P3908R0: constexpr from_chars<float> / to_chars<float>

This paper makes float-to-chars and chars-to-float conversion functionality in <charconv> constexpr compatible.

Motivation

This is the missing piece of <charconv>. Its absence forces users to write their own conversions and that's against the purpose of standardization. By allowing it, we will be able to reuse this code in constexpr formatting using std::format too.

Implementation

The implementation of the conversion algorithms can be done constexpr compatible with current language without resorting to any magic. Implementation in libc++ needs to just move the code from .cpp files to headers. Same thing applies for libstdc++.

Modern and fast algorithms

fast_float

It's a library done by Daniel Lemire which implements proposed functionality fully in constexpr, and it has over 1.9 stars on Github, it's used in production code.

Zmij

Recently a modern approach named Zmij was discovered by Victor Zverovich, it's only ~1kLoC. Modifying it to be constexpr showed there is no performance difference. It uses only integer operations and std::bit_cast.

In terms of compile time performance of just including this code, the additional time to including this header is negligable.

Compilation throughtput concerns

If the complation performance is still a concern, especially for a big tables, this can be improved to delay the instantiation by making it a template which is instantiated when first used (including the constant evaluation of its values). This can be done by template <typename = void> trick, which lazily instantiate the table only if it's needed. This is used in fast_float library already.

Alternative approach

Alternative approach is to provide builtins doing the same operations. They can be implemented with charconv code itself. Or compiler can detect and bypass from_chars / to_chars instead of using explicit builtins.

Wording

The change is simple adding (as usual) a few of constexpr here and there.

28.2.1 Header <charconv> synopsis [charconv.syn]

When a function is specified with a type placeholder of integer-type, the implementation provides overloads for char and all cv-unqualified signed and unsigned integer types in lieu of integer-type.

When a function is specified with a type placeholder of floating-point-type, the implementation provides overloads for all cv-unqualified floating-point types ([basic.fundamental]) in lieu of floating-point-type.

🔗

namespace std { // floating-point format for primitive numerical conversion enum class chars_format { scientific = unspecified, fixed = unspecified, hex = unspecified, general = fixed | scientific }; // [charconv.to.chars], primitive numerical output conversion struct to_chars_result { // freestanding char* ptr; errc ec; friend bool operator==(const to_chars_result&, const to_chars_result&) = default; constexpr explicit operator bool() const noexcept { return ec == errc{}; } }; constexpr to_chars_result to_chars(char* first, char* last, // freestanding integer-type value, int base = 10); to_chars_result to_chars(char* first, char* last, // freestanding bool value, int base = 10) = delete; constexpr to_chars_result to_chars(char* first, char* last, // freestanding-deleted floating-point-type value); constexpr to_chars_result to_chars(char* first, char* last, // freestanding-deleted floating-point-type value, chars_format fmt); constexpr to_chars_result to_chars(char* first, char* last, // freestanding-deleted floating-point-type value, chars_format fmt, int precision); // [charconv.from.chars], primitive numerical input conversion struct from_chars_result { // freestanding const char* ptr; errc ec; friend bool operator==(const from_chars_result&, const from_chars_result&) = default; constexpr explicit operator bool() const noexcept { return ec == errc{}; } }; constexpr from_chars_result from_chars(const char* first, const char* last, // freestanding integer-type& value, int base = 10); constexpr from_chars_result from_chars(const char* first, const char* last, // freestanding-deleted floating-point-type& value, chars_format fmt = chars_format::general); }

The type chars_format is a bitmask type ([bitmask.types]) with elements scientific, fixed, and hex.

The types to_chars_result and from_chars_result have the data members and special members specified above.

They have no base classes or members other than those specified.

28.2.2 Primitive numeric output conversion [charconv.to.chars]

All functions named to_chars convert value into a character string by successively filling the range [first, last), where [first, last) is required to be a valid range.

If the member ec of the return value is such that the value 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.

Otherwise, the member ec has the value errc::value_too_large, the member ptr has the value last, and the contents of the range [first, last) are unspecified.

