Proposal: Improve std::tm formatter for libstdc++

[phprus]

libstdc++ has a non standard facet std::__timepunct. It is used in std::time_put:
https://github.com/gcc-mirror/gcc/blob/2a62b047eca5859142f024c339cd2686635cf5b1/libstdc%2B%2B-v3/include/bits/locale_facets_nonio.tcc#L1175-L1213

When using libstdc++ we can replace the std::time_put facet with std::__timepunct,
which will can format tm into the preallocated memory buffer. This will avoid the std::basic_ostringstream overhead.

std::__timepunct is a part of libstdc++ ABI.

What do you think about this replacement?

[vitaut]

Thanks for the suggestion but I don't think it's worth using non-standard APIs for this specific case. There might be other ways to optimize locale-specific formatting such as providing a custom stream.