Bug#187599: marked as done (Cannot subtract from streampos)
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and subject line Not a bug: Cannot subtract from streampos
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From: Bart Samwel <bsamwel@liacs.nl>
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Subject: Cannot subtract from streampos
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Package: g++
Version: 3.2.3-20030331
If I have the following code:
#include <sstream>
int foo()
{
std::streampos pos;
pos - 1;
}
When I compile this with "g++ -Wall test.cc", this gives me:
test.cc: In function `int foo()':
test.cc:6: ISO C++ says that `std::fpos<_StateT>
std::fpos<_StateT>::operator-(long int) [with _StateT = mbstate_t]'
and `
operator-' are ambiguous even though the worst conversion for the
former is
better than the worst conversion for the latter
I think this means that there are both a global operator- and a member
operator- defined for std::fpos<_StateT>. I can work around this (I can
do pos -= 1 and afterwards pos += 1), but I'd much rather be able to
simply subtract from the streampos.
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Received: (at 187599-done) by bugs.debian.org; 16 Nov 2003 20:17:31 +0000
>From doko@cs.tu-berlin.de Sun Nov 16 14:17:30 2003
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From: Matthias Klose <doko@cs.tu-berlin.de>
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Message-ID: <16311.55804.590960.837521@gargle.gargle.HOWL>
Date: Sun, 16 Nov 2003 21:11:40 +0100
To: 187599-done@bugs.debian.org
CC: Phil Edwards <pme@gcc.gnu.org>, martin@v.loewis.de
Subject: Not a bug: Cannot subtract from streampos
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[ok, re-found this (german) message in my mailbox, trying to translate
it ... and closing the report]
g++-3.4 reports:
bug-187599.cc: In function `int foo()':
bug-187599.cc:6: error: ambiguous overload for 'operator-' in 'pos - 1'=
/usr/include/c++/3.4/bits/postypes.h:236: note: candidates are: std::fp=
os<_StateT> std::fpos<_StateT>::operator-(const std::streamoff&) const =
[with _StateT =3D mbstate_t]
/usr/include/c++/3.4/bits/postypes.h:248: note: std::st=
reamoff std::fpos<_StateT>::operator-(const std::fpos<_StateT>&) const =
[with _StateT =3D mbstate_t]
There exists a conversion from streampos to streamoff, so that the
expression can be interpreted as
=09(streamoff)pos - 1
or
=09pos.operator-(1)
In both cases conversions are used (from streampos to streamoff, and
from int to long). The ambiguouity can be resolved be writing
=09pos - 1L
Martin wrote:
> Kein Bug (glaube ich). Ich glaube (ohne es noch mal kontrolliert
> zu haben), dass ISO C++ diese Operation tats=E4chlich als mehrdeutig=20=
> ablehnt. Der Submitter hat =FCbersehen, dass es eine Konvertierung
> von streampos nach streamoff gibt, so dass man das ganze als
>=20
> (streamoff)pos - 1
>=20
> (streamoff ist ein primitiver Typ) oder als
>=20
> pos.operator-(1)
>=20
> verstehen kann. In beiden F=E4llen sind Konvertierungen im Spiel:
> Einmal von streampos nach streamoff, beim anderen von int nach
> long. Der Submitter wird die Mehrdeutigkeit los, indem er schreibt
>=20
> pos - 1L
>=20
> Er sollte ber=FCcksichtigen, dass das Ergebnis jeweils ein anderes
> ist: Mal ein streampos, und mal ein streamoff.
>=20
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