constexpr is one of the magic keywords in modern C++. You can use it to create code, that is then executed before the compilation process ends. This is the absolute upper limit for software performance.

Design and evolution of constexpr in C++

by Evgeny Shulgin

From the article:

The authors suffered greatly from the inability to use STL containers and wrote the std::vector and std::map analogues. Inside, these analogues have std::array that can work in constexpr. Proposal [P0784] Standard containers and constexpr studies the possibility of inputting STL containers in constexpr evaluations. Note. It's important to know what an allocator is. STL containers work with memory through it. What kind of an allocator — is specified through the tempte argument. If you want to get into the topic, read this article.