Mutual class instances in C++

Question

What is the issue with this code? Here we have two files: classA.h and classB.h

classA.h:

#ifndef _class_a_h_
#define _class_a_h_

#include "classB.h"

class B; //????

class A
{
public:
    A() {
        ptr_b = new B(); //????
    }
    
    virtual ~A() {
        if(ptr_b) delete ptr_b; //????
                    num_a = 0;
    }
    
    int num_a;
    B* ptr_b; //????
};

#endif //_class_a_h_

classB.h:

#ifndef _class_b_h_
#define _class_b_h_

#include "classA.h"

class A; //????

class B
{
public:     
    B() { 
        ptr_a = new A(); //????
                    num_b = 0;
    }
    
    virtual ~B() { 
        if(ptr_a) delete ptr_a; //????
    }
            
    int num_b;
    A* ptr_a; //????
};

#endif //_class_b_h_

when I try to compile it, the compiler (g++) says:

classB.h: In constructor ‘B::B()’:

classB.h:12: error: invalid use of incomplete type ‘struct A’

classB.h:6: error: forward declaration of ‘struct A’

classB.h: In destructor ‘virtual B::~B()’:

classB.h:16: warning: possible problem detected in invocation of delete operator:

classB.h:16: warning: invalid use of incomplete type ‘struct A’

classB.h:6: warning: forward declaration of ‘struct A’

classB.h:16: note: neither the destructor nor the class-specific operator delete will be

called, even if they are declared when the class is defined.

Answer 1

You cannot create instances of an incomplete type (the compiler doesn't know anything about the class!)

You need to move the definitions of your functions (the constructor of A and B) into a C++ file that can include both headers (or into several C++ files, if you follow the convention that you have one class per file).

That having been said, your code as written has a serious problem: every A creates an instance of B and every B creates and instance of A. You will end up with an infinite recursion and you will eventually run out of memory.

Two minor nitpicks: you do not need to test whether a pointer is null before calling delete on it (it is safe to delete a null pointer), and you need to change your include guards (names beginning with an underscore in the global namespace are reserved to the implementation).