Re: [boost] [MPL] Making Generators

From: David Abrahams (dave_at_[hidden])
Date: 2002-12-05 12:06:27

"David A. Greene" <greened_at_[hidden]> writes:

> [Posted to boost because MPL is not yet released.  At what
>   point should these questions go to boost-users?]
>
> Say I have a type my_type:
>
> template<typename T, typename U, typename V>
> struct my_type { ... }
>
> Now let's say I want to create a generator that
> binds T to some type but leaves U and V free to
> be filled in later.   Basically, I want something
> like this:
>
> template<typename Bound>
> struct my_type_generator {
>     template<typename U, typename V>
>     struct apply {
>        typedef my_type<Bound, U, V> type;
>     };
> };

Looks good to me.

> Is there a convenient way to create this with MPL?  

You want it to be more convenient than that?!

    template <class T, class U, class V>
    struct my_type_generator
    {
        typedef my_type<T,U,V> type;
    };

lambda<my_type_generator> does it, unless of course your compiler
needs BOOST_MPL_AUX_LAMDA_SUPPORT.

I don't think it's much of a savings, though.

> I thought lambda might do the trick, but apparently not:
>
> typedef my_type<int, mpl::_1, mpl::_2> my_type_generator;
>
> (This will be passed to an MPL algorithm so I didn't
> bother with lambda<>).
>
> This causes a static assertion in mpl::arg<2> where it
> sees a void type for V (I think).
>
> Is the problem that my_type doesn't contain a ::type
> member?  

Maybe. What compiler are you using?

> my_type is not a metafunction so maybe it just can't be used
> conveniently with mpl.  It's easy enough to manually define
> my_type_generator, but I think this is a pretty common pattern and
> it might be useful as a library component.

Hmmmm...

-- 
                       David Abrahams
   dave_at_[hidden] * http://www.boost-consulting.com
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