Access private elements of object of same class

问题

Is this legal? If not, will the following code allow this?

class Foo
{
    friend class Foo;
}

回答1:

That's redundant. Foo already has access to all Foo members. Two Foo objects can access each other's members.

class Foo {
public:
  int touchOtherParts(const Foo &foo) {return foo.privateparts;}
private:
  int privateparts;
};

Foo a,b;
b.touchOtherParts(a);

The above code will work just fine. B will access a's private data member.

回答2:

Yes it is legal for an object of class Foo to access the private members of another object of class Foo. This is frequently necessary for things like copy construction and assignment, and no special friend declaration is required.

回答3:

It is redundant and unnecessary. Moreover, I get the following warning in g++

warning: class ‘Foo’ is implicitly friends with itself

回答4:

Classes friending themselves makes sense if they're templates, as each instantiation with distinct parameters is a different class.

来源：https://stackoverflow.com/questions/3832613/access-private-elements-of-object-of-same-class