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

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 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?

[...]

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template struct twice : apply::type > {};

BOOST_MPL_ASSERT(( is_same,int>::type, int**> ));

template struct quasi : plus::type > {};

BOOST_MPL_ASSERT(( equal_to,int_<3> >, int_<7> > ));

This works because quasi inherits from plus that inherits from int_.

Yep.

...

template struct quasi2 : apply >, F, A >::type {};

Here ::type makes quasi2 inherit from int_, too, ...

...

BOOST_MPL_ASSERT(( equal_to,int_<3> >, int_<7> > ));

... so this works again.

...

template struct twice2 : apply >, F, A >::type {};

BOOST_MPL_ASSERT(( is_same,int>::type, int**> ));

This has a number of bugs from copying quasi2. Something that works would be:

template struct twice2 : apply< apply1<_1, apply1<_1, _2> >, F, A > {};

BOOST_MPL_ASSERT(( is_same,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, -- Aleksey Gurtovoy MetaCommunications Engineering