Improve number-to-string conversion

[nlohmann]

The dump_integer function is naive custom code and the dump_float function uses a Grisu2 implementation. This leaves a lot of room for improvements.

In C++17, we could use <charconv>'s std::to_chars as pointed out by u/STL and live happily ever after. Unfortunately, we are still sticking around with C++11.

Examples for code to try are:

• https://gist.github.com/Veedrac/069dd35f896de4df4e14b881a455ca47 (CC0, see also https://www.reddit.com/r/programming/comments/7ljzty/how_to_print_integers_really_fast_with_source_code/dro1umo)
• https://github.com/ulfjack/ryu (Boost / Apache2)
• https://pvk.ca/Blog/2017/12/22/appnexus-common-framework-its-out-also-how-to-print-integers-faster/ (Apache2)

Maybe there are other projects, too.

It would be great to try different implementations. The largest piece of work should usually be to make it run as single header, and to adjust it to the interface.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nlohmann]

#1411 tackled the integer part.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[victor-varjo]

In C++17, we could use 's std::to_chars as pointed out by u/STL and live happily ever after.

I wish it was so, but even the __cpp_lib_to_chars feature test macro doesn't guarantee that to_chars works with floats. If you ever decide to implement it with some sort of feature probing or gating here is a test case for you:
0x1.e69a800000000p+0
Grisu2 -> "1.9007949829101563"
std::to_chars -> "1.9007949829101562"