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Friend classes and methods

Posted on 2013-01-23
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Last Modified: 2013-01-24
Hello Experts!
Please have a look at the code below:

#include <iostream>
using namespace std;

class F;

class X {
  friend class F { };//error, it makes sense, friend declaration is not supposed to declare class
};

class A {
  void g();
};

void z() {
  class B {//we define class within method... weird but legal
    friend void f() { }; //error, it makes sense, friend declaration is not supposed to declare method
  };
}

class C {
  friend void A::g() { } //error, it makes sense, friend declarion is not supposed to declare method
  friend void h() { }//now I am confused! It is legal! Why? What logic is behind it? How does compiler understand it?
};

int main(){
  return 0;
}

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Question by:panJames
8 Comments
 
LVL 31

Expert Comment

by:farzanj
ID: 38810261
Your friend class declaration should not have braces at the end
friend class F { };

Should be
friend class F;

Same is the problem with the friend member function.  You are only supposed to give a prototype preceded by word friend not the definition so you cannot have {}

With non member function (where you said is confusing), you are giving a definition as well and it will be a non member function but being friend will be able to access the protected and private data of the class.
0
 
LVL 22

Accepted Solution

by:
ambience earned 300 total points
ID: 38810318
void z() {
  class B {//we define class within method... weird but legal
    friend void f() { }; //error, it makes sense, friend declaration is not supposed to declare method
  };
}

Actually, the reason it fails is because a friend function is not permitted in local class. This is what the standard says

A function can be defined in a friend declaration of a class if and only if the class is a non-local class

class C {
  friend void h() { }//now I am confused! It is legal! Why? What logic is behind it? How does compiler understand it?
};

C is a non-local class therefore it works.
0
 
LVL 30

Expert Comment

by:Zoppo
ID: 38810322
Hi panJames,

farzanj is right, removing the { } solves the friend class F in class X and the friend void A::g() in class C.

Somthing like friend void f(); doesn't make much sense since this declares the function f from the same class as friend which is meaningless. You can i.e. declare a global function as friend:
void f(); // prototype

void z() {
	class B {//we define class within method... weird but legal
		friend void ::f(); //ok
	};
}

void f() { /*do something*/ }

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Hope that helps,

ZOPPO
0
 
LVL 22

Expert Comment

by:ambience
ID: 38810337
Oh and please note this is about definition - for example

void h() {}

void z() {
  class B {
      friend void h();    // works
        friend void f() { }; // does not work
  };
}
0
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Author Comment

by:panJames
ID: 38813447
@ambience


class C {
  friend void h() { }//now I am confused! It is legal! Why? What logic is behind it? How does compiler understand it?
};

C is a non-local class therefore it works.

Still confused...
To me C looks like a local class because I can see it's declaration. Why is it not local?

Another thing.

friend void h() { }

for me it looks like declaration of method 'h', method that does nothing so...

1. does compiler assume that somewhere in the code there is method:

void h() { }

?

2. or

class C {
  friend void h() { cout<<"C++ is weird";}
};

does it mean that we can do such a thing:

class C {
  int x, y;
  friend void h() { cout<<"C++ is weird";}
};

h();

**** result: ****
C++ is weird
0
 
LVL 30

Assisted Solution

by:Zoppo
Zoppo earned 200 total points
ID: 38813488
Hi panJames,

a local class is a class which is declared within another class or a function, i.e.:
class A // none-local class
{
 class B{}; // local class
};

 void f() {
  class C{}; // local class
}

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Next: friend void h() { } declares and implements a method in the class. The friend keyword is meaningless here since a method always has access to private members of their class.

What you can do declaring a global function as friend is to allow a specific function to access private members, something like this:
class C {
	int x; // private
	friend void f();
};

void f()
{
	C c;
	c.x = 1; // OK
}

void h()
{
	C c;
	c.x = 1; // Error: cannot access private member
}

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Hope that helps,

ZOPPO
0
 
LVL 22

Expert Comment

by:ambience
ID: 38813547
>>  a local class is a class which is declared within another class or a function

Half true - a local class is one declared inside a function. A class inside another class is nested but not local. You can certainly do

class A { public: class B { }; };

A::B b;

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This is nested, not local.

>>  The friend keyword is meaningless here since a method always has access to private members of their class.
>> for me it looks like declaration of method 'h', method that does nothing so...

NO - its not meaningless and its NOT Method, its a function (that is not gobal). The following does not compile, because h is not a method - take out the friend and it compiles.

class C 
{
  friend void h() { 
	  cout << "Not a method so this is not defined " << this << std::endl;      << ERROR!
  }

public:
	void method()
	{
		h();
	}
};

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>> does it mean that we can do such a thing:

No, since its not in global scope - but you can use it like in the example above.

>>.C++ is weird

When you dont know what you are doing its always weird.
0
 
LVL 30

Expert Comment

by:Zoppo
ID: 38813567
ok, sorry, ambience is right. Only within a function it's a local class. And the 'h()' is a function which is not a method. Honestly I have to admit I didn't know that's even possible :(
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