[boost] [fusion] Instantiating elements of nested sequences

From: Carlos Rafael Giani (e0325834_at_[hidden])
Date: 2008-03-16 08:50:17

Hi,

I want to store boost signals in fusion sequences. This does not work 
directly, since signals are noncopyable. I figured I could use 
shared_ptr's of the signals, and all that needs to be done is to iterate 
through the sequences, instantiate objects, and assign them to the 
shared_ptr's.

These are the structures for the signals:

        template < typename Element >
        struct element_signals
        {
                typedef boost::signal < void(model const &, Element const &) > 
event_signal_t;
                typedef boost::signal < void(model const &, Element const &, 
std::vector < std::string > const &) > modified_signal_t;

                typedef boost::fusion::vector <
                        std::tr1::shared_ptr < event_signal_t >, // add
                        std::tr1::shared_ptr < event_signal_t >, // remove
                        std::tr1::shared_ptr < modified_signal_t > // modified
		> signals_t;

                typedef boost::fusion::pair < Element, signals_t > signals_pair_t;
        };

        typedef boost::fusion::map <
                element_signals < vertex_iterator > ::signals_pair_t,
                element_signals < edge_iterator > ::signals_pair_t
	> signals_t;

Now, I am trying to do something like this pseudo-code:

signals_t signals;

for_each(signals,
        for_each(_1,
                _1 = typeof(_1)(new typeof(_1)::element_type)
        )
);

Problems:

1) How do I get the type of the current element in the for_each loop 
without resorting to functors?
2) I tried to use a MPL placeholder for the _1 in the outer for_each 
loop, but it didn't seem to work. Does Fusion have its own placeholders, 
or what am I missing?

Regards,