We can use a structure and a class, which are defined by myself, as key of std::map.
If we want to do so, we need define comparison operator < for defined structure or class, because std::map stores data as binary tree. If we don’t define <, computer can not decide which one is bigger or smaller.
I show the sample code below. In this sample, a structure, which has an int and char as its member, works a key of std::map.