C++ fmt library, formatting a single argument with just format specifier

Question

Using the C++ fmt library, and given a bare format specifier, is there a way to format a single argument using it?

// example
std::string str = magic_format(".2f", 1.23);

// current method
template <typename T>
std::string magic_format(const std::string spec, const T arg) {
    return fmt::format(fmt::format("{{:{}}}", spec), arg);
}

While the above implementation does what I want, I would prefer a way that doesn't require me to build a new temporary string in the process.

Answer 1

You can do this by using formatter<T> directly:

template <typename T>
std::string magic_format(fmt::string_view spec, const T& arg) {
  fmt::formatter<T> f;
  fmt::format_parse_context parse_ctx(spec);
  f.parse(parse_ctx);
  std::string str;
  auto out = std::back_inserter(str);
  using context = fmt::format_context_t<decltype(out), char>;
  auto args = fmt::make_format_args<context>(arg);
  context format_ctx(out, args, {});
  f.format(arg, format_ctx);
  return str;
}

Godbolt: https://godbolt.org/z/Ras_QI