Re: [boost] Config for template templates?

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2000-09-16 16:10:40

I think it would be dangerous for boost to get on the slippery slope of 
banning certain C++ language features, especially when the workaround 
adversely effects the interface to a lib.  In this particular example, 
the interface of the partial specialization workaround is not near as 
elegant as the template template interface.  I do not see a compelling 
reason to artificially dumb-down the interface of boost libs for those 
with conforming compilers.  I do however, think it is worthwhile to offer 
an optional dumbed-down interface for something as sparsely supported as 
template templates.

Motivating example:

Consider the compile-time list structure of 10.10.5 of "Generative 
Programming".  I might want to create a list, and then sort it using a 
predicate.  Maybe something like:

template <class T, class U>
struct by_size
{
        static const bool value = sizeof(T) <= sizeof(U);
};
...
        typedef make_list<short, int, char, long>::type MyList;
        typedef sort<MyList, by_size>::type SortedList;

It is very nice to be able to give the predicate as a template template 
parameter.  No doubt you can figure out how to do this without template 
templates.  But gee, I really like the interface here.

Heck, I'm quickly trying to think of a "predicate_tag" style workaround 
here and I'm coming up blank.  But I'm sure someone will...

-Howard

jsiek_at_[hidden] wrote on 9/16/2000 2:13 AM
>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;
>
>
>Daryle Walker writes:
> > Should we have a #define for template templates support in 
>boost/config.hpp?
> > This could give some flexibility for people who may want to use template
> > templates as the best solution, but give alternate solutions if needed. 
> (I
> > was think of template templates for getting the standard algorithms to 
>work
> > with the range stuff.)
> > 
> > -- 
> > 
> > 
> > 
> > 
> > 
> > 
>
>
>