Re: [boost] Config for template templates?

From: David Abrahams (abrahams_at_[hidden])
Date: 2000-09-16 07:52:31

----- Original Message -----
From: <jsiek_at_[hidden]>

>
> Template templates are never really needed, since they
> can be replaced by using partial specialization. Instead
> of having two versions of the code, one with template templates
> and one with the partial spec. version, I think its better
> to just maintain the one version that is more portable,
> the partial spec. version. Also, the partial spec can
> be replaced with something even more portable, but that
> starts to get really messy...
>
>
> with template templates:
>
>   template<class U, template <class T> Container>
>   struct bar {
>     typedef Container<U> foo;
>     ...
>   };
>
>   bar<int, std::vector> b;
>
> with partial spec:
>
>   template <class ContainerTag, class U>
>   struct container_generator { };
>
>   template <class U, class ContainerTag>
>   struct bar {
>     typedef typename container_generator<ContainerTag, U>::type fooX;
>     ...
>   };
>
>   struct vector_tag { };
>
>   template <class T>
>   struct container_generator<vector_tag, T> {
>     typedef std::vector<T> type;
>   };
>
>   bar<int, vector_tag> b;

Actually, this is a perfect candidate for "poor man's partial
specialization", as I like to call it, and IMO  not too messy. OTOH, I am
forced to use VC++, so of course I think it's worth it ;)

withOUT partial spec:

  template <class ContainerTag>
  struct container_ {
    template <class U>
    struct generator {
        typedef void type;
    };
  };

  template <class U, class ContainerTag>
  struct bar {
    typedef typename container_<ContainerTag>::generator<U>::type fooX;
    ...
  };

  struct vector_tag { };

  template <>
  struct container_<vector_tag> {
    template <class T>
    struct generator {
        typedef std::vector<T> type;
    }
  };

  bar<int, vector_tag> b;

------

if you wanted to keep container_generator<C, U> as an available syntax, you
could always add:

template <class C, class U>
struct container_generator : container_<C>::generator<U> {};

"Poor man's partial spec." can be used any time you need to specialize on a
match of some, but not all, of the template parameters. It is useless for
such important things as specializing an algorithm for all pointer types,
const references, arrays, etc.

-Dave