c++/2831: operator ?: requires storage for initialized static member constants

[fspam@sourceware.cygnus.com]

>Number:         2831
>Category:       c++
>Synopsis:       operator ?: requires storage for initialized static member constants
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue May 15 12:26:00 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Forest Wilkinson
>Release:        gcc version 3.0 20010515 (prerelease)
>Organization:
>Environment:
Red Hat Linux 7.1 (kernel 2.4.2-2smp, dual processor i686)
>Description:
When compiling an executable from the included source code, g++ produces the following error messages:

/tmp/cce1LkFy.o: In function `main':
/tmp/cce1LkFy.o(.text+0xd): undefined reference to `foo_class::flag1    '
/tmp/cce1LkFy.o(.text+0x19): undefined reference to `foo_class::flag2    '
collect2: ld returned 1 exit status


I verified this with the gcc online compiler submission form at http://www.codesourcery.com/gcc-compile.shtml
The form said:
"The compiler used in this form was last updated from the nightly build on 2001-05-15 . 
The sources were checked out from the gcc-3_0-branch branch of the GCC CVS repository."


The same problem occurs in the gcc that ships with red hat linux 7.1:  gcc version 2.96 20000731 (Red Hat Linux 7.1 2.96-81)
>How-To-Repeat:

// undefref.cpp

/*
When compiled with gcc version 2.96 20000731 (Red Hat Linux 7.1 2.96-81),
or gcc version 3.0 20010515 (prerelease),
the compiler produces the following error message:

/tmp/cceI5nKb.o: In function `main':
/tmp/cceI5nKb.o(.text+0xd): undefined reference to `foo_class::flag1'
/tmp/cceI5nKb.o(.text+0x19): undefined reference to `foo_class::flag2'
collect2: ld returned 1 exit status
*/

class foo_class
	{
	public:
		// flag1 & flag2 are initialized static integral constants,
		// as described in Stroustrup 3: section 10.4.6.2: Member Constants.
		// The are never used in a way that requires them to be stored as
		// objects in memory, so they should not require a separate definition.
		static const int flag1 = (1 << 0);
		static const int flag2 = (1 << 1);
	};

// According to Stroustrup, these should not be necessary:
//const int foo_class::flag1;
//const int foo_class::flag2;

int main( int argc, char *argv[])
	{
	// This line produces the "undefined reference" error:
	int flags = (argc == 1) ? foo_class::flag1 : foo_class::flag2;

	// This line is just fine:
	int aflags = foo_class::flag1;

	return 0;
	}

>Fix:
I can work around this problem by avoiding the ?: operator when accessing my static member constants.

Another workaround for this example code would be to define storage for those constants, but this becomes ugly when they belong to a template class.  (Besides, according to Stroustrup, I shouldn't have to do it.)
>Release-Note:
>Audit-Trail:
>Unformatted:
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