How to make a function friend to both base and derived class Post: 302146334

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Top Forums Programming How to make a function friend to both base and derived class Post 302146334 by ennstate on Tuesday 20th of November 2007 03:24:00 AM

11-20-2007

Registered User

How to make a function friend to both base and derived class

Hi,
I have a base class and derived a class from the base class, i want to print & read the data for the object created for the derived class,so i have overloaded both the << and >> operators and also have done the foward declaration.

Below is the code snippet,

Code:

#include <iostream>

class Line; // Forward Declaration for friend

class Point {
 private:
   int x,y;
 public:
   virtual void draw() ;
   friend std::ostream& operator<< ( std::ostream& , const Line&);
   friend std::istream& operator>> ( std::istream& ,  Line&);
};


class Line : public Point {
 private:
  int color;
 public :
   void draw () { std::cout<< "Drawing a Line" << std::endl; }
   friend std::ostream& operator<< ( std::ostream& , const Line&);
   friend std::istream& operator>> ( std::istream& ,  Line&);
};

inline
std::ostream& operator<< ( std::ostream &os , const Line &out ) {
  os<<"Line: [" << out.x << "," << out.y << " ," << out.color << " ]" << std::endl;
  return os;
}

inline
std::istream& operator>> ( std::istream &is , const Line &in ) {
  std::cout<<"Line x , y , color info: " << std::endl;
  is >> in.x >> in.y >> in.color;
  return is;
}

int main(int argc, char *aargv) {
 Line new_line;
 std::cin>>new_line;
 std::cout<<new_line;
 return(0);
}

Am using g++ and on compiling i get the below errors, how to fix this

Code:

g++ friends.cpp
friends.cpp: In function `std::istream& operator>>(std::istream&, const Line&)   ':
friends.cpp:8: error: `int Point::x' is private
friends.cpp:34: error: within this context
friends.cpp:8: error: `int Point::y' is private
friends.cpp:34: error: within this context
friends.cpp:18: error: `int Line::color' is private
friends.cpp:34: error: within this context

Thanks
Nagarajan G

ennstate

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statevarview

StateVarView(3U)					    InterViews Reference Manual 					  StateVarView(3U)

NAME

       StateVarView - state variable view base class

SYNOPSIS

       #include <Unidraw/stateview.h>

DESCRIPTION

       StateVarView  is  the base class for state variable views.  State variable views provide a graphical interface to examining and potentially
       modifying a state variable subject.  The StateVarView base class is an abstract class derived from MonoScene.  Like MonoScene,  the  State-
       VarView	class  is not instantiated; instead, subclasses add state and behavior appropriate for displaying and editing their subject.  Like
       other MonoScene subclasses, StateVarView subclasses use an interactor composition to define their appearance.

PUBLIC OPERATIONS

       virtual void Update()
	      Update the state variable view in response to a change in state it depends on (typically the subject's).	This  operation  does  not
	      normally need redefinition if Init and Stale (described below) are redefined.

PROTECTED OPERATIONS

       StateVarView(StateVar*)
	      Subclass	constructors  normally take an instance of the corresponding subject as an argument.  The base class constructor automati-
	      cally attaches the view to the subject.

       virtual void Init()
	      Initialize the view based on information in the subject.	Subclasses redefine this operation according to their semantics;  it  does
	      nothing by default.

       virtual boolean Stale()
	      Return  whether the view is in any way inconsistent with its subject.  This operation always returns true by default; subclasses can
	      redefine it to make a more discerning decision.

SEE ALSO

       MonoScene(3I), StateVar(3U)

Unidraw 							   6 August 1990						  StateVarView(3U)