Can we use custom formatters for std strings?

Question

I wish to add a custom std::string formatter, so it takes precedence over the default one and I don't have to use formatters for custom types like std::format("{}", Upper(s)).

#include <iostream>
#include <format>
#include <string>

template <>
struct std::formatter<std::string> {
  constexpr auto parse(std::format_parse_context& ctx) {
    // Simplified for this example, actually I want to implement a specific {:}
    return ctx.begin();
  }

  auto format(std::string s, std::format_context& ctx) const {
    for (char& c : s) {
      c = std::toupper(static_cast<unsigned char>(c));
    }
    return std::format_to(ctx.out(), "{:s}", s);
  }
};

int main() {
  const std::string s = "hello world";
  std::cout << std::format("{}", s) << "\n";
}

I expect the output HELLO WORLD, but the actual output is hello world.

Answer 1

You are not allowed to specialize std::formatter<std::string> because it does not rely on a program-defined type based on [namespace.std].

One workaround without providing std::formatter specialization is to transform the string-like object via views::transform and format it as a string, i.e., ":s":

constexpr auto to_upper = 
  std::views::transform(
    [](char c) -> char { return std::toupper(static_cast<unsigned char>(c)); });

int main() {
  const std::string s = "hello world";
  std::cout << std::format("{:s}", to_upper(s)) << "\n";
}