Re: [boost] [foreach] cheap_copy crisis?

From: David Abrahams (dave_at_[hidden])
Date: 2005-12-03 16:22:35

"Eric Niebler" <eric_at_[hidden]> writes:

> David Abrahams wrote:
>> Eric Niebler <eric_at_[hidden]> writes:
>> 
>>>
>>>Put the overload in an associated namespace of the type. Or for maximum 
>>>portability, you could put the overload at global scope.
>> 
>> 
>> Really?  If invoked from within a template, global scope overloads
>> won't be found unless an argument is in the global namespace or the
>> overloads happen to appear before the point of definition of the
>> template.
>
>
> That was the idea, yes.
>
>
>> If you want a way to make this portable, consider adding a dummy
>> parameter that comes from namespace Boost and asking people to
>> overload there (the Ramey trick).
>
>
> I'm not sure how this helps on compilers that don't do ADL. This is
> a macro. BOOST_FOREACH(foo, bar) will expand to something that makes
> a non-qualified call to boost_foreach_has_cheap_copy *in the context
> of the BOOST_FOREACH invocation*.  This could be any context, in any
> namespace. 

Right.  Including from within a template, which could foil lookup in
the global namespace.

> If I want this to work on compilers that don't do ADL,
> requiring people to make their overloads visible at global scope
> seems necessary, but I've likely overlooked something.

namespace boost
{
  template <class T>
  struct foreach_has_cheap_copy
  {
       char (& test(...)) [2];
       char test(mpl::true_*);
       typedef mpl::bool_<
          sizeof(
              test(
                  boost_foreach_has_cheap_copy(
                      (T*)0,
                      boost::whatever()) ))
       > type;
  };
}

Now all compilers will look in namespace boost, whether or not they
support ADL.

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