c++ - [ ]operator for custom object : '[' illegal for class? -

here code try access element of class edge:

#include <iostream>  // sort() #include <algorithm> #define pi 3.14159265358979323846  struct point{     point(int xx, int yy): x(xx), y(yy) { } int x; int y; };  // class edge: representing lines segments of poly-line struct edge{     // constructor     edge(point p0, point p1) : start(p0), end(p1){          if (p0.x == p1.x && p0.y == p1.y) throw std::invalid_argument("edge: identical points!");      }      // operator< defined sorting increasing ordinate of end point     bool operator<(const edge& e){ return (end.y < e.end.y); }     // data members: start point , end point of line point start; point end; };  static void generatepoints(vector<point>& p){ p.push_back(point(50,50)); p.push_back(point(200,50)); p.push_back(point(200,200)); p.push_back(point(50,200)); } //------------------------------------------------------------------------------------------------ int main(){  // generate points poly-line vector<point> polypoints; generatepoints(polypoints);  vector<edge> polyedges; point first = polypoints(0); point last = polypoints(polypoints.size()-1); polyedges.push_back(edge(last, first)); (size_t = 1; < polypoints.size(); ++i) polyedges.push_back(edge(polypoints[i-1], polypoints[i]));   int ycoordinate = polyedges[i].end.y;  return 0; } 

now, have vector of edges, so:

vector<edge> polyedges; 

and when try access specific member polyedges[i].end.y, following error message:

  'vector' : undeclared identifier   'point' : illegal use of type expression   see declaration of 'point'   'p' : undeclared identifier   'generatepoints' : function-style initializer appears function definition   vector' : undeclared identifier   see declaration of 'point'   'polypoints' : undeclared identifier   'point' : illegal use of type expression   'polypoints' : undeclared identifier   'generatepoints': identifier not found   'vector' : undeclared identifier   edge' : illegal use of type expression   see declaration of 'edge'   error c2088: '[' : illegal class   polyedges' : undeclared identifier   'polypoints': identifier not found   'polypoints' : undeclared identifier   left of '.size' must have class/struct/union   'point' : illegal use of type expression   error c2228: left of '.end' must have class/struct/union   error c2228: left of '.y' must have class/struct/union 

it must related overloading of []operator.

question:

should overload [] operator , if so, how it?

try idea code. no need override [] operator. uses [] operator vector class. not of class

#include <iostream> #include <vector>  struct point {     int x;     int y;     point(int xx, int yy) : x(xx), y(yy) { }    };  struct edge {     point start;     point end;     edge(point p0, point p1) : start(p0), end(p1) {         if (p0.x == p1.x && p0.y == p1.y) {             throw std::invalid_argument("edge: identical points!");         }     }     bool operator<(const edge& e) { return (end.y < e.end.y); }      };  int main() {         point p1(0, 1);     point p2(2, 3);     edge e1(p1,p2);      std::vector<edge> polyedges;     polyedges.push_back(e1);      int = 0;     std::cout << polyedges[i].end.y << std::endl;     system("pause");      //output "3" }