C++

Forum

 
 
Friendship behaves different with nested classes

ncomputersorg (67)
Hello everybody!

We've found that friendship could behave different with nested classes.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
 
#include<iostream>
using namespace std;

class B{
	int b;

	B():b(123456){}

	friend class C;
};

class C:B{
	void asd(){
		cout<<B::b<<endl;
	}

	friend struct A;
};

struct A{
	A(){
		C c;
		c.asd();
	}
};

int main(){
	A a;
}

Only when class B and class C are nested, class B and class C can have the same friend struct A.

When they are nested, class C must be declared forward when the friend of class B is class C.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
 
#include<iostream>
using namespace std;

struct A{
	//class C;

	class B{
		int b;

		B():b(123456){}

		friend struct A;
		//friend class C;
	};

	class C:B{
		void asd(){
			cout<<B::b<<endl;
		}

		friend struct A;
	};

	A(){
		C c;
		c.asd();
	}
};

int main(){
	A a;
}

Why does friendship behave different with nested classes?

Especially: Why are nested classes implicit friends, when they are only friends of the class, that nests them?

Thank you very much!

Best regards,

http://ncomputers.org
Last edited on
MiiNiPaa (8886)
Only when class B and class C are nested, class B and class C can have the same friend struct A.
I don't get you.
1
2
3
4
5
6
 
class C {friend struct A;};

class B {friend struct A;};

struct A {};
..Both B and C have the same friend A

Why are nested classes implicit friends
They are members of enclosing class and have the same access to enclosing class members as any other member.

Answers to all "why" questions:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf
ncomputersorg (67)
MiiNiPaa:

How are you doing?

Thank you very much for your time and your answer!

The link you posted is very useful!

Only when class B and class C are nested, class B and class C can have the same friend struct A.
I don't get you.

I try to express me better:

Only when class B and class C are nested and they have the same friend struct A, class B and class C are implicit friends.

Best regards,

http://ncomputers.org
Cubbi (4774)
As the link given by MiiNiPaa explains, under 11.3[class.friends]/2

Declaring a class to be a friend implies that the names of private and protected members from the class granting friendship can be accessed in the base-specifiers and member declarations of the befriended class.

Because B granted friendship to A, the declaration of C (which is a member declaration of A) can access B's private member B::b.

Note that B doesn't need to be a member of A for this to happen, it's not "only when class B and class C are nested", but rather "only when class C is nested"

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
 
#include <iostream>

class B{
    int b;
    B() : b(123456){}
    friend struct A; // this lets A::C::asd() see B::b
};

struct A{
    class C:B{
        void asd() { std::cout << B::b << '\n'; }
        friend struct A; // this lets A::A() see A::C::asd()
    };
    A() { C c; c.asd(); }
};

int main()
{
    A a;
}
Last edited on
ncomputersorg (67)
Cubbi:

Thank you very much for your answer and your time!

Wow! I understand now friendship with nested classes better.

Best regards,

http://ncomputers.org
Last edited on
Topic archived. No new replies allowed.