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

In order to use a data type in an std::map, it must have a particular ordering called a strict weak ordering (https://en.wikipedia.org/wiki/Weak_ordering). This means that the inequality operator (<) obeys a very specific set of rules. The operator you specified however is not a weak ordering. In particular, given two screenPoints, a and b constructed from (1,2) and (2,1) respectively, you will see that it is false both that a < b and that b < a. In a strict weak ordering, this would be required to imply that a == b, which is not true!

Because your inequality operator does not meet the requirement of a strict weak ordering, map ends up doing unexpected things. I recommend reading up more details on what this ordering is, and reading/thinking about why map requires it. In the short term, you can redefine your operator as follows:

bool operator<(const screenPoint& left, const screenPoint& right){
  if (left.x != right.x) return left.x < right.x;
  else return (left.y < right.y);
}

Answer 2

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.