friend class in TC++ book

hello,

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

Code:

---

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,

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

Quote:

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Originally Posted by wysota

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:

Code:

---

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?)

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

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

Code:

---

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

---

Or where, please?

Quote:

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Originally Posted by mickey

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.

Code:

---

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.
}

---

hello,
i don't understand:

Code:

---

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...

Could you prepare a minimal compilable example of the above?

Quote:

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Originally Posted by mickey

double d2 = _parent->_x; //it compiles....

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The standard says:

Quote:

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A nested class is a member and as such has the same access rights as any other member.

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Which means that nested classes behave as if they were friends of nesting class.

Quote:

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Originally Posted by wysota

Could you prepare a minimal compilable example of the above?

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

Quote:

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Originally Posted by mickey

how come TC++ vol2 write those 2 lined commented in the post #7 (if they are unuseful) ???

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Not everything what's written in a book is correct.

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".