Forward declaration usage/problem

[iammilind]

Pointer types can be used without error by fwd declarations.
In following code,.
Is there any way to use A::print() inside B::B() ?
===========================================

struct A;
struct B {
  B(A *pA) {
    cout<<"pA = "<<pA<<endl;
    cout<<"pA->print() = "<<pA->print()<<endl; // throwing error
  }
};
struct A {
  void print() { cout<<"Hello World\n"; }
};
int main()
{
  B obj(new A);
}
 

===========================================
Error:
In constructor `B::B(A*)':
error: invalid use of undefined type `struct A'
error: forward declaration of `struct A'

[Banfa]

The problem is that you are trying to use the structure A at line 5 before it has been defined. At that point the compiler has only seen a forward declaration and that is only good for declaring pointers and references to the structure not for using it.

You can easily use A::print() inside B::B(A *pA) by only declaring B::B(A *pA) in the declaration of B and putting the definition of B::B(A *pA) later after A has been declared.

struct A;    // Forward declare A
 
struct B {  // Declare B
 
  B(A *pA); // Declare B:B(A *pA)
};
 
struct A {  // Declare A
  void print() { cout<<"Hello World\n"; }
};
 
// Define B::B(A *pA) in the presence of the declaration of A
B::B(A *pA) {
      ....
}
 

BTW Please remeber to put [code] ... [/code] tags round your code.

[iammilind]

Thanks Banfa.
Actually later, I had used the similar technique only. It seems that, by language parsing rules, we can't use anything other than pointers.

My code of struct A and struct B were in 2 different header files.
Later on I moved the definitions to the .cpp file to solve it.

[Banfa]

You are not restricted to pointers (or references), as long as A references B but B does not reference A then A can contain a nested B simply by including B header file

// Header a.h
#include "b.h"
 
struct A
{
    int anInt;
    B myB;
};
 

// b.h
struct B
{
    int anInt;
    float aFloat;
};
 

You are restricted to using pointers or references where A references B and B references A.