From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id CD4A63858D20; Tue,  5 Mar 2024 17:23:49 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org CD4A63858D20
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1709659429;
	bh=rbKMydew8UVHR+VlI/woWdxZFNmG39iojZM+di/27qc=;
	h=From:To:Subject:Date:From;
	b=R5q1tfy+ct2ztINiv5wc79Y5NiH54r/Daf4f1xi6DZzCY2i4ZS4w5boVoyRWmUKSt
	 6XYZ54w61wyHAxSd75ypCiAS0dQruCG7gLLRQrSBRTJBubft+NAgMM1zFChp3Z+fsq
	 485PfouFoqSvV5Oku6zrLchAXO+TCPSzpuuX2VjM=
From: "howard.hinnant at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/114244] New: Need to use round when parsing
 fractional seconds
Date: Tue, 05 Mar 2024 17:23:49 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: howard.hinnant at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-114244-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114244

            Bug ID: 114244
           Summary: Need to use round when parsing fractional seconds
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: howard.hinnant at gmail dot com
  Target Milestone: ---

I'm not sure if this is a bug or just QOI.  But consider:

#include <chrono>
#include <string>
#include <iostream>


using time_point_t =3D std::chrono::sys_time<std::chrono::milliseconds>;

time_point_t decrypt(std::string s) {
    time_point_t tp;
    std::istringstream in(s);
    in >> parse("%Y%m%d-%T", tp);
    return tp;
}

int main() {
    std::cout << decrypt("20240304-13:00:00.002");
}

I expect the output to be:

2024-03-04 13:00:00.002

but it is:

2024-03-04 13:00:00.001

I believe the issue is the use of duration_cast<milliseconds> or
time_point_cast<milliseconds> somewhere under the parse (not sure exactly
where).  Since 0.002 is not exactly representable in floating point, we're
getting a truncate towards zero because this parses as just barely under 0.=
002.

I highly recommend the use of round<Duration> when converting floating point
based durations and time_points to integral based.=