[boost] Re: Customization via Overloading in Namespace

From: David Abrahams (dave_at_[hidden])
Date: 2004-02-19 23:35:46

Brian McNamara <lorgon_at_[hidden]> writes:

> On Thu, Feb 19, 2004 at 04:54:36PM -0500, David Abrahams wrote:
>> Unless I'm just observing a bug in Comeau, I've just discovered
>> something.
> ...
>> It seems that's true for qualified names, but when ADL is disabled
>> with parentheses, names aren't bound until the point of instantiation:
>> 
>>    // lib.hpp
>>    namespace lib
>>    {
>>       template <class T> void f(T);
>>       template <class T> void g(T x)
>>       {
>>          (f)(x);       // not bound here
>>       }
>>    }
>> 
>>    // 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)

>  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.

> In the example, it looks like it is not ADL that is
> disabled, but rather template hygiene.
>
>
> (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.

> All designs based on overloading or ADL will end in tears!")

You're preaching to the choir.  Read the paper I referenced if you
want to know more.

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