[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[pbristow at hetp dot u-net.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122

Paul A. Bristow <pbristow at hetp dot u-net.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pbristow at hetp dot u-net.com

--- Comment #18 from Paul A. Bristow <pbristow at hetp dot u-net.com> ---
Working on test for Boost.Multiprecision UDTs, I have just stumbled on this,
expecting loopback to 'just work' as it does on Clang libc++ and MSVC.

LWG 2381 is still (currently open) and seems to have stalled?

Is there any way to get this resolved for GCC libstd++?

[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[maxim.yegorushkin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122

--- Comment #19 from Maxim Egorushkin <maxim.yegorushkin at gmail dot com> ---
(In reply to Jonathan Wakely from comment #17)

> At this time LWG 2381 is still open. Until the proposed fix (or some other
> fix) is resolved by making a change to the draft standard, I do not want to
> change libstdc++.

May be enable hexfloat round-trip if a user defines a macro?

The standards often follow existing practice, waiting on the standard may lead
to a deadlock.

[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122

--- Comment #20 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'll ask the new LWG chair to bump the issue.

[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[pbristow at hetp dot u-net.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122

--- Comment #21 from Paul A. Bristow <pbristow at hetp dot u-net.com> ---
I also note that 

 https://en.cppreference.com/w/cpp/io/manip/fixed
    Notes
    Hexadecimal floating-point formatting ignores the stream precision
specification,
    as required by the specification of std::num_put::do_put.
  https://en.cppreference.com/w/cpp/locale/num_put/put
    Additionally, if floatfield != (ios_base::fixed | ios_base::scientific),
    then (since C++11) precision modifier is added, set to str.precision()

"When formatting a floating point value as hexfloat (i.e., when floatfield ==
(std::ios_base::fixed | std::ios_base::scientific)), the stream's precision is
not used; instead, the number is always printed with enough precision to
exactly represent the value."     (since C++11)

but this is not what MSVC currently 16.7.0.Preview 3 is implementing as noted

https://developercommunity.visualstudio.com/content/problem/520472/hexfloat-stream-output-does-not-ignore-precision-a.html

*It would be extremely unfortunate if various compilers implemented this
differently.*

I see no case for precision to change the output from std::hexfloat because it
specifies a *decimal* precision, not a hexadecimal one.  std::hexfloat output
should always be enough precision to *exactly represent the value*, however
large.

It would be nice if std::showpoint could also respect output of trailing zeros,
so that the default std::showpoint  output displays neat columns, and that
std::noshowpoint made output with minimum size (for example, minimizing
serialized data).

(In passing, I note that std::scientific always includes (often many) trailing
zeros, ignoring showpoint.  Changing this would be logical but a breaking
change.)

[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[dvirtz at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122

Dvir Yitzchaki <dvirtz at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |dvirtz at gmail dot com

--- Comment #22 from Dvir Yitzchaki <dvirtz at gmail dot com> ---
any update on this now that CWG 2381 has been approved?

source: https://wg21.link/lwg2381

[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|SUSPENDED                   |NEW

[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
   Target Milestone|---                         |13.0
             Status|NEW                         |ASSIGNED

[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|13.0                        |13.2

--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 13.1 is being released, retargeting bugs to GCC 13.2.

[Bug libstdc++/81122] [DR 2381] parsing f stopped after '0' when reading std::hexfloat >> f;

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81122

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|13.2                        |13.3

--- Comment #24 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 13.2 is being released, retargeting bugs to GCC 13.3.