[Boost-users] mpl::inherit_linearly question

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2006-12-29 10:39:31

Hi!

I am trying to create a static visitor using metapgramming. This approach worked fine in some test
code, but now I face some problems.

Let me first show some example:

I've define an item_visitor template class:

template <class Pair> //mpl::pair obtained from map sequence
struct item_visitor
{
public:
        typedef typename Pair::second	value_type;

        virtual void visit(value_type const& t)
        {
                assert(0 && "This function must be overriden in the concrete handler class!");
        }
};

template<class Seq>
struct base_sequence_visitor
        : public boost::mpl::inherit_linearly
                 <
                        Seq
                 ,	boost::mpl::inherit<boost::mpl::_1, item_visitor<boost::mpl::_2> >
		 >::type
{};

This example works fine and I can dispatch my type.

Now I have some additional type, which provides a view to my structure. And I would like my visit
to implemented as:

virtual void visit(GenericView<Pair::second> const& t)
{ ... }

In such a case I have to introduce a second parameter to the item_visitor template:

template<class Pair, template<class> SomeView>
struct item_visitor { ... };

Therefore the base_sequence_visitor must be updated as well to forward the View template:

template<class Seq, template<class> SomeView>
struct base_sequence_visitor
        : public boost::mpl::inherit_linearly
                 <
                        Seq
                 ,	boost::mpl::inherit<boost::mpl::_1, item_visitor<boost::mpl::_2, SomeView> >
		 >::type
{};

Using this construct I get an error, that mpl::arg<2> does not have a type second. That means that
the placeholder _2 does not get expanded. So I tried to "code around" and used the following
trick:

template<class>
calss SomeView;

template<class Pair>
struct some_visitor : item_visitor<Pair, SomeView>
{};

Then I added to the base_sequence_visitor template an additional argument, which specifies an item
visitor class to be used:

template<class Seq, template<class> ItemVisitor>
struct base_sequence_visitor
        : public boost::mpl::inherit_linearly
                 <
                        Seq
                 ,	boost::mpl::inherit<boost::mpl::_1, ItemVisitor<boost::mpl::_2> >
		 >::type
{};

And it compiles fine! But when I try to cast the base_sequence_visitor to some complete
item_visitor type, which is really in the sequence I receive a compiler error, that
cannot convert from 'base_sequence_visitor<Seq,ItemVisitor>' to
'ClassWhereSimpleVisitorIsDefined::simple_visitor<PairType> &'

Do I miss something?

With Kind Regards,

Ovanes Markarian