an example of how to write a custom std::formatter

custom_std_formatter.cpp

#include <iostream>
#include <format>
#include <iterator>

class Thing
{
public:
	std::string s{ "some text" };
};

template<class CharT>
struct std::formatter<Thing, CharT> : std::formatter<std::string, CharT>
{
	/*
    Example of a Custom std::formatter for an example class called Thing.
    Writes "Thing{" + Thing::s + "}".
  */
  
  // Define format() by calling the base class implementation with the wrapped value
	template<class FormatContext>
	auto format(Thing t, FormatContext& fc) 
	{
		std::formatter<std::string, CharT>::format("Thing{", fc);
		std::formatter<decltype(Thing::s), CharT>::format(t.s, fc);
		*fc.out()++ = '}';
		return fc.out();
	}
};

int wmain(int argc, wchar_t* args[])
{
	Thing t;
	std::cout << std::format("{}", t);
  // writes: Thing{some text}
}