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Message-ID: <123201be6f14$1abd1160$e63d2509@jonpryor.raleigh.ibm.com>
From: "Jonathan Pryor" <jonpryor AT vt DOT edu>
To: "Jonathan Pryor" <jonpryor AT cowpie DOT acm DOT vt DOT edu>,
        "Mumit Khan" <khan AT xraylith DOT wisc DOT EDU>
Cc: "Cygwin Mailing List" <cygwin AT sourceware DOT cygnus DOT com>
Subject: Re: egcs 1.1.1: odd warning message with template friend functions 
Date: Mon, 15 Mar 1999 13:45:43 -0500
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Mumit Khan <khan AT xraylith DOT wisc DOT EDU> writes:

>"Jonathan Pryor" <jonpryor AT vt DOT edu> writes:
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>> Note: I'm not complaining about the resulting executable; I'm
>> complaining about the warning message itself.
>>
>> When I compile the attached source code, I get the
>> following warning:
>>
>>     mi.cpp:7: warning: friend declaration `bool operator ==(
>>         const class meta_info<T> &, const class meta_info<T> &)'
>>     mi.cpp:7: warning:   declares a non-template function
>>     mi.cpp:7: warning:   (if this is not what you intended, make sure
>>     mi.cpp:7: warning:   the function template has already been declared,
>>     mi.cpp:7: warning:   and add <> after the function name here)
>>     mi.cpp:8: warning: friend declaration `class meta_info<T> metaid(
>>         const T &)'
>>     mi.cpp:8: warning:   declares a non-template function
>
>Please see "Guiding Declarations" in the standard. EGCS is correct; MSVC
>doesn't implmenent this correctly. This was added after Stroustrup book
>came out (at least my copy of it).
>
>Regards,
>Mumit

If it was added after Stroustrup, that would cure some of my
confusion...  But I'm always confused.  ;-)  So....

(In reference to a later note you sent to Bob Aglietti:)
Please, bore me with guiding declarations!  :-)

As for Dejanews -- I tried searching for "guiding declarations"
and "guiding decl", but didn't find too much that helped.

In all seriousness, I'm currently looking at the draft C++ standard
available at http://www.cygnus.com/iso/ .  Hopefully this is current
enough... (though with my luck, it's incorrect and the only way
to know for sure is to spend the $18 (?) to get a real copy of
the standard -- which might not be a bad idea, actually...)

However, I tried searching for "Guiding Declarations", and didn't
find anything.  What I did find was section 14.5.3 -- Friends --
and a lot of confusion.

Assuming I'm reading this correctly (always a good question), then
in the following template class:

    // prototypes
    template<class T> class task;
    template<class T> task<T>* preempt(task<T>*);

    template<class T> class task {
     //  ...
     friend void next_time();
       // all task specializations have the non-template function
       // next_time() as a friend.

     friend void process(task<T>*);
       // "each specialization of the task class template has an
       // appropriately typed function process as a friend, and
       // this friend is not a function template specialization"
       //
       // OK, it's not a specialization; but is it still a template?
       // I think this declaration matches my original report.

     friend task<T>* preempt<T>(task<T>*);
       // "each specialization of the task class template has the
       // appropriate specialization of the function template
       // preempt as a friend"
       //
       // This would match the syntax egcs was looking for.

     template<class C> friend int func(C);
       // "each specialization of the task class template has all
       // specializations of the function template func as friends".
       //
       // Oddly enough, this actually compiles find with both
       // egcs and MSVC...

     //  ...
    };

The 2nd friend declaration resembles what I was curious about.
But the standard doesn't seem to specify whether or not the function
must be a template; "an appropriately typed function" sounds like
a template, but I'd just like to verify this.

If "an appropriately typed function" is in fact a template, then
I shouldn't be getting a warning for the following

    template <typename T>
    class meta_info {
      friend const meta_info<T>& metaid ( const T& );
      meta_info ();
    public:
    };

    template <typename T>
    const meta_info<T>& metaid ( const T& ) {
      meta_info<T> mi;
      return mi;
    }

So what's the scoop here?  What's "an appropriately typed function"?
Does it mean it'll look for a non-template function with the same
types at compile time?  If a non-template function with the correct
types isn't found, but a template function with the same prototype
exists, will a standard-conforming compiler use that?

If the last question is true, then why (again) give a warning about
declaring a non-template function?  Is it "safe" to ignore this
warning in this case, then?

Thanks,
 - Jon


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