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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LEARN ABOUT SUSE

tk::derived

Derived(3)						User Contributed Perl Documentation						Derived(3)

NAME

       Tk::Derived - Base class for widgets derived from others

SYNOPSIS

	   package Tk::MyNewWidget;

	   use Tk::widgets qw/ BaseWidget, list of Tk widgets /;
	   use base qw/ Tk::Derived Tk::BaseWidget /;

	   Construct Tk::Widget 'MyNewWidget';

	   sub ClassInit {
	       my( $class, $mw ) = @_;
	       #... e.g., class bindings here ...
	       $class->SUPER::ClassInit( $mw );
	   }

	   sub Populate {
	       my( $self, $args ) = @_;

	       my $flag = delete $args->{-flag};
	       if( defined $flag ) {
		   # handle -flag => xxx which can only be done at create
		   # time the delete above ensures that new() does not try
		   # and do  $self->configure( -flag => xxx );
	       }

	       $self->SUPER::Populate( $args );

	       $self = $self->Component( ... );

	       $self->Delegates( ... );

	       $self->ConfigSpecs(
		   '-cursor'	=> [ SELF, 'cursor', 'Cursor',	 undef ],
		   '-something' => [ METHOD, dbName,  dbClass, default ],
		   '-text'	=> [ $label, dbName,  dbClass, default ],
		   '-heading'	=> [ {-text => $head},
				       heading, Heading,  'My Heading' ],
	      );
	  }

	  sub something {
	      my( $self, $value) = @_;
	      if ( @_ > 1 ) {
		 # set it
	      }
	      return # current value
	  }

DESCRIPTION

       Tk::Derived is used with Perl's multiple inheritance to override some methods normally inherited from Tk::Widget.

       Tk::Derived should precede any Tk widgets in the class's base class definition.

       Tk::Derived's main purpose is to apply wrappers to "configure" and "cget" methods of widgets to allow the derived widget to add to or
       modify behaviour of the configure options supported by the base widget.

       The derived class should normally override the "Populate" method provided by Tk::Derived and call "ConfigSpecs" to declare configure
       options.

       The public methods provided by Tk::Derived are as follows:

       ->ConfigSpecs(-key => [kind, name, Class, default], ...)

SEE ALSO

       Tk::ConfigSpecs Tk::mega Tk::composite

perl v5.12.1							    2007-05-05								Derived(3)