What is this use of std::map doing? - c++

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.

Answer 2

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);
}