source file, or inline after the definition of B:

class A{
    // ...
    void test();
};

class B{
    // ...
};

inline void A::test(){
    // ...
}