Warning: returning reference to temporary

[Dr. Andreas Stuebinger]

consider the following C++ snippet:

-----------------------------------------------------
#define REPBITS 32

class foo {
public:
  unsigned short set_width;

  unsigned long* pbits;
  unsigned long bits;

  unsigned long& word() {
    return set_width <= REPBITS ? bits : *(pbits);
  }

  const unsigned long& word() const {
    return set_width <= REPBITS ? bits : *(pbits);
  }
};
-----------------------------------------------------

Compiling it with the latest egcs-20000306 version on RedHat 6.0 results in:

460> /opt/egcs-20000306/bin/g++ -v -c foo.C
Reading specs from /opt/egcs-20000306/lib/gcc-lib/i686-pc-linux-gnu/2.96/specs
gcc version 2.96 20000306 (experimental)
 /opt/egcs-20000306/lib/gcc-lib/i686-pc-linux-gnu/2.96/cpp -lang-c++ -v -D__GNUC__=2 -D__GNUG__=2 -D__GNUC_MINOR__=96 -D__GNUC_PATCHLEVEL__=0 -D__cplusplus -D__ELF__ -Dunix -Dlinux -D__ELF__ -D__unix__ -D__linux__ -D__unix -D__linux -Asystem(posix) -D__EXCEPTIONS -Acpu(i386) -Amachine(i386) -Di386 -D__i386 -D__i386__ -D__tune_pentiumpro__ foo.C /tmp/cclFDDnL.ii
GNU CPP version 2.96 20000306 (experimental) (cpplib)
 (i386 Linux/ELF)
ignoring nonexistent directory `/usr/local/include'
#include "..." search starts here:
#include <...> search starts here:
 /opt/egcs-20000306/include/g++-3
 /opt/egcs-20000306/i686-pc-linux-gnu/include
 /opt/egcs-20000306/lib/gcc-lib/i686-pc-linux-gnu/2.96/include
 /usr/include
End of search list.
 /opt/egcs-20000306/lib/gcc-lib/i686-pc-linux-gnu/2.96/cc1plus /tmp/cclFDDnL.ii -quiet -dumpbase foo.cc -version -o /tmp/ccW5XMEr.s
GNU C++ version 2.96 20000306 (experimental) (i686-pc-linux-gnu) compiled by GNU C version 2.96 20000306 (experimental).
foo.C: In method `const long unsigned int &foo::word () const':
foo.C:19: warning: returning reference to temporary
 as -V -Qy -o foo.o /tmp/ccW5XMEr.s
GNU assembler version 2.9.1 (i386-redhat-linux), using BFD version 2.9.1.0.23


Why the warning about returning reference to temporary? If the const
version is changed to

    if(set_width <= REPBITS)
      return bits;
    else
      return *(pbits);

no warning is emitted.

-- 
MfG,
	Andreas Stübinger
--------------------------------------------------------------------------
Dr. Andreas Stübinger            Email: Andreas.Stuebinger@mchp.siemens.de
Siemens AG, ZT SE 4              Phone: ++49-89/636-51338
D-81730 München, Germany         Fax:   ++49-89/636-42284