friend class in TC++ book

[mickey]

hello,

I read TC++ vol2. (html ed 2004) at page 456 (chap design patterns) "the inner class idiom".
In the class "Outer" it writes:

class Outer {
.....
class Inner1;
friend class Outer::Inner1;
...........
};

I tried to get out those two lines and it compiles; why? What sense have they, please?

thanks,

[wysota]

It means oobjects of the Outer::Inner1 class can access private members of Outer class instances.

[mickey]

It means oobjects of the Outer::Inner1 class can access private members of Outer class instances.
Sorry, I don't understand how expolit this two lines; see this ,please:

class Shape {
class Type;
friend class Shape::Type;

int _number;
double _x, _y;
class Type {
Shape* _parent;
public:
Type(Shape* sh) : _parent(sh) {
double d = _parent->compute();
double d2 = _x; //is this an access ? It works only with "parent->_x"
}
};
Type _type;
public:
Shape(double x = 0, double y = 0) : _type(this), _x(x), _y(y) { }
double compute() { return _x * _y; }
};
Where can I access to provate member of Shape ? (and how?)

[wysota]

Instances of Shape::Type can access _number, _x, _y and _type from instances of Shape.

[mickey]

hello,
I don't find a way to access those private members; from where? from main?

//main.cpp
Shape s;
Shape::Type t(&s);

Or where, please?

[jacek]

I don't find a way to access those private members; from where? from main?
Since Shape::Type is a friend of Shape (it works only one way), then any object of Shape::Type can access member variables of any Shape object.
Shape::Type::Type(Shape* sh) : _parent(sh) {
...
double d2 = _parent->_x; // "this" is an object of Shape::Type class, so it can access
// _x in _parent which points to an instance of Shape.
}

[mickey]

hello,
i don't understand:


class Shape {
//class Type;
//friend class Shape::Type;
int _number;
double _x, _y;

class Type {
Shape* _parent;
public:
Type(Shape* sh) : _parent(sh) {
double d = _parent->compute();
double d2 = _parent->_x; //it compiles....
}
};
public:
Type _type;
Shape(double x = 0, double y = 0) : _type(this), _x(x), _y(y) { }
double compute() {
return _x * _y; }
};

if i comment those lines, it complies anyway...

[wysota]

Could you prepare a minimal compilable example of the above?

[jacek]

double d2 = _parent->_x; //it compiles....
The standard says:
A nested class is a member and as such has the same access rights as any other member.
Which means that nested classes behave as if they were friends of nesting class.

[mickey]

Could you prepare a minimal compilable example of the above?

the example is compilable;
Thw question was: how come TC++ vol2 write those 2 lined commented in the post #7 (if they are unuseful) ???

[jacek]

how come TC++ vol2 write those 2 lined commented in the post #7 (if they are unuseful) ???
Not everything what's written in a book is correct.

[wysota]

I'd say this code in the book might be an explicit declaration of an implicit feature of the standard. Especially that we know some compilers tend to create their own "standards".