MPL and apply_if trouble

19 May 2003

      I encountered the following problem today which I've reduced down to a
simple example. Basically what I want to do is recurse through a
typelist (mpl::vector) popping an element each time and end recursion by
using apply_if with mpl::empty<List> as the condition. I've made a
simple Reverse class template to illustrate the problem. (The Reverse
class is of course intended to reverse the contents of a typelist....)

--------------------- begin code ---------------------

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

using namespace boost::mpl;

template <typename List>
struct Reverse
{
    typedef typename front<List>::type           FrontElem;
    typedef typename pop_front<List>::type       Rest;
    typedef typename apply_if<
                        empty<Rest>,
                        vector<FrontElem>,
                        typename push_back::type
                             >::type             Type;
};

typedef vector        List;
typedef Reverse<List>::Type         RevList;

--------------------- end code -----------------------

When attempting to create the RevList type however, gcc spits the
following at me:

reverse.cpp: In instantiation of
`boost::mpl::pop_frontboost::mpl::void_ >':
reverse.cpp:15:   instantiated from
`Reverseboost::mpl::void_ >'
reverse.cpp:21:   instantiated from `Reverse'
reverse.cpp:21:   instantiated from `Reverse >'
reverse.cpp:21:   instantiated from `Reverse<List>'
reverse.cpp:25:   instantiated from here
reverse.cpp:15: base class `
   boost::mpl::pop_front_traits
...
::algorithmboost::mpl::void_ >' has incomplete
type
reverse.cpp: In instantiation of
`Reverseboost::mpl::void_ >':
reverse.cpp:21:   instantiated from `Reverse'
reverse.cpp:21:   instantiated from `Reverse >'
reverse.cpp:21:   instantiated from `Reverse<List>'
reverse.cpp:25:   instantiated from here
reverse.cpp:15: no type named `type' in `struct 
   boost::mpl::pop_frontboost::mpl::void_ >'
reverse.cpp:21: no type named `type' in `struct 
   boost::mpl::pop_frontboost::mpl::void_ >'
reverse.cpp: In instantiation of `Reverse >'
reverse.cpp:21:   instantiated from `Reverse<List>'
reverse.cpp:25:   instantiated from here
reverse.cpp:21: no type named `Type' in `struct 
   Reverseboost::mpl::void_ >'
reverse.cpp: In instantiation of `Reverse >':
reverse.cpp:21:   instantiated from `Reverse<List>'
reverse.cpp:25:   instantiated from here
reverse.cpp:21: no type named `Type' in `struct 
   Reverse'
reverse.cpp: In instantiation of `Reverse<List>':
reverse.cpp:25:   instantiated from here
reverse.cpp:21: no type named `Type' in `struct 
   Reverse >'
reverse.cpp:25: syntax error before `;' token
I cannot see why it reaches the point where it tries to instantiate
Reverse with an empty typelist - the apply_if should stop that (at least
that's my intention. Am I being a complete ass somewhere in this code
snippet? I've become blinded by now, so if anyone can point out what's
wrong I would greatly appreciate it.

Sincerely,
--
Tarjei

MPL and apply_if trouble

Tarjei Knapstad