c++ - Error when trying to pass '==' overload to std::map comparator -

i'm trying pass overload of custom class comparator of std::map.

here's code , error me :

here position.hpp

#ifndef position_hpp_ # define position_hpp_  class   position { public:   int   _x;   int   _y;    position(){}   position(int x, int y)   {     _x = x;     _y = y;   }    void  setposition(position &newpos)   {     _x = newpos._x;     _y = newpos._y;   }    int   getx()   {     return _x;   }    int   gety()       return _y;   }   void  setx(int x)   {     _x = x;   }   void  sety(int y)   {     _y = y;   }    struct cmpissame{     inline bool operator==(const position& pos)     {       if (pos._x == _x && pos._y == _y)         return true;       return false;     }   };    inline position&      operator=(const position& pos)   {     _x = pos._x;     _y = pos._y;     return *this;   } };  #endif 

and here map declaration in gameengine.hh

private:   std::map<position, case, position::cmpissame> _cases; 

and here error:

position.hpp: in member function ‘bool position::cmpissame::operator==(const position&)’: position.hpp:17:7: error: invalid use of non-static data member ‘position::_x’    int _x;        ^ position.hpp:52:21: error: location        if (pos._x == _x && pos._y == _y)                      ^ position.hpp:18:7: error: invalid use of non-static data member ‘position::_y’    int _y;        ^ position.hpp:52:37: error: location        if (pos._x == _x && pos._y == _y)                                      ^ 

any pleasure?

first of have use either functional object or static function. in case approach wrong because comparator shall satisfy strict weak ordering principle correcponds operator <.

from c++ standard (23.2.4 associative containers)

3 phrase “equivalence of keys” means equivalence relation imposed comparison , not operator== on keys. is, 2 keys k1 , k2 considered equivalent if comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false. 2 keys k1 , k2 in same container, calling comp(k1, k2) shall return same value.