Re: [boost] Yet another FOREACH

From: Daniel James (daniel_at_[hidden])
Date: 2005-10-09 04:08:05

A. Azuma wrote:
> I use a subtle implementation of compile-time global counter to emulate
> auto/decltype, and subtly hides them in FOREACH macro and iterator syntax.
> Please see the attached code for more detail. The attached code has been tested
> on GCC3.4.2, MSVC7.1 and MSVC8.0 beta 2. All the implementations are
> compliant to the standard, I believe so at least.

Unfortunately, this technique is not standards compliant. The problem is 
in this code:

template<int n>
struct test_auto_range_count_injector
{
   friend char (&test_auto_range_count(auto_range_count_tag<n>))[n];
};

#define FOREACH_AUTO_RANGE_COUNT()                  \
sizeof(                                             \
   ::foreach_detail::test_auto_range_count(          \
     ::foreach_detail::auto_range_end_of_count_tag() \
   )                                                 \
)

On a strictly standards compliant compiler, a friend function that is 
only declared inside a class is found using argument dependant lookup - 
the compiler knows to look inside the class because one of it's 
parameters is the class (or a descendant of it). Here there is no such 
parameter so the compiler doesn't know that it needs to look in 
test_auto_range_count_injector<n>.

As far as I know the only compilers that work that way are EDG based 
compilers (eg. Intel, Comeau) in strict mode. This technique will work 
on EDG compilers if strict mode isn't on.

The same applies to the friend funcions in auto_iterator<id, Iterator>.

Daniel