Question

I am trying to share info between classes using information from here,
except that I am not able to get these two classes to compile and thus to
communicate with each other.

So, what I want is for ClassB to get a reference to ClassA, and
ClassB to be initialized in ClassA.

Compiler errors (using g++):

In member function 'void ClassA::foo()':
hello.cpp:9: error: 'ClassB' was not declared in this scope
hello.cpp:9: error: 'someB' was not declared in this scope
hello.cpp:9: error: expected type-specifier before 'ClassB'
hello.cpp:9: error: expected ';' before 'ClassB'

So then I try adding class ClassB; before ClassA,

like:

class ClassB; // just this here.

class ClassA; // all of classA here.

class ClassB; // all of classB here.

And I get:

hello.cpp: In member function ‘void ClassA::foo()’:
hello.cpp:11: error: invalid use of incomplete type ‘struct ClassB’
hello.cpp:3: error: forward declaration of ‘struct ClassB’
hello.cpp:12: error: invalid use of incomplete type ‘struct ClassB’
hello.cpp:3: error: forward declaration of ‘struct ClassB’

---

This is the code that am I trying to use based on the website above:

include stdio.h

class ClassA {
    public:
        void foo() {
            ClassB *someB = new ClassB();
            someB->setRelative(this);
        }

        void bar() {
            printf("B is communicating with me =)\n");
        }
};

---

class ClassB {

    ClassA *m_relative;

    public:
        void setRelative(ClassA *other) {
            this->m_relative = other;
        }

        void foo() {
            m_relative->bar();
        }
};

Answer 1

You’re obviously having a circular dependency here, something you can’t really solve using headers only (might be possible with some macro tricks, but I wouldn’t worry about).

What you should do is splitting your code into declarations (headers) and implementations.

a.h

// only include this file once per translation unit
#pragma once

// tell the compiler B is going to be a class,
// so B* is a pointer to a class - nothing more is needed for now.
class B;

// full declaration of A
class A {
    B *child;

public:
    A(void);
}

a.cpp

#include "a.h" // included whenever the structure/size of A has to be known
#include "b.h" // same for B

A::A(void) : child(new B(this)) {
    // do something with child or...
    B * = new B(this);
}

b.h

// only include this file once per translation unit
#pragma once

// again, just tell the compiler A is going to be a class
class A;

class B {
    A *parent;
public:
    B(A *pparent);
}

b.cpp

#include "a.h"
#include "b.h"

B::B(A *pparent) : parent(pparent) {
    // do something with parent
}