[boost] Re: Customization via Overloading in Namespace

From: David Abrahams (dave_at_[hidden])
Date: 2004-02-20 12:25:49

Brian McNamara <lorgon_at_[hidden]> writes:

> On Thu, Feb 19, 2004 at 11:35:13PM -0500, David Abrahams wrote:
>> Brian McNamara <lorgon_at_[hidden]> writes:
>> > On Thu, Feb 19, 2004 at 04:54:36PM -0500, David Abrahams wrote:
> ...
>> >>    // user.cpp
>> >>    #include "lib.hpp"
>> >>    #include "user.hpp" // defines some user::f overload
>> >> 
>> >>    int main()
>> >>    {
>> >>       user::my_class x;
>> >>       lib::g(x);  // calls user::f()
>> >>    }
>> >
>> > The return of name-capture... I dunno enough about the standard to know
>> > if this is a bug or not.  I am curious what you mean by "when ADL is
>> > disabled with parentheses": surely ADL is still needed for lib::g to
>> > call user::f? 
>> 
>> Ugh, of course.  Sorry, I should've said:
>> 
>>        lib::g(x);  // calls lib::f(user::my_class)
>
> And
>    #include "user.hpp" // defines some user::f overload
> should be
>    #include "user.hpp" // defines some lib::f overload
> ?  Just checking; I think yes.
>

Yes.  Sorry again.

>> >  I have seen parens around function names before as a
>> > means to something, but can you give me the reminder-summary of what
>> > the parens do?  
>> 
>> They disable ADL.
>
> Neat!

Well, ugly IMO.  I can understand why it'd appeal to someone who
usually invokes f this way, though:

        f x y z

;-)

>> > (Despite the fact that no one wants to hear it, I would feel remiss if I
>> > didn't play Chicken Little and say "Fool's gold!  Qualified calls and
>> > template specialization are the one true path to such customization!
>> 
>> You'd be wrong.  It'd be nice if it were true, but qualified calls
>> are subject to order dependencies, among other problems.
>
> I'm not clear about this.
>
>    // lib.hpp
>    namespace lib {
>       template <class T> void f(T);
>       template <class T> void g(T x) { lib::f(x); }
>    }
>    
>    // user.hpp 
>    #include "lib.hpp"
>    namespace user {
>       struct MyClass {};
>    }
>    namespace lib {
>       template <> void f( user::MyClass ) { /* print "foo" */ }
Not an overload.

>    } 
>       
>    // user.cpp
>    #include "user.hpp"
>    int main() {
>       user::MyClass m;
>       lib::g(m);
>    }
>    
> Would that not print "foo"?  What is the order dependency? 

That's a specialization.  Actually, there'd still be an order
dependency if g wasn't a function template, but that's beside the
point.

   // lib.hpp
   namespace lib {
      template <class T> void f(T);
      template <class T> void g(T x) { lib::f(x); }
   }
   
   // user.hpp 
   #include "lib.hpp"
   namespace user {
      struct MyClass {};
   }
   namespace lib {
      void f( user::MyClass ) { /* print "foo" */ } // AN OVERLOAD
   } 
      
   // user.cpp
   #include "user.hpp"
   int main() {
      user::MyClass m;
      lib::g(m);    // link error; no body for f<T> where T = user::MyClass.
   }
   

> (I don't
> think I have access to a compiler that supposedly does this right, or
> else I'd try it myself.) What are the other problems?  (I know one:
> function templates can't be "partially" specialized, which means you
> sometimes need to use a class helper, ugh.)

specialization interacts badly with overloading too:

  template <class T> int f(T);
  template <> int f(int*);
  template <class T> void f(T*);

  int x = 0;
  int y = f(&x); // error; can't convert void to int.
  

> (Apologies if this is a topic we've discussed before.)

See LWG 225,226,229, and the qn paper I referenced earlier.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com