Thread: circular header dependencies

Here is a problem of circular dependency. I want to allow a class B ctor friend access to private members of a class A object. But in order to tell class A this, class A needs to know about the existence of the class B::B(const A&) ctor, so a simple forward declaration will not work.

Code:

//A.h:
#ifndef A_H
#define A_H
#include "B.h"

class A {
public:
    A(int i);
    friend B::B(const A &);
private:
    int _i;
};

#endif

//B.h:
#ifndef B_H
#define B_H
#include "A.h"

class B {
public:
    B(const A &);
private:
    int deg;
};

#endif

//A.cpp:
#include "A.h"
A::A(int i) : _i(i) { }

//B.cpp:
#include "B.h"
B::B(const A &a) : deg(a._i) { }

My basic approach was to have each class header include the other class header so that they each know of the other's entire structure. But inevitably one header comes before the other (of course) so it seems that it is simply impossible to have each class know of the other classes' complete definition as it is being compiled.

So there is no main() here. Errors are displayed with just:

Code:

gcc B.cpp

Originally Posted by laserlight

Instead of including the header for class A in the header for class B, forward declare class A in the header for class B, just before the definition of class B.

Yes, that exposes the name A so that the B(const A &) ctor can be declared. I actually came across that solution after I posted my question.

Then, in order to compile B.cpp, one must still #include "A.h" either in B.h after class B definition, or in B.cpp before the definition of B::B(const A &a) ctor, so that a._i is known of. (actually, B.cpp could just #include "A.h" as that #includes "B.h", but i just let the hdr guards do their thing.)