Re: [Boost-users] [MPL/Fusion?]Call a 0-args template function on members of a sequence

21 Aug 2006

      Hi Dave,

Thanks for your reply.

David Abrahams wrote:
...
David Abrahams  writes:
...
Edd Dawson  writes:
...
Hello all,
I have the following:
template<typename T>
     void f() { /* ... */ }
typedef boost::mpl::vector my_vec;
I'm looking for a way to call f<T>() for all types T in my_vec. I initially 
hoped that either an mpl algorithm or fusion::for_each might be able to 
accomplish this for me, but after reading through their docs this doesn't appear 
to be the case.
  struct call_f
  {
     template <class Wrapper>
     void operator()(Wrapper)
Sorry, it's missing a "const" right here--^^^^
     {
         f<typename Wrapper::type>();
     }
  };
mpl::for_each >(call_f());
I didn't find this in the docs. Is it meant to be there?

Unfortunately, that didn't quite work for me, anyway. A requirement of for_each 
(according to my MSVC 8 diagnostics) is that each type in the sequence must be 
default constructible.

The (near) exact code I'm trying to compile is:

     struct register_cage
     {
         template<typename wrapper>
         void operator()(wrapper) const
         {
             // ...
         }
     };

     struct null_type { };

     // template
     template<
         BOOST_PP_ENUM_PARAMS_WITH_A_DEFAULT(BOOST_MPL_LIMIT_VECTOR_SIZE, 
typename T, null_type)
     >
     struct catch_exceptions
     {
         typedef typename boost::mpl::vector<
             BOOST_PP_ENUM_PARAMS(BOOST_MPL_LIMIT_VECTOR_SIZE, T)
         >::type exception_types;

         catch_exceptions()
         {
             using namespace boost::mpl;

             typedef boost::mpl::_1 my1;
             typedef boost::mpl::_2 my2;

             // Remove all null_type entries and then sort the remaining ones,
             // most derived to front.
             typedef remove_if<
                 exception_types,
                 boost::is_same<_, null_type>
             >::type unique_exception_types;

             typedef sort<
                 unique_exception_types,
                 boost::is_base_of
             >::type sorted_exception_types;

             for_each >(register_cage());
         }
     };

     // Elsewhere, in 'client' code

     catch_exceptions();

The most appropriate part of the (long) compiler error is:

1>c:\development\boost\include\boost-1_33_1\boost\utility\value_init.hpp(34) : 
error C2512: 'std::runtime_error' : no appropriate default constructor available

1> 
c:\development\boost\include\boost-1_33_1\boost\utility\value_init.hpp(34) : 
while compiling class template member function 
'boost::vinit_detail::non_const_T_base<T>::non_const_T_base(void)'

Assuming that I have interpreted the compiler errors correctly, is this 
default-constructible requirement really necessary? I can do the task using the 
trick I listed in my original post so I would expect a library of this kind to 
be able to.

Kind regards,

Edd