Is a class automatically a friend of itself

Question

Why does this work?

#include <stdio.h>

class ClassA
{
public:
    ClassA(int id) : my_id(id) { };

    ClassA * makeNewA(int id)
    {
        ClassA *a = new ClassA(id);
        printf("ClassA made with id %d\n", a->getId());
        return a;
    };

private:
    int getId() {
        return my_id;
    };

private:
    int my_id;
};

int main()
{
    ClassA a(1);
    ClassA *b = a.makeNewA(2);
    return 0;
}

Irrespective of whether or not it's a good idea, why does it work? The public function ClassA::makeNewA(int) instantiates a new ClassA and then calls a private function getId() using the new object. Is a class automatically a friend of itself?

Thanks

Maybe a better way to think about it is that C++ encapsulation is class level, not object level. — Nir Friedman, Sep 09 '16 at 06:27
@NirFriedman: Except that that isn't strictly true - You can't access `protected` members of another object, even from an object who has inherited the same members. — MSalters, Sep 09 '16 at 11:10

score 11 · Accepted Answer · answered Sep 09 '16 at 05:00

11

Yes, it is intentional that a class's public methods can access its own private members, even if that method is acting on a different instance.

I guess one could say that a class automatically is a "friend" of itself.

answered Sep 09 '16 at 05:00

BingsF

1,269
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score 6 · Answer 2 · edited May 23 '17 at 10:33

6

Basically, yes, the class is friend of itself, but better explanation would be that:

In C++ access control works on per-class basis, not on per-object basis.

(copied from there)

Note that this applies also to other languages, for example Java (in which you don't have friends) where access control also works on per-class basis.

edited May 23 '17 at 10:33

Community

1
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answered Sep 09 '16 at 06:59

PcAF

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score 2 · Answer 3 · answered Sep 09 '16 at 05:01

2

All class methods, static or not, are automatically "friends" of the class. Friend is used to allow external functions and classes access to a class. The class is always its own "friend".

answered Sep 09 '16 at 05:01

GAVD

1,977
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Is a class automatically a friend of itself

3 Answers3