What is this use of std::map doing?

Question

Can anyone explain the output I am getting from this simple program using std::map. Note that I insert p into the map, but not q yet i

Answer 1

The problem is with the way you defined the comparison functor, in this case. The two elements, p, and q, have the same x and y, just inverted. Your logic checks that the x of one is less than that of the other, as well as the ys. This can never evaluate to true, for these inputs.

Try this snippet:

int main()
{
    auto p = screenPoint(1,2);
    auto q = screenPoint(2,1);

   std::cout << std::boolalpha << (p < q) << " " << (q < p) << std::endl;
}

It will print out

false false

So p is not less than q, and q is not less than p. As far as the map is concerned, that makes them equivalent.