c++ adding struct var to set of vars causing error

Question

I have some pretty simple C++ code that creates points from a struct definition and tries to add those points to a set.

#include <stdio.h>       /* printf */
#include <bits/stdc++.h> /* vector of strings */
using namespace std;

struct point
{
    int x;
    int y;
};

int main(){

   for(int i = 0; i <= 6; i++){
        set<point> visited_points;
        point visited_point{4, 1};
        visited_points.insert(visited_point);

   }
}

But this code throws a large console error when I run it saying:

In file included from /usr/include/c++/7/string:48:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/istream:38,
                 from /usr/include/c++/7/sstream:38,
                 from /usr/include/c++/7/complex:45,
                 from /usr/include/c++/7/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/7/bits/stdc++.h:52,
                 from ex.cpp:2:
/usr/include/c++/7/bits/stl_function.h: In instantiation of ‘constexpr bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = point]’:
/usr/include/c++/7/bits/stl_tree.h:2038:11:   required from ‘std::pair<std::_Rb_tree_node_base*, std::

........

/usr/include/c++/7/bits/stl_function.h:386:20: note:   ‘const point’ is not derived from ‘const std::__cxx11::sub_match<_BiIter>’
       { return __x < __y; }
                ~~~~^~~~~

Is there a part of my code that I did wrong? I just want a way to keep track of multiple points in a list.

Answer 1

Sets are ordered, and so the elements need an ordering function. Your point class doesn't not have this. Add a suitable definition of

bool operator<(const Point& a, const Point& b);

For instance

bool operator<(const Point& a, const Point& b)
{
    return a.x < b.x || a.x == b.x && a.y < b.y;
}

But whatever ordering function you choose it must define a strict weak ordering