feature?: ignored characters `'_ / allowing prefixs 0x and 0b etc...

[Andersama]

This isn't a performance related feature, but I've found it odd that the charconv spec goes out of its way to avoid parsing what would otherwise be necessary to handle c++'s own floating point literals. What would you think about having some additional utility from_chars like functions to handle those formats?

Ages ago I modified microsoft's charconv to do this for myself, but it broke at some point because headers changed and some utility functions went missing. The core idea is roughly this:

if (fmt is prefixed) {
  //bump pointer forward past the prefix for the format
}

//continue parsing like charconv would except skip over specified characters
uint64_t mantissa = 0;
for (;begin != end; ++begin) {
if ((*begin == some_template_pack) || ...)
  continue;
const unsigned char digit = char_table[*begin];
if (digit >= base)
  break;
//so on...
}

Not used your library, but it seems like hex formatted floats have the enum for the format but aren't supported? Could also be the opportunity to add that in.

[lemire]

@Andersama We want the from_chars function to abide by the standard as much as possible. It should be close to a "drop-in" replacement for whatever the standard library provides.

However, we also have the from_chars_advanced function which is an extension. There is no limit to what we could do there.

Support for other data types and formats is certainly welcome. Please consider providing a pull request, or many pull requests.

[Andersama]

It looks like parse_number_string is the underlying function that from_char_advanced is using. Were you expecting the drop in to handle hex floats? Currently it seems like it doesn't, and it seems like I'm about to add a fairly large if / else block otherwise with an extra flag integer to parse_options. Pretty sure that'll be a bit of a performance hit.

[lemire]

We have decent benchmarks so we can assess performance issues. I would not necessarily be overly concerned at first.