Re: [boost] C++ millenium challenge solved?

From: Eric Niebler (eric_at_[hidden])
Date: 2005-12-17 13:17:34

Thorsten Ottosen wrote:
> Eric Niebler wrote:
> 
>>Here's my latest attempt. This works on gcc 3.4.4 (cygwin), but not on 
>>VC7.1 or (sadly) Comeau Online. Language lawyers, start your engines!
> 
> Eric, this is very cool :-)
> 
> I tried to simplify it a little:
> 
> template<typename T>
> struct probe
> {
>       operator T ();
>       operator T&() const;
> };
> 
> template<typename T>
> char is_volatile( const T& );
> 
> template<typename T>
> double is_volatile( T& );
> 
> That gave the same results.

I noticed this too, and decided it was a bug in gcc. I think that given

   template<typename T> struct conv {
       operator T() { return T(); }
   };
   template<typename T> void bar(T&) {}

the expression:

   typedef foo const cfoo;
   bar(true ? conv<foo>() : cfoo());

should compile, and that in bar, T should be deduced at "foo const". 
With gcc, it does *not* compile because the expression "true ? 
conv<foo>() : cfoo()" appears to have the type "rvalue of foo" instead 
of "rvalue of foo const". gcc is inadvertantly dropping cv-modifiers 
where it shouldn't.

But we're certainly free to use that to our advantage. :-)

> Then I did
> 
>  > template<typename T>
>  > probe<T> make_probe(T const &);
> 
> template<typename T>
> const probe<T> make_probe(T const &);
> 
> Now it works on vc7.1 too. Not como.

Whoa. If the probe is const-qualified, how can the non-const member ever 
get called on it? Seems like you've found a compiler bug. I would test 
with Comeau Online, but it's not responding at the moment.

Anyway, getting this to work with vc7.1 is a major advance. Thanks.

> I'm not a language specialist and my understanding of ?: comes mainly 
> from your article. That said, I find the solution very intuitive.

If we can make this work on the popular compilers, it'll be a big win 
for both boost.foreach and boost.typeof. It doesn't matter if we need to 
take advantage of compiler bugs to get it. I wonder if other compilers 
and other versions of gcc accept either my variant or yours.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com