Re: c++/3230: False warning for 'returning reference to temporary'

Re: c++/3230: False warning for 'returning reference to temporary'

[jason]

Synopsis: False warning for 'returning reference to temporary'

Responsible-Changed-From-To: gdr->jason
Responsible-Changed-By: jason
Responsible-Changed-When: Mon Mar 18 06:02:19 2002
Responsible-Changed-Why:
    mine
State-Changed-From-To: analyzed->closed
State-Changed-By: jason
State-Changed-When: Mon Mar 18 06:02:19 2002
State-Changed-Why:
    This is not a bug in the compiler.  The standard (5.16) says that the result of your ?: expression is an rvalue, because the two options do not have the same type.  As a result, the return value binds to that rvalue, which is a local temporary.  The EDG compiler gives the same warning.
    
    You may feel that this is a bug in the standard, and I'd be inclined to agree.  But until the standard is fixed, the compiler will continue to work this way.  If you'd like to submit a defect report against the standard, the place for that is comp.std.c++.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=3230

Re: c++/3230: False warning for 'returning reference to temporary'

[Gabriel Dos Reis]

The following reply was made to PR c++/3230; it has been noted by GNATS.

From: Gabriel Dos Reis <gdr@codesourcery.com>
To: jason@gcc.gnu.org
Cc: andrewp@andypo.net, gcc-bugs@gcc.gnu.org, gdr@gcc.gnu.org,
   gcc-gnats@gcc.gnu.org
Subject: Re: c++/3230: False warning for 'returning reference to temporary'
Date: 21 Mar 2002 09:17:51 +0100

 jason@gcc.gnu.org writes:
 
 | Synopsis: False warning for 'returning reference to temporary'
 | 
 | Responsible-Changed-From-To: gdr->jason
 | Responsible-Changed-By: jason
 | Responsible-Changed-When: Mon Mar 18 06:02:19 2002
 | Responsible-Changed-Why:
 |     mine
 | State-Changed-From-To: analyzed->closed
 | State-Changed-By: jason
 | State-Changed-When: Mon Mar 18 06:02:19 2002
 | State-Changed-Why:
 |     This is not a bug in the compiler.  The standard (5.16) says that the result of your ?: expression is an rvalue, because the two options do not have the same type.  As a result, the return value binds to that rvalue, which is a local temporary.  The EDG compiler gives the same warning.
 
 OK.
 
 |     You may feel that this is a bug in the standard, and I'd be
 | inclined to agree.  But until the standard is fixed, the compiler will
 | continue to work this way.
 
 I'll raise the issue on the core group refelector.
 
 -- Gaby

Re: c++/3230: False warning for 'returning reference to temporary'

[gdr]

Synopsis: False warning for 'returning reference to temporary'

Responsible-Changed-From-To: unassigned->gdr
Responsible-Changed-By: gdr
Responsible-Changed-When: Sat Aug 11 13:19:49 2001
Responsible-Changed-Why:
    Analyzed.
State-Changed-From-To: open->analyzed
State-Changed-By: gdr
State-Changed-When: Sat Aug 11 13:19:49 2001
State-Changed-Why:
    Confirmed as a bug. A regression over 2.95.2

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view&pr=3230&database=gcc

c++/3230: False warning for 'returning reference to temporary'

[andrewp]

>Number:         3230
>Category:       c++
>Synopsis:       False warning for 'returning reference to temporary'
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    unassigned
>State:          open
>Class:          rejects-legal
>Submitter-Id:   net
>Arrival-Date:   Mon Jun 18 09:26:06 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Andrew Pollard
>Release:        3.0 20010616 (prerelease)
>Organization:
>Environment:
System: Linux odie.andypo.net 2.2.19-7.0.1smp #1 SMP Tue Apr 10 01:46:39 EDT 2001 i686 unknown
Architecture: i686

	
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: ../gcc-3_0-branch/configure --prefix=/usr/local/gcc-3.0-i686-pc-linux-gnu --with-gnu-as --with-gnu-ld --enable-threads=posix --enable-version-specific-runtime-libs --enable-languages=c++,java --with-dwarf2 --enable-long-long=yes --enable-shared
>Description:
Gcc-3.0 generates an incorrect warning about returning a reference to a temporary when a ?: expression is used directly in a return statement. If another temporary is used, no warning is given.
>How-To-Repeat:
a.cc:
------------------------------------------------------------------------------
const int* bar();

const int&
foo1()
{
        static int empty;
        const int* x = bar();
        return (x ? *x : empty);
}

const int&
foo2()
{
        static int empty;
        const int* x = bar();
        const int& r = (x ? *x : empty);
        return (r);
}
------------------------------------------------------------------------------
% /usr/local/gcc-3.0-i686-pc-linux-gnu/bin/g++ -c a.cc
a.cc: In function `const int& foo1()':
a.cc:8: warning: returning reference to temporary

Whereas foo2() compiles successfully.

gcc-2.95.3 and egcs-1.1.2 do not give this warning.
>Fix:
Workaround simple, just use another temporary to store the returned result and return that instead

	const int& r = (x ? *x : empty);
	return (r);

instead of

	return (x ? *x : empty);
>Release-Note:
>Audit-Trail:
>Unformatted: