[Boost-users] Re: C++ Template Metaprogramming - Exercise 3-6

From: Edward Diener (eddielee_at_[hidden])
Date: 2005-03-13 20:36:53

Aleksey Gurtovoy wrote:
> Bruno Martínez Aguerre writes:
>> Hi,
>
> Hi Bruno,
>
> First of all, sorry for the late reply on this.
>
>> I'm *partially* answering to myself.
>>
>> On Thu, 03 Mar 2005 22:54:56 -0200, Bruno Martínez Aguerre
>> <br1_at_[hidden]> wrote:
>>
>>> I'm stuck at exercise 3-6 of the book.  I have to write twice (see
>>> below) as a lambda expression.  I don't understand why the same
>>> transformation that works for quasi fails for twice.  Any help,
>>> please?
>>>
>
> [...]
>
>>>
>>> template <class F, class A>
>>> struct twice : apply<F, typename apply<F, A>::type > {};
>>>
>>> BOOST_MPL_ASSERT(( is_same<twice<add_pointer<_1>,int>::type, int**>
>>> ));
>>>
>>> template <class F, class A>
>>> struct quasi : plus<F, typename plus<F, A>::type > {};
>>>
>>> BOOST_MPL_ASSERT(( equal_to<quasi<int_<2>,int_<3> >, int_<7> > ));
>>
>> This works because quasi inherits from plus that inherits from int_.
>
> Yep.
>
>>
>>> template <class F, class A>
>>> struct quasi2 : apply<plus<_1, plus<_1,_2> >, F, A >::type {};
>>
>> Here ::type makes quasi2 inherit from int_, too, ...
>>
>>> BOOST_MPL_ASSERT(( equal_to<quasi2<int_<2>,int_<3> >, int_<7> > ));
>>
>> ... so this works again.
>>
>>> template <class F, class A>
>>> struct twice2 : apply<apply<_1, apply<_1,_2> >, F, A >::type {};
>>>
>>> BOOST_MPL_ASSERT(( is_same<twice2<add_pointer<_1>,int>::type,
>>> int**> ));
>>
>> This has a number of bugs from copying quasi2.  Something that works
>> would be:
>>
>> template <class F, class A>
>> struct twice2 : apply< apply1<_1, apply1<_1, _2> >, F, A > {};
>>
>> BOOST_MPL_ASSERT(( is_same<twice2<add_pointer<_1>,int>::type, int**>
>> ));
>>
>> Now, this isn't right yet.  If I replace apply1 with apply, it
>> breaks. Could anybody point out why?
>
> That's a subtle pitfall that at very least should be explicitly
> mentioned in the documentation. I've put it on my TODO list; sorry
> you've had to struggle with it!
>
> In essence, the problem here is that variadic (unnumbered) 'apply'
> form
> is never a valid placeholder expression because it always has
> BOOST_MPL_LIMIT_METAFUNCTION_ARITY + 1 number of template parameters
> (metafunction to invoke + maximum number of arguments), which is in
> conflict with the Placeholder Expression concept's requirements --
> http://www.boost.org/libs/mpl/doc/refmanual/placeholder-expression.html.
>
> When I first realized this, I was thinking about bumping up
> BOOST_MPL_LIMIT_METAFUNCTION_ARITY internally, so that, in fact,
> the maximum supported arity of placeholder expressions (but not
> everything else) would be BOOST_MPL_LIMIT_METAFUNCTION_ARITY + 1, but
> unfortunately never got to it. I'll consider fixing this for 1.33.
>
> Thanks for your feedback,

Why not have the answers to the exercises in the book on-line so that
readers can look at them to further learn the library ?