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

#include "always40.hpp"
#include <format>
#include <iostream>

struct std::formatter<Always40>
  // parse the format string for this type:
  constexpr auto parse(std::format_parse_context& ctx) {
    return ctx.begin();    // return position of } (hopefully there)

  // format by always writing its value:
  auto format(const Always40& obj, std::format_context& ctx) const {
    return std::format_to(ctx.out(), "{}", obj.getValue());