P0067R5: Elementary string conversions

[cjdb]

Link: https://wg21.link/p0067r5

BEGIN-RST-NOTES
For integer types, ``std::(to|from)_chars`` has been available since v7; for ``float`` and ``double``, ``std::to_chars`` since v14 and ``std::from_chars`` since v20. Support is complete except for ``long double``.
END-RST-NOTES

[llvmbot]

@llvm/issue-subscribers-c-17

Author: Christopher Di Bella (cjdb)

[h-vetinari]

I'm glad there are finally issues for these topics; that makes it much easier to collect publicly available information about them, rather have them scattered in the heads of core libcxx contributors.

• [libc++] std::to_chars precision loss for 80-bit long double #56261
• Unexpected error when using libc++ (call to deleted function 'from_chars') #59374
• Floating point std::to_chars has a pretty large code size impact #64180
• Any plan for libc++ to support std::from_chars for floating points? #92711

Also some context I had kept from long ago

@h-vetinari: Stumbled over this while looking for the status of P0067R5; took me a while to unearth some old comments on this topic in https://reviews.llvm.org/D70631:

@h-vetinari: I was under the impression that 87c0160 did most of the work for from_chars?

@mordante: Thanks for both links I wasn't aware of this effort!
At the moment I'm too busy with <format> to look at from_chars, but this is useful when somebody starts working on from_chars. (I might look at it when I finished the larger projects I'm working on.)

[lemire]

The from_chars issue could be largely solved with https://github.com/fastfloat/fast_float

It is what GCC/glibc++ does.

I also left a comment in #92711

I'd be happy to help.

[h-vetinari]

The implementation of the remaining bits landed in #91651, which completes this paper except for long-double support (needs #113237). Feature test macro wasn't set yet.

[frederick-vs-ja]

Looks like that we're now very close to setting FTM on long double-eq-double platforms.