Bug#478734: g++-4.2: refuses to compile valid C++ syntax
On Thu, May 01, 2008 at 10:39:24AM -0500, Jason Kraftcheck wrote: Daniel Jacobowitz wrote: On Wed, Apr 30, 2008 at 05:51:32PM -0500, Jason Kraftcheck wrote: Why can't I take a reference to an rvalue? Because you can't modify rvalues. This is the definition of the C++ language. The next major revision of C++ will have T rref, the two ampersands declaring an rvalue reference instead of the normal lvalue kind. Are you certain that the temporary created by invoking the copy constructor is an rvalue? If so, then why does the following syntax work? std::vectorint v; std::vectorint w( std:vectorint(v) ); The copy constructor also takes a reference. The only difference between constructing 'w' and calling 'swap' on it is that the former takes a const reference. You are permitted to bind rvalues to const references, even when it requires creation of a temporary. This is 8.5.3 [dcl.init.ref] in the C++ standard. -- Daniel Jacobowitz CodeSourcery -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Bug#478734: g++-4.2: refuses to compile valid C++ syntax
The same syntax is accepted by the only other C++ compiler I have access too: Sun's. I guess what I don't understand is why, if I create a temporary by explicitly calling the copy constructor, that temporary is treated as an rvalue. So how do you want to proceed? If you're certain that you're correct then I have no objection to you closing the bug report. Please do so with my apologies for being a nuisance, as I'm certainly not sure that I'm correct. Or if you prefer, I could post the question to comp.lang.c++. thanks, - jason -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Bug#478734: g++-4.2: refuses to compile valid C++ syntax
* Jason Kraftcheck: The same syntax is accepted by the only other C++ compiler I have access too: Sun's. I guess what I don't understand is why, if I create a temporary by explicitly calling the copy constructor, that temporary is treated as an rvalue. The standard says so in section 3.10 (Lvalues and rvalues): | 6 An expression which holds a temporary object resulting from a cast to | a nonreference type is an rvalue (this includes the explicit creation | of an object using functional notation (5.2.3)). The type you're casting to is not a reference type, so it's an rvalue according to section 5.4 (Explicit type conversion (cast notation)). -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Bug#478734: g++-4.2: refuses to compile valid C++ syntax
Package: g++-4.2
Version: 4.2.3-4
Severity: grave
Justification: renders package unusable
g++ refuses to compile the following syntax commonly used to force a
std::vector to release allocated memory:
#include vector
int main()
{
std::vectorint v;
v.clear();
v.swap( std::vectorint(v) );
return 0;
}
I emmits the following error message:
bug.cc: In function 'int main()':
bug.cc:6: error: no matching function for call to 'std::vectorint,
std::allocatorint ::swap(std::vectorint, std::allocatorint )'
/usr/include/c++/4.2/bits/stl_vector.h:728: note: candidates are: void
std::vector_Tp, _Alloc::swap(std::vector_Tp, _Alloc) [with _Tp =
int, _Alloc = std::allocatorint]
-- System Information:
Debian Release: 4.0
APT prefers stable
APT policy: (990, 'stable'), (500, 'unstable'), (500, 'testing')
Architecture: amd64 (x86_64)
Shell: /bin/sh linked to /bin/bash
Kernel: Linux 2.6.18-6-amd64
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Versions of packages g++-4.2 depends on:
ii gcc-4.2 4.2.3-4The GNU C compiler
ii gcc-4.2-base 4.2.3-4The GNU Compiler Collection (base
ii libc6 2.7-10 GNU C Library: Shared libraries
ii libstdc++6-4.2-dev4.2.3-4The GNU Standard C++ Library v3 (d
g++-4.2 recommends no packages.
-- no debconf information
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Processed: Re: Bug#478734: g++-4.2: refuses to compile valid C++ syntax
Processing commands for [EMAIL PROTECTED]: severity 478734 normal Bug#478734: g++-4.2: refuses to compile valid C++ syntax Severity set to `normal' from `grave' thanks Stopping processing here. Please contact me if you need assistance. Debian bug tracking system administrator (administrator, Debian Bugs database) -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Bug#478734: g++-4.2: refuses to compile valid C++ syntax
severity 478734 normal thanks On Wed, Apr 30, 2008 at 11:55:19AM -0500, Jason Kraftcheck wrote: Severity: grave This is not grave, g++ is perfectly usable for other code. I emmits the following error message: bug.cc: In function 'int main()': bug.cc:6: error: no matching function for call to 'std::vectorint, std::allocatorint ::swap(std::vectorint, std::allocatorint )' /usr/include/c++/4.2/bits/stl_vector.h:728: note: candidates are: void std::vector_Tp, _Alloc::swap(std::vector_Tp, _Alloc) [with _Tp = int, _Alloc = std::allocatorint] I'm pretty sure GCC is correct to refuse this. The result of a cast is an rvalue, so you can not take a reference to it. -- Daniel Jacobowitz CodeSourcery -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Bug#478734: g++-4.2: refuses to compile valid C++ syntax
Daniel Jacobowitz wrote: severity 478734 normal thanks On Wed, Apr 30, 2008 at 11:55:19AM -0500, Jason Kraftcheck wrote: Severity: grave This is not grave, g++ is perfectly usable for other code. I emmits the following error message: bug.cc: In function 'int main()': bug.cc:6: error: no matching function for call to 'std::vectorint, std::allocatorint ::swap(std::vectorint, std::allocatorint )' /usr/include/c++/4.2/bits/stl_vector.h:728: note: candidates are: void std::vector_Tp, _Alloc::swap(std::vector_Tp, _Alloc) [with _Tp = int, _Alloc = std::allocatorint] I'm pretty sure GCC is correct to refuse this. The result of a cast is an rvalue, so you can not take a reference to it. Why can't I take a reference to an rvalue? The temporary is guaranteed to exist until the completion of the function it is passed to. Further, the error message doesn't indicate that g++ interpreted the argument as const. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
Bug#478734: g++-4.2: refuses to compile valid C++ syntax
On Wed, Apr 30, 2008 at 05:51:32PM -0500, Jason Kraftcheck wrote: Why can't I take a reference to an rvalue? Because you can't modify rvalues. This is the definition of the C++ language. The next major revision of C++ will have T rref, the two ampersands declaring an rvalue reference instead of the normal lvalue kind. the error message doesn't indicate that g++ interpreted the argument as const. Const is not the same as lvalue/rvalue. It says you tried to pass a std::vectorint where a std::vectorint was required and the fact that that was an error suggests that taking a reference is not possible here. -- Daniel Jacobowitz CodeSourcery -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of unsubscribe. Trouble? Contact [EMAIL PROTECTED]
