preprocessor/5240: Unicode L## doesn't work in all cases for macros

preprocessor/5240: Unicode L## doesn't work in all cases for macros

[dstarr]

>Number:         5240
>Category:       preprocessor
>Synopsis:       Unicode L## doesn't work in all cases for macros
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    unassigned
>State:          open
>Class:          ice-on-legal-code
>Submitter-Id:   net
>Arrival-Date:   Wed Jan 02 00:56:00 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     dstarr@opera.com
>Release:        versions 3.0 and later
>Organization:
>Environment:
All
>Description:
This is best described by first looking at the code.
When using the code which is listed in the How-To-Repeat section, a compiler error occurs stating that LMyTestString was not declared.
This to me makes it seam that the preprocessor is expanding UNI_L(MyTestString) to LMyTestString instead of L##MyTestString.
I'm not 100% sure that this is perfectly valid code, however MSVC, CodeWarrior, Intel (list goes on a long while) all handle this code as intended.
>How-To-Repeat:
#define UNI_L(string) L##string
#define MyTestString "Hello World!"

int main(int argc, char **argv)
{
...
    CallToUnicodeFunction(UNI_L(MyTestString));
...
}
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:

Re: preprocessor/5240: Unicode L## doesn't work in all cases for macros

[neil]

Synopsis: Unicode L## doesn't work in all cases for macros

State-Changed-From-To: open->closed
State-Changed-By: neil
State-Changed-When: Wed Jan  2 01:40:47 2002
State-Changed-Why:
    Not a bug - GCC is correct.
    
    The standard states that arguments are expanded, UNLESS the parameter is preceded by a # or ##, or followed by a ##.  Your parameter is preceded by a ##.
    
    I find it *very* hard to believe that other compilers get it wrong.  In particular, Intel uses the EDG front end.  Comeau also uses the EDG front end, and if you go to
    
    http://www.comeaucomputing.com/tryitout/
    
    and give the on-line compiler your input, with a command line of -E, the output you will get is
    
    int main(int argc, char **argv)
    {
    ...
        CallToUnicodeFunction(LMyTestString);
    ...
    }
    
    which is what GCC is giving.
    
    It is possible that all the compilers you list other than MSVC have a work-around for MSVC brokenness when on Windows, but I doubt it because even selecting WindowsNT on the Comeau home page doesn't affect the result.  Maybe it requires an --enable-brokenness flag of some kind.  *shrug*.
    
    To get the result you want, you need to have an extra level of indirection so that the argument is expanded according to the rule I give above:
    
    #define UNI_L2(string) L##string
    #define UNI_L(string) UNI_L2(string)
    #define MyTestString "Hello World!"
    
        CallToUnicodeFunction(UNI_L(MyTestString));

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=5240