P3395R0
Formatting of std::error_code

1. Introduction

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

2. Motivation

error_code has a rudimentary ostream inserter. For example:

std::error_code ec;
auto size = std::filesystem::file_size("nonexistent", ec);
std::cout << ec;

This works and prints generic:2.

However, the following code doesn’t compile:

std::print("{}\n", ec);

Unfortunately, the existing inserter has several issues, such as I/O manipulators applying only to the category name rather than the entire error code, resulting in confusing output:

std::cout << std::left << std::setw(12) << ec;

This prints:

generic     :2

Additionally, it doesn’t allow formatting the error message and introduces potential encoding issues, as the encoding of the category name is unspecified.

3. Proposal

This paper proposes adding a formatter specialization for std::error_code to address the problems discussed in the previous section.

The default format will produce the same output as the ostream inserter:

std::print("{}\n", ec);

Output:

generic:2

It will correctly handle width and alignment:

std::print("[{:>12}]\n", ec);

Output:

[   generic:2]

Additionally, it will allow formatting the error message:

std::print("{:s}\n", ec);

Output:

No such file or directory

(The actual message depends on the platform.)

The main challenge lies in the standard’s lack of specification for the encodings of strings returned by error_category::name and error_code::message / error_category::message (syserr.errcat.virtuals):

virtual const char* name() const noexcept = 0;

Returns: A string naming the error category.

virtual string message(int ev) const = 0;

Returns: A string that describes the error condition denoted by ev.

In practice, implementations typically define category names as string literals, meaning they are in the ordinary literal encoding.

However, there is significant divergence in message encodings. libc++ and libstdc++ use strerror[_r] for the generic category which is in the C (not "C") locale encoding but disagree on the encoding for the system category: libstdc++ uses the Active Code Page (ACP) while libc++ again uses strerror / C locale on Windows. Microsoft STL uses a table of string literals in the ordinary literal encoding for the generic category and ACP for the system category.

The following table summarizes the differences:

Obviously none of this is usable in a portable way through the generic error_category API because encodings can be and often are different.

To address this, the proposal suggests using the C locale encoding (execution character set), which is already employed in most cases and aligns with underlying system APIs. Microsoft STL’s implementation has a number of bugs in std::system_category::message ([MSSTL-3254], [MSSTL-4711]) and will likely need to change anyway. This also resolves [LWG4156].

An alternative approach could involve communicating the encoding from error_category. However, this introduces ABI challenges and complicates usage compared to adopting a single encoding.

4. Wording

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

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

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

template<class charT> struct formatter<error_code, charT> {
  constexpr typename basic_format_parse_context<charT>::iterator
    parse(basic_format_parse_context<charT>& ctx);

  template<class FormatContext>
    typename FormatContext::iterator
      format(const error_code& ec, FormatContext& ctx) const;
};

constexpr typename basic_format_parse_context<charT>::iterator
  parse(basic_format_parse_context<charT>& ctx);

Effects: Parses the format specifier as a error-code-format-spec and stores the parsed specifiers in *this.

error-code-format-spec:
  fill-and-align_opt width_opt s_opt

where the productions fill-and-align and width are described in [format.string].

Returns: An iterator past the end of the error-code-format-spec.

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

Effects: Let s be ec.message() transcoded into the ordinary literal encoding, with maximal subparts of ill-formed subsequences substituted with U+FFFD REPLACEMENT CHARACTER per the Unicode Standard, Chapter 3.9 U+FFFD Substitution in Conversion, if the s option is used, otherwise format("{}:{}", ec.category().name(), ec.value()). Writes s into ctx.out(), adjusted according to the error-code-format-spec.

Returns: An iterator past the end of the output range.

Modify [syserr.errcat.virtuals]:

...

virtual string message(int ev) const = 0;

Returns: A string of multibyte characters in the executon character set that describes the error condition denoted by ev.

5. Implementation

The proposed formatter for std::error_code has been implemented in the open-source {fmt} library ([FMT]).