Question

How would I implement a operator< for line segments that consist of a start and a end Point. I want to insert the line segments into a map so the order doesn’t need to be semantic, but it should work for all cases.

Answer 1

Order everything lexicographically:

struct Point { int x; int y; };

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

Or use the ready made comparator from tuple:

#include <tuple>

// ...

return std::tie(a.x, a.y) < std::tie(b.x, b.y);

Or actually use a std::tuple<int, int> for your points and do nothing at all!

Then, do the same for the lines:

struct LineSegment { Point x; Point y; };

// repeat same code as above, e.g.

bool operator<(LineSegment const & a, LineSegment const & b)
{
    return std::tie(a.x, a.y) < std::tie(b.x, b.y);
}

To repeat, the no-work-at-all solution just uses tuples all the way:

typedef std::tuple<int, int> Point;
typedef std::tuple<Point, Point> LineSegment;
// everything "just works"