The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License. Creative Commons License

// raw code

#ifndef TRUTH_HPP
#define TRUTH_HPP

struct Truth {
  int value;
  constexpr Truth() : value{42} {  // enable compile-time initialization

constexpr Truth theTruth;          // force compile-time initialization

#endif // TRUTH_HPP