P0682R0: Repairing elementary string conversions

Introduction

This paper proposes a few repairs for the elementary string conversions introduced into C++17 via P0067R5 "Elementary string conversions, revision 5". In particular, the following issues were raised in e-mail discussions:

• Move the functions into a separate header; do not cram them into <utility> (suggestion by Zhihao Yuan: <textproc>)
• Depending on std::error_code may cause an indirect dependency on std::string; use std::errc instead (LWG 2955).
• Clarify the behavior when parsing "-" as a signed number. The outcome should be errc::invalid_argument.
• Rounding behavior for floating-point to_chars / from_chars (Edward Catmur via std-discussion)

Discussion

Separate header

• <string> — but to_chars / from_chars don't use std::string
• <cstring> — but adding C++ names to headers adapted from C is probably unwise
• <cstdlib> — but adding C++ names to headers adapted from C is probably unwise

• <textproc> (Zhihao Yuan) — but there is not a lot of text processed
• <textconv> — but might be confused with encoding conversions
• <charconv> — but more than one character is converted
• <stringconv> — but might be confused with encoding conversions
• <fromto_chars> — a bit long and artificial
• <chars>

Dependency on std::error_code (LWG 2955)

This dependency brings in functions returning a std::string, which is at a higher level than the functions to_chars and from_chars. For a low-level facility which might also be made available on freestanding environments, such a dependency seems to violate boundaries between abstraction levels.

The recommendation is to directly use the std::errc enumeration (22.5.1 [system_error.syn]), However, the usage pattern of the functions deteriorates to something like:

  if (auto [ptr, ec] = to_chars(p, last, 42); ec != std::errc()) {
    // failure case
  }

Clarify the behavior when parsing "-" as a signed number

Rounding behavior for floating-point to_chars / from_chars

"For example 0x1.0000000000001p0 is approx. 1.000000000000000222045, so is 1.0000000000000003 an acceptable output from to_chars, or only 1.0000000000000002?" The suggested resolution is to prefer the output that has the smallest difference to the original floating-point value.

Should the current rounding mode be considered in from_chars? For example, 1e23 might be read as 0x1.52d02c7e14af6p76 or as 0x1.52d02c7e14af7p76. If the current rounding mode is, in fact, considered, the follow-on question is whether the round-trip guarantee only applies when both to_chars and from_chars operate under the same rounding mode, or regardless of rounding mode. For applications such as interval arithmetic, it seems worthwhile if from_chars does consider the rounding mode, but to_chars should certainly not be forced to output a long digit string for 1e23 to disambiguate.

Wording changes

  struct to_chars_result {
    char* ptr;
    error_codeerrc ec;
  };
[...]
  struct from_chars_result {
    const char* ptr;
    error_codeerrc ec;
  };
  

... If the member ec of the return value is such that the value, when converted to bool, is false is equal to the value of a value-initialized errc, the conversion was successful and the member ptr is the one-past-the-end pointer of the characters written. ..

The functions that take a floating-point value but not a precision parameter ensure that the string representation consists of the smallest number of characters such that there is at least one digit before the radix point (if present) and parsing the representation using the corresponding from_chars function using the same rounding mode (29.4 [cfenv]) recovers value exactly. If there are several such representations, the representation with the smallest difference to the floating-point argument value is chosen, resolving any remaining ties using the current rounding mode. [ Note: ... ]

All functions named from_chars analyze the string [first, last) for a pattern, where [first, last) is required to be a valid range. If no characters match the pattern, value is unmodified, the member ptr of the return value is first and the member ec is equal to errc::invalid_argument. [ Note: If the pattern allows for an optional sign, but the string has no digit characters following the sign, no characters match the pattern. ] ... Otherwise, value is set to the parsed value, after rounding according to the current rounding mode (29.4 [cfenv]), and the member ec is set such that the conversion to bool yields false value-initialized.