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 "always42.hpp"
#include <format>

//*** formatter for type Always42:
// - use standard int formatter
struct std::formatter<Always42> : std::formatter<int>
  auto format(const Always42& obj, std::format_context& ctx) {
    // delegate formatting of the value to the standard formatter:
    return std::formatter<int>::format(obj.getValue(), ctx);