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

Hi, 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?

#include #include

#include #include #include #include #include #include #include

using namespace boost; using namespace boost::mpl; using namespace boost::mpl::placeholders;

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_.

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? The first g++ error message is: ej3_6.cpp: In instantiation of `boost::mpl::apply_wrap2, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::add_pointer, int>': ej3_6.cpp:22: instantiated from `boost::mpl::apply2, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::add_pointer, int>' ej3_6.cpp:22: instantiated from `boost::mpl::apply, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::add_pointer, int, mpl_::na, mpl_::na, mpl_::na>' ej3_6.cpp:22: instantiated from `twice2, int>' ej3_6.cpp:22: instantiated from here ej3_6.cpp:22: no class template named `apply' in `struct boost::mpl::apply, mpl_::na, mpl_::na, mpl_::na, mpl_::na>' Thank you, Bruno