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Difference in hexadecimal floating-point (a) format between fmt and std isn't documented #4657
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while (print_xdigits > 0 && xdigits[print_xdigits] == '0') --print_xdigits; buf.push_back('0'); buf.push_back(specs.upper() ? 'X' : 'x'); buf.push_back(xdigits[0]);it's a design choice most probably
Good catch, thanks! Let me investigate why
std::to_charsdoesn't print0xlikeprintf.Happy to help
- changed the title
[-]Difference in hexadecimal floating-point (`a`) format bewteen `fmt` and `std` isn't documented[/-][+]Difference in hexadecimal floating-point (`a`) format between `fmt` and `std` isn't documented[/+]on Jan 22, 2026 I think it's an unintentional change in
std::formatcaused by switching tostd::to_charsin one of the revisions of P0645. The reference implementation ofstd::formatalways printed0xlike pretty much every other facility and I missed the fact thatstd::to_charsdoes something different. The<charconv>paper doesn't mention it so it's unclear why it went with this "innovation". @statementreply, could you by any chance submit an LWG issue?Reacted by statementreplyLWG issue: https://cplusplus.github.io/LWG/issue4515
https://godbolt.org/z/TaK4s5YTM
When formatting floating-point numbers to hexadecimal (
a) format,fmtincludes the0xprefix (similar toprintf%a), butstddoesn't (specified in terms ofto_chars).Perhapsstdshould be fixed.