Two Classes Befriending Each Other - c++

Question

I'm trying to make two classes friend with each other, but I keep getting a "Use of Undefined type A" error message.
Here is my code:
I've tried to add class A; as shown on top but still the same.
#include <iostream>
class A;
class B
{
private:
int bVariable;
public:
B() :bVariable(9){}
void showA(A &myFriendA)
{
std::cout << "A.aVariable: " << myFriendA.aVariable << std::endl;// Since B is friend of A, it can access private members of A
}
friend class A;
};
class A
{
private:
int aVariable;
public:
A() :aVariable(7){}
void showB(B &myFriendB){
std::cout << "B.bVariable: " << myFriendB.bVariable << std::endl;
}
friend class B; // Friend Class
};
int main() {
A a;
B b;
b.showA(a);
a.showB(b);
system("pause");
return 0;
}
I'm trying to make class A access class B and vice versa via the friendship.

Answer 1

You can't access myFriendA.aVariable because the compiler doesn't know it exists. All it knows is that a class A exists (because of the forward declaration on the second line), but it hasn't been fully defined so it has no idea what its members/methods are.
If you wanted to make this work, showA() would have to be declared outside of the class scope.
class A;
class B
{
private:
int bVariable;
public:
B() :bVariable(9){}
void showA(A &myFriendA);
friend class A;
};
class A
{
private:
int aVariable;
public:
A() :aVariable(7){}
void showB(B &myFriendB){
std::cout << "B.bVariable: " << myFriendB.bVariable << std::endl;
}
friend class B; // Friend Class
};
// Define showA() here
void B::showA(A &myFriendA)
{
std::cout << "A.aVariable: " << myFriendA.aVariable << std::endl;
}
int main() {
A a;
B b;
b.showA(a);
a.showB(b);
system("pause");
return 0;
}

Answer 2

As #user888379 pointed out, moving the implementation of showA and showB methods after both classes have been completely declared will solve your problem. Following is working code:
#include <iostream>
class A;
class B
{
private:
int bVariable;
public:
B() :bVariable(9){}
void showA(A &myFriendA);
friend class A; // Friend Class
};
class A
{
private:
int aVariable;
public:
A() :aVariable(7){}
void showB(B &myFriendB);
friend class B; // Friend Class
};
void B::showA(A &myFriendA) {
std::cout << "A.aVariable: " << myFriendA.aVariable << std::endl; // Since B is friend of A, it can access private members of A
}
void A::showB(B &myFriendB) {
std::cout << "B.bVariable: " << myFriendB.bVariable << std::endl; // Since A is friend of B, it can access private members of B
}
int main() {
A a;
B b;
b.showA(a);
a.showB(b);
return 0;
}
Read this answer for more detailed analysis.