为什么在定义类之前声明它的对象会在友元类中产生错误而在友元函数中不会产生错误

Question

class B;
class A {
private:
  int numA;
public:
  A(): numA(12) { }
  // friend function declaration
  friend int add(A, B);
  };

this does not give any error on declaring object of class B in friend function,,but this gives,,as firstly class B is declared

 class Apple;
 class B {
 private:
 int b;

public:
void showA(Apple d)
{
    // Since B is friend of A, it can access
    // private members of A
    cout << "A::a=" ;
}
};
};

this gives an error of incomplete type for object d,,why this is happening though we already declared class apple before,

Answer 1

Why does the first example compile?

In your first example, you have a forward declaration of class B followed by a declaration of a friend function that uses it in its parameter list:

class B;
class A {
  ...
  friend int add(A, B);
};

This is allowed because, although B is incomplete, we are not defining add yet, only declaring an intention to eventually do so.

Why does the second example not compile?

In the second example, we have a forward declaration of class Apple , followed by a definition of showA :

class Apple;
class B {
  ...
  void showA(Apple d)
  {
    ...
  }
};

This time, since we are defining the function, the compiler is obligated to generate code for it. But because Apple is incomplete, the compiler cannot know, for example, how much space in memory to reserve to hold the parameter d . Therefore this is an error.

The question When can I use a forward declaration? explains some of what can and cannot be done with an incomplete (forward-declared) type.

The use of 'friend' is irrelevant here

The friend keyword is basically irrelevant here. friend primarily affects access control (in the sense of public and private ), but that's not the issue here.

A detail: friend also affects scoping. Because of friend , add is not a member of class A , but rather refers to a member of the global scope without actually introducing one (it's weird). But that does not change whether an incomplete type can be used as a parameter.