Re: [Boost-Users] Re: mpl::if_ or mpl::apply_if?

From: Aleksey Gurtovoy (alexy_at_[hidden])
Date: 2003-02-25 07:07:22

Markus Werle wrote:
> >> Q3: (OT) Is there a mpl-equivalent to run-time case?
> > 
> > :-)
> > 
> > Aleksey and I have been discussing some shorthands for
> > 
> >   if (c1) then t1
> >   else if (c2) then t2
> >   else if (c3) then t3
> >   ...
> >   else if (c4) then t4
> > 
> > but that's not quite the same as a case statement.
> 
> A std::map equivalent would help here a lot.
> It should be possible to search within a typelist of 
> std::pair<condition, type> ...
> Unfortunately I am still stumbling through the mpl
> source [getting mad about all those  
> WORKAROUND_FOR(STUPID_COMPILER_VENDOR)]
> 
> Could You please give me an idea how to 
> search within such a typelist?

With the current CVS, you don't have to do it by yourself:

#include "boost/mpl/vector.hpp"
#include "boost/mpl/switch.hpp"
#include "boost/mpl/project1st.hpp"
#include "boost/mpl/pair.hpp"
#include "boost/mpl/always.hpp"
#include "boost/mpl/bool_c.hpp"
#include "boost/mpl/assert_is_same.hpp"

#include "boost/type_traits/remove_reference.hpp"
#include "boost/type_traits/add_const.hpp"
#include "boost/type_traits/is_pointer.hpp"
#include "boost/type_traits/is_reference.hpp"

using namespace boost::mpl;

typedef vector<
      pair< boost::is_pointer<_1>, _1 >
    , pair< boost::is_reference<_1>, boost::remove_reference<_1> >
    , pair< always<true_c>, boost::add_const<_1> >
    > switch_body;

typedef switch_< switch_body, char& >::type t1;
typedef switch_< switch_body, int* >::type t2;
typedef switch_< switch_body, int >::type t3;

BOOST_MPL_ASSERT_IS_SAME(t1, char);
BOOST_MPL_ASSERT_IS_SAME(t2, int*);
BOOST_MPL_ASSERT_IS_SAME(t3, int const);

Does it cover your needs?

Aleskey