Linker error and how to access members

Question

#include <iostream>
#include <stdlib.h>

class B;

class A {
    int x;
public:
    B it();
    friend class B;
};

class B {
    int y;
    A a;
public:
    B(A aa):y(99), a(aa) {};
    int get_y() {
    return this -> y;
    }
};

using namespace std;

int main() {
    A md;
    cout << md.it().get_y() << endl;

return 0;
}

Ignore encapsulation and other details, my problem is that I want to use "md.it().get_y()", but my compiler gives me this linker error: Undefined symbols for architecture x86_64: "A::it()", referenced from: _main in main.o ld: symbol(s) not found for architecture x86_64. How can I solve this ?

Answer 1

EDIT2: After realizing that

B it();

is NOT a function returning B, but an object inside class A: You have a cyclic depenency, where A uses B and B uses A. The compiler is unable to resolve this. Forward declares like

class N;

ONLY work for pointers or references, where the actual object size does not have to be known

EDIT: I just realized I misread the error message. You are missing the function definition for the it() method.

The forward declare propably does not work for

B it();

changing the code to

B& it();

should fix your issue

Note that B is now returned by refernce instead of by value. So you might actually do something like

const B& it() const;