[Bug c++/55239] New: Spurious "unused variable" warning on function-local objects with a destructor and an initializer

[Bug c++/55239] New: Spurious "unused variable" warning on function-local objects with a destructor and an initializer

[bisqwit at iki dot fi]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55239

             Bug #: 55239
           Summary: Spurious "unused variable" warning on function-local
                    objects with a destructor and an initializer
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bisqwit@iki.fi


In the code below, a function-local object is declared with a destructor whose
role is to ensure that some action is taken at the end of the scope, no matter
which route the function is exited.

    #include <stdio.h>
    void LoadSomeFile(const char* fn)
    {
        /* Open file */
        FILE* fp = fopen(fn, "rb");
        /* Ensure that the file is automatically closed no matter which path
this function is exited */
        struct closer { FILE* f;  ~closer() { if(f) fclose(f); }
                      } autoclosefp = {fp};
        /* Some code here that deals with fp, and may include several "return;"
clauses */
    }
    int main() { LoadSomeFile(__FILE__); } // test

Bug GCC gives a spurious "unused variable 'autoclosefp'" for this code,
implying that autoclosefp has no function. It does. Without it, the file would
not be closed and resources would be leaked.

The problem also occurs, when the code is rewritten like this:

    #include <stdio.h>
    void LoadSomeFile(const char* fn)
    {
        /* Open file */
        FILE* fp = fopen(fn, "rb");
        /* Ensure that the file is automatically closed no matter which path
this function is exited */
        struct closer { FILE* f;  ~closer() { if(f) fclose(f); } };
        closer autoclosefp = {fp};
        /* Some code here that deals with fp, and may include several "return;"
clauses */
    }
    int main() { LoadSomeFile(__FILE__); } // test

Changing the "= {fp}" into C++11 style "{fp}" does not take away the warning,
either.

Only changing the initialization-by-initializer into an member-assignment takes
away the warning.

    #include <stdio.h>
    void LoadSomeFile(const char* fn)
    {
        /* Open file */
        FILE* fp = fopen(fn, "rb");
        /* Ensure that the file is automatically closed no matter which path
this function is exited */
        struct closer { FILE* f;  ~closer() { if(f) fclose(f); } } autoclosefp;
        autoclosefp.f = fp;
        /* Some code here that deals with fp, and may include several "return;"
clauses */
    }
    int main() { LoadSomeFile(__FILE__); } // test

I would argue that this is inconvenient, and wrong behavior on GCC.

Tested and verified on GCC 3.3 through 4.7.1. The -Wunused-variable (or -Wall)
option is required.

[Bug c++/55239] Spurious "unused variable" warning on function-local objects with a destructor and an initializer

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55239

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-11-08
     Ever Confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-11-08 14:46:56 UTC ---
Yes, annoying.

Giving the type a user-defined constructor makes the warning go away.  A
user-defined destructor should have the same effect.

[Bug c++/55239] Spurious "unused variable" warning on function-local objects with a destructor and an initializer

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55239

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-08 15:04:05 UTC ---
I fixed this in mainline, didn't I?

[Bug c++/55239] Spurious "unused variable" warning on function-local objects with a destructor and an initializer

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55239

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |DUPLICATE

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-08 15:06:14 UTC ---
Yes.

*** This bug has been marked as a duplicate of bug 10416 ***

[Bug c++/55239] Spurious "unused variable" warning on function-local objects with a destructor and an initializer

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55239

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-11-08 15:14:25 UTC ---
Oops, thanks, Paolo!

[Bug c++/55239] Spurious "unused variable" warning on function-local objects with a destructor and an initializer

[bisqwit at iki dot fi]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55239

--- Comment #5 from Joel Yliluoma <bisqwit at iki dot fi> 2012-11-08 15:16:48 UTC ---
Nice. I had no idea this was first reported in 2003 and fixed in 2012 in a
version recent enough to be still unreleased :)

[Bug c++/55239] Spurious "unused variable" warning on function-local objects with a destructor and an initializer

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55239

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-08 15:20:37 UTC ---
Understandable: the kind of work I have been doing lately at times is pretty
weird ;)

[Bug c++/55239] Spurious "unused variable" warning on function-local objects with a destructor and an initializer

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55239

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-11-08 15:21:40 UTC ---
Paolo's been very busy recently, cleaning up loads of longstanding bugs like
that