Why does std::set seem to force the use of a const_iterator?

Question

Consider the simple program below, which attempts to iterate through the values of a set using NON-const references to the elements in it:

#include 

        

Answer 1

std::set uses the contained values to form a fast data structure (usually, a red-black tree). Changing a value means the whole structure needs to be altered. So, forcing constness, std::set prevents you from pushing it into a non-usable state.