[Bug c++/54047] New: unused variable warning not for std::string

[Bug c++/54047] New: unused variable warning not for std::string

[plasmahh at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54047

             Bug #: 54047
           Summary: unused variable warning not for std::string
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: plasmahh@gmx.net


Compiling the following code:

#include <string>

struct A { };

void foo()
{
        A z;
        std::string z0;
}


I get a warning for unused variable z, but not for variable z0. It seems that
std::string is somehow magical here, but I can't see how or why.

[Bug c++/54047] unused variable warning not for std::string

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54047

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-07-20 12:09:01 UTC ---
It's the case for all objects with a non-trivial constructor, not only
std::string.

Constructors might have side-effects, so you might not need to "use" the
variable, all you want is to construct it.

e.g. you don't want an unused variable warning for 'l' here:

  void f(std::mutex& m, int& i) {
    std::lock_guard<std::mutex> l(m);
    i = 1;
  }

I think this behaviour is intentional and correct.

[Bug c++/54047] unused variable warning not for std::string

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54047

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com

--- Comment #2 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-07-20 14:07:55 UTC ---
(In reply to comment #1)
> I think this behaviour is intentional and correct.

I completely agree with that.

[Bug c++/54047] unused variable warning not for std::string

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54047

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Closing.