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[std-proposals] Redefine std::to_string in terms of std::format

From: ஜெய்கணேஷ் குமரன் <jaiganesh.kumaran_at_[hidden]>
Date: Fri, 15 Mar 2024 22:54:52 +0000
Hello all,

Currently std::to_string cannot be overloaded for custom user-defined types. While you could follow the convention of having a to_string free function in the enclosing namespace of a particular type and then rely on ADL to resolve the function, it obviously would not work with fundamental types unless you using-ed namespace std or std::to_string.

I would prefer being able to std::to_string to stringily any type if possible. I believe that it should be based on std::format so that the author of a typer can simply specialise std::formatter<T> to get both std::to_string and std::format/print working.

std::string to_string(std::formattable<char> auto&& value) { return std::format("{}", value); }

Granted, specialising std::formatter<T> is a bit more verbose than simply having a to_string free or member function. Perhaps you could allow formatting, with no format-arguments, a type that just has a to_string free function.

template <typename T>
concept has_free_to_string = requires(T const& t)
{ to_string(t) };

template <has_free_to_string T>
struct std::formatter<T>
auto constexpr parse(std::format_parse_context& context) { return context.begin(); }

auto format(T const& str, std::format_context& context) const
return std::format_to(context.out(), "{}", to_string(str));

The same thing shall apply for wide strings.

Note: I do not have the time to participate in standardisation in order to open formal proposals, but I wish someöne else does based on my ideas.

Jaiganésh Kumaran.

Received on 2024-03-15 22:54:57