Re: [boost] Re: A replacement to any_cast

From: Rob Stewart (stewart_at_[hidden])
Date: 2004-12-10 12:16:30

From: David Abrahams <dave_at_[hidden]>
> Rob Stewart wrote:
> > From: "Yuval Ronen" <ronen_yuval_at_[hidden]>
> > 
> > But you can't.  You have to write the following, as I showed
> > above:
> > 
> >    int a = some_any.template get<int>();
> 
> ?? There are no dependent types in that utterance.  Unless some_any is
> not known to be a boost::any (?)  Am I missing something?

I tried to track down the relevant section in the standard to
confirm what I wrote, but I couldn't.  I was relying on C++PL,
3rd Ed, App C, section C.13.6, "Template as a Qualifier."

The example is (reduced):

   struct Memory
   {
      template <typename T> T * get_new();
   };

   template <typename Allocator>
   void
   f(Allocator & m)
   {
      int * p1 = m.get_new<int>(); // error
      int * p2 = m.template get_new<int>();
   }

This Memory::get_new() is used within a function template, but
Stroustrup doesn't mention that it is precisely because get_new()
is dependent that "template" is needed, so I didn't think that it
was a dependent type.

Thinking further, I realized that m.get_new<int>() actually means
m.Allocator::get_new<int>(), so I now see that it is a dependent
type according to 14.6.2.1/1.

In the OP's case, the invoked member function would be any<T>'s,
not T's, so it isn't dependent and "template" isn't needed, which
is what you implied.

-- 
Rob Stewart                           stewart_at_[hidden]
Software Engineer                     http://www.sig.com
Susquehanna International Group, LLP  using std::disclaimer;