Re: template template parameters (was Re: [boost] Re: Special functions, octonions, quaternions in the CVS)

From: Larry Evans (jcampbell3_at_[hidden])
Date: 2001-06-25 15:31:43

Vesa Karvonen wrote:

>
> Anyway, I'm interested in seeing plausible uses of template template
> parameters. I think that this subject might be more appropriate for a C++
> newsgroup, except that I'm mainly interested in seeing actual use of template
> template parameters in existing libraries.

The smart pointer garbage collection classes in the zip files in:
   http://groups.yahoo.com/group/boost/files/GarbageCollectByTmplSpecializa/
use template template parameters.  I'd be interested in knowing if they can
be avoided.

>
> ...
>
> At least in template metaprogramming, template template parameters seem to
> have limited use and member templates seem much more powerful. The only truly
> plausible "use" I know of, is that of adapting a metafunction into a
> "metafunction object" (is there a better name for this concept?):
>
>     template
>     < template<class> class fun
>     >
>     struct make_fun_obj
>     { template
>       < class T
>       >
>       struct code : fun<T>
>       {
>       };
>     };
>
> Metafunction objects, which rely on member templates make it possible to:
> - bind parameters (and curry):
>
>     template
>     < class fun_obj
>     , class p1
>     >
>     struct bind_2nd
>     { template
>       < class p0
>       >
>       struct code : fun_obj::template code<p0,p1>
>       {
>       };
>     };
>
> - return a metafunction object or store it in a type data structure.
>
> These techniques are quite powerful. For instance, I'm using lists that

This technique was also used in the subjprox_vec and subjprox_one templates
in GarbageCollectByTmplSpecializa to enable the smart pointer analog to
vector of vectors and pointers to pointers.