Question

In STL containers, like map and set – How the unique key is determined for user-defined types?

In C#, this is done by overriding GetHashCode and Equals methods.

Answer 1

A map, for example, takes 4 parameters as its template arguments. Most of the time, developers use just 2 of them i.e. the Key type and the Value type. But, one can also provide a functor that compares objects of the Key type (whatever type it would be).

From cplusplus.com:

Compare: Comparison class: A class that takes two arguments of the key type and returns a bool. The expression comp(a,b), where comp is
an object of this comparison class and a and b are key values, shall
return true if a is to be placed at an earlier position than b in a
strict weak ordering operation. This can either be a class
implementing a function call operator or a pointer to a function (see
constructor for an example). This defaults to less, which returns
the same as applying the less-than operator (a

http://www.cplusplus.com/reference/stl/map/

It is true for almost all cases (when using for sorting, or for any other container) and this principle holds across quite a few languages and platforms.