[Boost-users] fusion sequences, type erasure across translation units

Subject: [Boost-users] fusion sequences, type erasure across translation units
From: Hicham Mouline (hicham_at_[hidden])
Date: 2010-01-29 00:14:16

Hello,

I have a .cpp file which a sort of top level driver.
I have say 100 different 
  struct param_i {...};
where 1<=i<=100

Each param_i is defined in a param_i.hpp and param_i.cpp invokes metafunctions to obtain
   typedef fusion< variant< vector<double>, triplet<double>, pair<double>, double>, 
                          variant<...>, ..., 
                          variant<...> > 
   seq_i_t;
this sequence type is visible only in the param_i.cpp, not the header file. Generation of this sequence is 
The size of these sequences = number of members of param_i.

The toplevel driver and other .cpp need to move instances of these types around to other .cpp.
To avoid having the compiler call the metafunctions for these middle translation units, but only at the generating .cpp and at the consuming .cpp, I created a tag 

   struct fusion_seq_tag {};

   typedef <typename Arg>
   struct make_fusion_seq_i {
      typedef <undefined> type;
   };

   typedef <typename Arg>
   struct make_fusion_seq_tag {
     typedef typename make_fusion_seq_i<Arg>::type base_type;
     struct type : fusion_seq_tag, base_type {
        type(...) : base_type(...) {}
     };
   };
     
   return smart_ptr<fusion_seq_tag>( new make_fusion_seq_tag<...>::type );

// this was the generating .cpp
The function returns smart_ptr<fusion_seq_tag> which can move around in other .cpp without any metaprogramming
until reaching the consuming .cpp

Is this type erasure?

I don't know how I can get the actual type back in the consuming .cpp if I don't know the originator but if I know only the shape of the fusion sequence. How to dynamic cast the type back to its unerased type?

If I know the originator .cpp then I would know the exact fusion sequence and therefore I would just dynamic_cast the fusion_seq_tag.

Regards,