A class has incomplete type [duplicate]

Question

#include<bits/stdc++.h>
using namespace std;

class B;
class A{
    public: 
        int a_i=10;
        void A_lookup(B b){
            cout << b.b_i <<endl;
        }
};
class B{
    public:
        int b_i=20;
        void B_lookup(A a){
            cout << a.a_i <<endl;
        }
};

int main(void){
    A a;
    B b;
    a.A_lookup(b);

In this case, I have two classes both of which would visit each other members. When compiling such a .cpp file, there are two piece of information:

sample2.cpp:8:25: error: 'b' has incomplete type void A_lookup(B b){

sample2.cpp:4:7: note: forward declaration of 'class B' class B;

Why could class A still not visit the member b_i of class B after class B has been declared ahead of class A. How should I fix such a error?