Can't use classes not defined yet in c++

Question

So I am coding a program in c++, I got a version of the program that returns the same error:

#include <iostream>

class A
{
public:
    B foo()
    {
        return B();
    }
};

class B
{
public:
    A bar()
    {
        return A();
    }
};

int main(int argc, char const *argv[])
{
    A x = A();
    B y = x.foo();
    return 0;
}

But whenever I compile it, it gives me two errors:

main.cpp:6:5: error: unknown type name 'B'
    B foo()
    ^
main.cpp:8:16: error: use of undeclared identifier 'B'
        return B();

However I can't just move B above A because I will just get a different error:

main.cpp:6:5: error: unknown type name 'A'
    A bar()
    ^
main.cpp:8:16: error: use of undeclared identifier 'A'
        return A();

I am completely stumped on how to solve this so any help would be appreciated!

Use forward declarations (and split the function definition and declaration for `A::foo` so it is after `B` has been fully defined) — UnholySheep, Feb 23 '22 at 22:51
Thanks! Seems to have worked! Would this work with operator overloading too because that is what I am using? — Grigori Iakimenko, Feb 23 '22 at 22:53
related/dupe: https://stackoverflow.com/questions/625799/resolve-build-errors-due-to-circular-dependency-amongst-classes — NathanOliver, Feb 23 '22 at 22:53

score 2 · Accepted Answer · answered Feb 23 '22 at 22:54

You need to split declaration and definition. Also, you need to declare B as a class before A uses it.

#include <iostream>

class B;

class A {
public:
    B foo();
};

class B {
public:
    A bar();
};

B A::foo() {
    return B();
}
A B::bar() {
    return A();
}

int main(int argc, char const *argv[]) {
    A x = A();
    B y = x.foo();
    return 0;
}

Can't use classes not defined yet in c++

1 Answers1