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Declaring friend overloaded operators in templates

Posted on 2003-11-06
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Last Modified: 2010-04-01
Hi, everyone,

I'm compiling the following code with VC++ SP5, and running into strange errors:

(line 5 is the one with the friend operator* declaration)
-------------------------------------------------------
template< class T > class aa
{
public:
      T a;
      friend aa<T> operator*<> ( const aa<T>&, const aa<T>& );
};

template< class T> aa<T> operator* ( const aa<T>& i, const aa<T> & j)
{
      return i*j;
}

int main()
{
      return 0;
}
---------------------------------------------------------------
This compiles fine on MinGW.


I have STLPort installed, but I've moved the directories well down, so they should not create problems.
Errors in VC++ are:
###########
new.cxx(5) : error C2143: syntax error : missing ';' before '<'
see reference to class template instantiation 'aa<T>' being compiled
###########
After this, there are more errors as cl assumes that the friend being declared is data, rather than function, and so on.

My question: What am I doing wrong?
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Question by:narkie
10 Comments
 
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Expert Comment

by:Sys_Prog
Comment Utility
>>>>>>   friend aa<T> operator*<> ( const aa<T>&, const aa<T>& );

What do u mean by putting <> after *

0
 

Author Comment

by:narkie
Comment Utility
http://gcc.gnu.org/faq.html#friend

To quote:

"the angle brackets must be present, otherwise the declaration will be taken as a non-template function"
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Sys_Prog earned 130 total points
Comment Utility
Hi narkie

I am not very sure, but that may be specifc to compilers

refer the following link for the same

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccelng/htm/tions_31.asp

HTH
Amit
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Expert Comment

by:Sys_Prog
Comment Utility
I think removing the <> after * would do the job

I would have tried it but I do not currently have access to a VC compiler.

HTH

Amit
0
 
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Expert Comment

by:SteH
Comment Utility
looking at the faq you posted the function has to be forward declared:

template <class T>
class aa;

template <class T>
aa<T> opertor*<> (const aa<T>&, const aa<T>&);

template <class T> class aa {
....
}

before defining it.
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Author Comment

by:narkie
Comment Utility
Sorry for being so late guys (arrg. sleep :().

@SteH:
Doing a declaration as you rightly noticed (my mistake - didn't read the FAQ properly) merely transfers the error from the friend declaration to the first declaration. MinGW seems to handle it fine, though. I wonder what's happening...

@Sys_Prog:
Removing the  angle brackets makes it a template class with a non-template function declaration. Strange thing is, if I do that, and instantiate two objects of class aa<int> and try to multiply them, cl compiles them happily. gcc (rightly, I think) starts yelling that there is no operator*( const aa<int>&, const aa<int>& ). I'm confused.

Thanks for the help!
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Expert Comment

by:_ys_
Comment Utility
Slight adjustment to what SteH posted.

template <class T>
class aa;

template <class T>
aa<T> opertor* (const aa<T>&, const aa<T>&);  // *** no <>

template <class T> class aa {
...
    friend template <class T>
    aa<T> opertor*<> (const aa<T>&, const aa<T>&);
...
}
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Expert Comment

by:efn
Comment Utility
For what it's worth, I think I dealt with a similar problem a few years ago and came to the conclusion that there was no formulation that would compile in both gcc and Visual C++.  I ended up with compiler-specific code selected with #ifdefs.

--efn
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Expert Comment

by:Sys_Prog
Comment Utility
I agree woth efn

That's what the link in my previous post says
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Author Comment

by:narkie
Comment Utility
Ah well, thank you.
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