PMR (polymorphic memory resources) are now fully described in:

C++17 - The Complete Guide

by Nico Josuttis

About the article:

This includes:

• How to use standard memory resources
• How to define own memory resources (such as a sophisticated new tracker)
• How to provide PMR support for own types

With this, using a map or unordered_map with elements located close to each other is really easy now.

And if you have to avoid heap memory allocation, this chapter is a must.