Basic, but complex.

What is an Allocator?

by Vanand Gasparyan

From the article:

This is one of the many beautiful questions you can think on or be asked at interviews if your native coding language is C++. The beautiful thing about this question is that you answer it again and again as you grow and gain more experience. Similarly, when you’re being interviewed for an internship, knowing that there is such a thing somewhere in STL might be enough, whereas later you’ll be expected to write your own custom allocator...