c++ - error while performing set_intersection with a map containing key and a structure -

i want find intersection between 2 maps. map has structure map<int,line>, line structure. problem when use set_intersection performing intersection following error represented in image.

below code

#include <string> #include <iostream> #include <map> #include <algorithm> using namespace std; typedef pair<int, int> point; struct line {     int lineid;     point starting, ending; }; int main(int argc, char* argv[]) {     typedef map<int, line> mymap;        line w,x,y,z;     w.lineid = 1;     w.starting = { 5, 1 };     w.ending = { 5, 100 };     x.lineid = 2;     x.starting = { 20, 56 };     x.ending = { 120, 56 };     y.lineid = 3;     y.starting = { 100, 150 };     y.ending = { 100, 200 };     z.lineid = 4;     z.starting = { 330, 50 };     z.ending = { 330, 150 };      mymap bin1;     bin1.insert({ w.lineid, w });     bin1.insert({ x.lineid, x });     bin1.insert({ y.lineid, y });      mymap bin2;     bin2.insert({ x.lineid, x });     bin2.insert({ y.lineid, y });     bin2.insert({ z.lineid, z });      mymap out;     mymap::iterator out_itr(out.begin());     set_intersection(bin1.begin(), bin1.end(), bin2.begin(), bin2.end(),                                                  inserter(out, out_itr));      cout << "" << out.size();                         return 0;     } 

any solve issue helpful.

the compiler complaining not finding operator< compare lines object (you need compare std::pair<int, line>, , comparator needs compare lines).

you must provide comparator lines :

example :

  bool operator<(const line& other) const   {       return other.lineid < lineid; // or whatever logic want compare lines.   } 

live example here

note :

alternatively, can directly provide comparator set_intersection, e.g. here lambda :

 set_intersection(bin1.begin(),                    bin1.end(),                    bin2.begin(),                    bin2.end(),                   inserter(out, out_itr),                   [] (const std::pair<int, line>& p1, const std::pair<int, line>& p2) { return p1.first != p2.first; } );