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 it says it found them both, but also says there is only 1 element in the map!

#include <map>
#include <iostream>

struct screenPoint {
  float x = 0, y = 0;
  screenPoint(float x_, float y_): x{x_}, y{y_}{}
};

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

std::map<screenPoint, float> positions;

int main(int argc, const char * argv[]) {

  auto p = screenPoint(1,2);
  auto q = screenPoint(2,1);
  positions.emplace(p,3);

  auto f = positions.find(p);
  auto g = positions.find(q);

  if (f == positions.end()){
    std::cout << "f not found";
  } else {
    std::cout << "f found";
  }

  std::cout << std::endl;

  if (g == positions.end()){
    std::cout << "g not found";
  } else {
    std::cout << "g found";
  }

  std::cout << std::endl;

  std::cout << "number elements: " << positions.size() << "\n";
  return 0;
}

Output:

f found
g found
number elements: 1

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.