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;
  ...........
};

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

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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;  }
};

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;  }
};

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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);

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

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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.
}

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.
}

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[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;  }
};

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;  }
};

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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".