Question

In question Using char as key in stdmap there is advised to use custom compare function/functor:

struct cmp_str
{
   bool operator()(char const *a, char const *b)
   {
      return std::strcmp(a, b) < 0;
   }
};

map<char *, int, cmp_str> BlahBlah;

This allows map to detect if key A is less than key B. But for example map<>::find() returns end if element is not found, and iterator to it if it is found. So map knows about equivalence, not only less-than. How?

Answer 1

The equality condition for two keys a and b are that a<b and b<a are both false. The map itself is commonly implemented as a balanced binary tree^*, so the less-than comparison is used to traverse the map from the root node until the matching element is found. When searching for a key k, less-than comparison is used until the first element for which the comparison is false is found. If the inverse comparison is also false, k has been found. Otherwise, k is not in the map. The map only uses the less-than comparison to this purpose.

Note also that std::set uses exactly the same mechanism, the only difference being that each element is it’s own key.

^* strictly speaking, the C++ standard does not specify that std::map be a balanced binary tree, but the complexity constraints it places on operations such as insertion and look-up mean that implementations chose structures such as red-black tree.