std::map, custom comparator's design constraints

Question

I have been trying defining a custom comparator for std::map container.

The question is : can I relay on == and != operators or would that break strict weak order ? Am I

Answer 1

You cannot rely on self consistent equals or not equals because these do not establish a strict weak ordering. The comparator return value should change if you switch the left and right arguments. It is essential for the map to be able to establish an ordering of elements, not just be able to distinguish whether two elements are equal.

It is very important that, is A < B is true, then B < A is false. Furthermore, if A < B and B < C are both true, then A < C is also true.

If you do not want to or cannot establish a strict weak ordering for your types, then, then you could use a map that doesn't require it: std::unordered_map¹, which is a hash map. However, this will require that you provide a hashing function and an equality comparison. It also requires that your compiler have C++11 support.

¹ As @JohnDibling points out in comments, std::unordered_map is unfortunately named. It should have been std::hash_map but apparently that name could have clashed with hash_maps from other libraries. In any case, the intent is not to have a map that is not ordered, but to have one with constant time look-up.