P3885R0
Add a formatter for std::error_category

1. Introduction

This paper proposes making std::error_category formattable using the formatting facility introduced in C++20 (std::format).

2. Motivation

std::error_category currently has no standard way to be formatted or printed directly. For example none of this compiles:

std::cout << std::generic_category();
std::print("{}", std::generic_category());

Additionally the encoding of std::error_category::name() was unspecified making it difficult to use it reliably - this is addressed by [P3395].

3. Proposal

The current paper proposes making std::error_category formattable:

std::print("{}", std::generic_category());

This prints generic.

It will correctly handle width and alignment:

std::print("[{:>10}]\n", std::generic_category());

Output:

[   generic]

Similarly to other recent cases, an ostream inserter is not proposed but it is possible to stream the string returned by std::error_categor::name() or the output of std::format.

4. Wording

Add to "Header <system_error> synopsis" [system.error.syn]:

// [system.error.fmt], formatter
template<class charT> struct formatter<error_category, charT>;

Add a new section "Formatting" [system.error.fmt] under "Class error_category" [syserr.errcat]:

template<class charT> struct formatter<error_category, charT>
    : formatter<basic_string_view<charT>, charT> {
  template<class FormatContext>
    typename FormatContext::iterator
      format(const error_category& cat, FormatContext& ctx) const;
};

template<class FormatContext>
  typename FormatContext::iterator
    format(const error_category& cat, FormatContext& ctx) const;

Let name be cat.name() if charT is char and cat.name(), transcoded to wide literal encoding otherwise.

Returns: formatter<basic_string_view<charT>, charT>::format(name, ctx).