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 recovers value exactly.

[Note 1:

This guarantee applies only if to_chars and from_chars are executed on the same implementation.

— end note]

If there are several such representations, the representation with the smallest difference from the floating-point argument value is chosen, resolving any remaining ties using rounding according to round_to_nearest ([round.style]).

The functions taking a chars_format parameter determine the conversion specifier for printf as follows: The conversion specifier is f if fmt is chars_format::fixed, e if fmt is chars_format::scientific, a (without leading "0x" in the result) if fmt is chars_format::hex, and g if fmt is chars_format::general.

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constexpr to_chars_result to_chars(char* first, char* last, integer-type value, int base = 10);

Preconditions: base has a value between 2 and 36 (inclusive).

Effects: The value of value is converted to a string of digits in the given base (with no redundant leading zeroes).

Digits in the range 10..35 (inclusive) are represented as lowercase characters a..z.

If value is less than zero, the representation starts with '-'.

Throws: Nothing.

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constexpr to_chars_result to_chars(char* first, char* last, floating-point-type value);

Effects: value is converted to a string in the style of printf in the "C" locale.

The conversion specifier is f or e, chosen according to the requirement for a shortest representation (see above); a tie is resolved in favor of f.

Throws: Nothing.

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constexpr to_chars_result to_chars(char* first, char* last, floating-point-type value, chars_format fmt);

Preconditions: fmt has the value of one of the enumerators of chars_format.

Effects: value is converted to a string in the style of printf in the "C" locale.

Throws: Nothing.

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constexpr to_chars_result to_chars(char* first, char* last, floating-point-type value,
                         chars_format fmt, int precision);

Preconditions: fmt has the value of one of the enumerators of chars_format.

Effects: value is converted to a string in the style of printf in the "C" locale with the given precision.

Throws: Nothing.

28.2.3 Primitive numeric input conversion [charconv.from.chars]

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 1:

If the pattern allows for an optional sign, but the string has no digit characters following the sign, no characters match the pattern.

— end note]

Otherwise, the characters matching the pattern are interpreted as a representation of a value of the type of value.

The member ptr of the return value points to the first character not matching the pattern, or has the value last if all characters match.

If the parsed value is not in the range representable by the type of value, value is unmodified and the member ec of the return value is equal to errc::result_out_of_range.

Otherwise, value is set to the parsed value, after rounding according to round_to_nearest ([round.style]), and the member ec is value-initialized.

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constexpr from_chars_result from_chars(const char* first, const char* last,
                                       integer-type& value, int base = 10);

Preconditions: base has a value between 2 and 36 (inclusive).

Effects: The pattern is the expected form of the subject sequence in the "C" locale for the given nonzero base, as described for strtol, except that no "0x" or "0X" prefix shall appear if the value of base is 16, and except that '-' is the only sign that may appear, and only if value has a signed type.

Throws: Nothing.

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constexpr from_chars_result from_chars(const char* first, const char* last, floating-point-type& value,
                             chars_format fmt = chars_format::general);

Preconditions: fmt has the value of one of the enumerators of chars_format.

Effects: The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except that

(6.1)
the sign '+' may only appear in the exponent part;
(6.2)
if fmt has chars_format::scientific set but not chars_format::fixed, the otherwise optional exponent part shall appear;
(6.3)
if fmt has chars_format::fixed set but not chars_format::scientific, the optional exponent part shall not appear; and
(6.4)
if fmt is chars_format::hex, the prefix "0x" or "0X" is assumed.
[Example 1:
The string 0x123 is parsed to have the value 0 with remaining characters x123.
— end example]

In any case, the resulting value is one of at most two floating-point values closest to the value of the string matching the pattern.

Throws: Nothing.

See also: ISO/IEC 9899:2024, 7.24.2.6, 7.24.2.8

Feature test macro

17.3.2 Header <version> synopsis [version.syn]

#define __cpp_lib_constexpr_charconv 202207L20????L          // // freestanding, also in <charconv>