Re: [boost] [mpl] vararg metafunction classes?

From: David Abrahams (dave_at_[hidden])
Date: 2006-10-14 16:20:10

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

> Imagine a template defined like this:
>
>    template< class T, class U = void >
>    struct foo
>    {};
>
> I would like to turn this into a metafunction class F such that:
>
>    mpl::apply< F, int >::type  ==>  foo<int, void>
>
> and
>
>    mpl::apply< F, int, float >::type ==> foo<int, float>
>
> Can this be done? If F is foo< _1, _2 >, then the first apply above 
> won't compile because it hasn't been given enough arguments.

  template <class L, class D>
  struct default2nd
  {
      template <class A0, class A1 = D>
      struct apply
        : mpl::apply<L,A0,A1>
      {}
  };

  default2nd< foo<_,_>, void>        

?

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