[boost] [MPL] for_each broken with empty list<>'s

From: Thomas Wenisch (twenisch_at_[hidden])
Date: 2003-07-07 13:21:34

Hi,

for_each seems to be unable to deal with empty lists, or lists that
are built by push_front on an empty list.  However, vectors work
fine.  Here is code which demonstrates the problem.  Replacing list with
vector makes the code compile.

#include <iostream>
#include <boost/static_assert.hpp>
#include <boost/mpl/push_front.hpp>
#include <boost/mpl/list.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/for_each.hpp>
#include <boost/mpl/alias.hpp>

typedef mpl::list<> empty;
typedef mpl::list<float> single;
typedef mpl::push_front<single, int>::type two;
typedef mpl::push_front<empty, int>::type push_on_empty;

BOOST_STATIC_ASSERT(mpl::size<empty>::value == 0); //OK
BOOST_STATIC_ASSERT(mpl::size<single>::value == 1); //OK
BOOST_STATIC_ASSERT(mpl::size<two>::value == 2); //OK
BOOST_STATIC_ASSERT(mpl::size<push_on_empty>::value == 1); //OK

struct type_printer
{
    type_printer(std::ostream& s) : f_stream(&s) {}
    template< typename U > void operator()(U &)
    {
        *f_stream << typeid(U).name() << '\n';
    }

 private:
    std::ostream* f_stream;
};

int main() {
  mpl::for_each< empty >( type_printer(std::cout) ); //won't compile
  mpl::for_each< single >( type_printer(std::cout) ); //OK
  mpl::for_each< two >( type_printer(std::cout) ); //OK
  mpl::for_each< push_on_empty >( type_printer(std::cout) ); //won't compile
}

Regards,
-Tom Wenisch
Computer Architecture Lab
Carnegie Mellon University