Why not modify key of associative container?

Question

I know that it\'s a terrible idea to change the key of an object in an associative container, but I wonder where exactly the standard forbids me to do so. Consider:

Answer 1

This injects Undefined Behavior in your program if you modify the values in a way that the comparison of any two keys is different after the change according to the comparator you specified.

Per Paragraph 23.2.4/3 of the C++11 Standard ([associative.reqmts]):

The phrase “equivalence of keys” means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false. For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value.