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: unassig...@gcc.gnu.org
ReportedBy: bisq...@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.
