Re: [boost] [MPL] Making Generators

From: David Abrahams (dave_at_[hidden])
Date: 2002-12-05 13:25:00

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

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

Oops, I meant

   lambda<my_type_generator<Bound,mpl::_1,mpl::_2> > 

of course!

> , unless of course your compiler
>> needs BOOST_MPL_AUX_LAMDA_SUPPORT.  I don't think it's much of a
>> savings, though.
>
> Not for one class, no, but when we're talking several classes
> with several binding requirements, I think there's a significant
> savings to be had.

So do you feel you need an additional library feature?

> Plus your solution here doesn't bind T to a type.  :)

Are you just pointing out my error?

>>>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?
>
> g++ 3.2.  The MPL paper and docs don't say anything about
> using placeholders or binders with classes that aren't
> metafunctions.  How would the binders know what to typedef
> as apply::type?

I don't know... well, it could detect whether there was a ::type
member, and if it were not present, it could just give you the outer
class. I think that's a bit of a hack, though.

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