Regarding forward declaration c++ class

Question

I have two classes like this:

class B;

class A {
public:
  int a, b;
  B *b;
public:
  int getctxt()
  {
     b->getctxt1();
  }
}

Class B {
public:  
  int c,d;
  A *a; 
  getctxt1()
  {
      /* something */
  }   
}

main()
{
  B *b = new B();
  A *a = new A();
  b->a = a;
  a->b = b;
}

But when I try to compile, it says

invalid use of incomplete type ‘struct A’.

Can anyone tell me how to solve this?

Answer 1

Inline class member function definitions are parsed as if they appeared right after the class definition. Of course B isn't defined at that point. So move the member function definition for A::getctxt out of the definition of class A:

class B;

class A { int getctxt(); /* ... */ };

class B { /* ... */ };

int A::getctxt()
{
    b->getctxt1();   // at this point, *b has complete type!
    return -35;
}

Answer 2

This is the reason why it is recommended to separate your classes in a .h file and a .cpp file. When you write class B;, you are telling the compiler that there is a class called B, but that's it, the compiler does not know what is inside this class. When the compiler reaches the line b->getctxt1();, it knows that b is an instance of B, but it does not know if getctxt1() is a method of B, because that is below. If you are writing all in a single file, you should first write all the declarations, and then all the implementations.

Answer 3

didn't anyone notice Class B syntax error? lowercase class is correct. and those two public members in A with the same name int b and B *b.

once you remove those and finish class definitions with semicolons, add a void type to the void getctxt1() that is missing it in B.

just define functions as they should be defined. not inline without a reason that you know which is!