Question

I’m trying to create a set of custom objects of the class Point

point.h

#ifndef POINT_H
#define POINT_H

class Point
{
      float x_coordinate,y_coordinate;
      public:
             Point(float x, float y):x_coordinate(x),y_coordinate(y)
             {}

             float get_x()
             {
                   return x_coordinate;
             }

             float get_y()
             {
                   return y_coordinate;
             }

             bool operator==(Point rhs)
             {
                  if( ((int)x_coordinate == (int)rhs.get_x()) && ((int)y_coordinate == (int)rhs.get_y()) )
                      return true;
                  else return false;
             }

             bool operator<(Point rhs)
             {
                  if((int)x_coordinate < (int)rhs.get_x())
                      return true;
                  else return false;
             }
};

#endif

I’ve just started to write the driver

#include<iostream>
#include<set>
#include "point.h"
using namespace std;



int main()
{
    Point p1(-10,-10),p2(-10,10),p3(10,10),p4(10,-10);
    set<Point> points_set = set<Point>();
    points_set.insert(p1);

    return 0;
}

but I get this error during compilation, I have overloaded the comparison and equality operator, what else must I do to get it to work?

In file included from /usr/include/c++/4.6/string:50:0,
                 from /usr/include/c++/4.6/bits/locale_classes.h:42,
                 from /usr/include/c++/4.6/bits/ios_base.h:43,
                 from /usr/include/c++/4.6/ios:43,
                 from /usr/include/c++/4.6/ostream:40,
                 from /usr/include/c++/4.6/iostream:40,
                 from driver.cpp:1:
/usr/include/c++/4.6/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = Point]’:
/usr/include/c++/4.6/bits/stl_tree.h:1267:4:   instantiated from ‘std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = Point, _Val = Point, _KeyOfValue = std::_Identity<Point>, _Compare = std::less<Point>, _Alloc = std::allocator<Point>]’
/usr/include/c++/4.6/bits/stl_set.h:410:29:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename _Alloc::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(const value_type&) [with _Key = Point, _Compare = std::less<Point>, _Alloc = std::allocator<Point>, typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename _Alloc::rebind<_Key>::other>::const_iterator = std::_Rb_tree_const_iterator<Point>, std::set<_Key, _Compare, _Alloc>::value_type = Point]’
driver.cpp:12:25:   instantiated from here
/usr/include/c++/4.6/bits/stl_function.h:236:22: error: passing ‘const Point’ as ‘this’ argument of ‘bool Point::operator<(Point)’ discards qualifiers [-fpermissive]

Answer 1

@Mat beat me to it to answer your specific question. So I’ll just point-out some issue with your un-idiomatic C++. You don’t need a conditional to return a boolean. Just return the value of what would have been your if statement:

bool operator==(const Point& rhs) const
{
    return ((int)x_coordinate == (int)rhs.get_x()) && ((int)y_coordinate == (int)rhs.get_y());
}

bool operator<(const Point& rhs) const
{
    return (int)x_coordinate < (int)rhs.get_x();
}

Also, it’s better to state the explicit casting mechanism in C++ rather than use a C-style case:

    return static_cast<int>(x_coordinate) < static_cast<int>(rhs.get_x());