friend class with forward class declaration does not compile

问题

This a basic program to understand how to use friend class in C++.

Class xxx has a class yyy object using friend. Since class yyy is defined after class xxx I have declared class yyy using forward declaration.

#include<iostream>
using std::cout;
using std::endl;

class yyy; //Forward Declaration of class yyy

class xxx{
private:
  int a;

public:
  xxx(){a=20;yyy y2;y2.show();}       //Error//
  void show(){cout<<"a="<<a<<endl;}
  friend class yyy; //Making class yyy as freind of class xxx
};
class yyy{
private:
  int b;

public:
  yyy(){b=10;}
  void show(){cout<<"b="<<b<<endl;}
};

int main(int argc, char *argv[])
{
  xxx x1; //creating xxx object and calling constructor
  x1.show();
  return 0;
}

When I compile the program I get this error:

error: aggregate ‘yyy y2’ has incomplete type and cannot be defined

I referred to this link recursive friend classes and someone had answered like this https://stackoverflow.com/a/6158833/2168706

I am following this approach but I am still unable to solve the issue. Please provide a solution if any exists and please let me know if I am wrong at any point in the code.

回答1:

Your're instantiating an object y2 of incomplete type yyy here:

{ a = 20; yyy y2; y2.show(); }

Move the implementation of constructor below the definition of yyy class:

class yyy;

class xxx {
  private:
    int a;

  public: 
    xxx();

    void show() { cout << "a=" << a << endl; }

    friend class yyy;
};

class yyy {
  private:
    int b;

  public:
    yyy() { b = 10; }

    void show() { cout << "b=" << b << endl; }
};

xxx::xxx() { a = 20; yyy y2; y2.show(); } // No error

As a result, at this point yyy is already defined, and you can instantiate y2.

To give you a logical explanation, why your variant didn't work: when you instantiate an object with automatic storage duration (on stack) like yyy y2;, compiler has to know at compile-time how much memory it should reserve for y2. Since the yyy type is incomplete (was only forward declared at the point of instantiation), the compiler is lost and reports an error.

NOTE: The best practice, is of course to separate definition of the class and its implementation by moving definition to header file (.hpp) and implementation to source file (.cpp). Don't forget to include headers properly. I don't feel like giving you an example here because it's very basic stuff and should be covered by any C++ book or a tutorial.

来源：https://stackoverflow.com/questions/16398416/friend-class-with-forward-class-declaration-does-not-compile

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