Re: [boost] [MPL] Making Generators

From: David A. Greene (greened_at_[hidden])
Date: 2002-12-05 15:56:40

David Abrahams wrote:

>>Your correction above makes everything clear to me now.
> 
> So do you  feel you need an additional library feature? ;-)

I suppose not.  What I really wanted was the ability to take a
regular old template class and create a generator out of it:

template<typename T, typename U, typename V>
struct my_type { ... }  // Note: no ::type member

typedef SHAZAM<my_type<int, _1, _2> > generator;

typedef generator::template apply<char, float>::type my_type_inst;

I want SHAZAM to be generic enough to take any template class,
not just my_type.  If I understand your solution correctly,
it requires a manually-constructed generator for my_type:

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

SHAZAM then becomes mpl::lambda.

The problem is, I'll have to create these trivial "generators"
(I hesitate to even call it that) many times over, once for each
class I want to bind.

But I can construct a generic trivial generator:

template<template<typename A, typename B, typename C> class Base,
          typename T, typename U, typename V>
struct trivial_generator {
    typedef Base<T, U, V> type;
};

Then I can use MPL lambda facilities.  Unfortunately, I need a
trivial_generator for every arity of class template.  But I
don't think there's any way around that.  MPL needs the same
thing for its placeholders.  It just seems a shame that this
enumeration needs to be repeated for this special case when
it already exists for _1, _2, etc.

>>>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.
>>
>>Agreed.  Urk...I'm not sure how to get around this problem without
>>requiring template template parameters (beyond what's used for
>>placeholders currently).
> 
> There's no way. So what? Your compiler supports them.

Of course.  But others don't.  I'm more concerned about the duplication
of effort described above.  But again, I don't think there's any way
around that.

                       -Dave

-- 
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