Re: [Boost-users] [Boost.MPL and Boost.Fusion] Combinatorial game with typelists

From: Chris Weed (chrisweed_at_[hidden])
Date: 2008-01-03 16:13:54

I personally solved this with BOOST.Preprocessor to generate the many
combinations, but I think you could make an MPL/Fusion solution.
Chris

On Jan 3, 2008 1:03 PM,  <dariomt_at_[hidden]> wrote:
> Hello boost users,
>
> I need some advice in using MPL and fusion.
>
> Lets take the following case for a typical Factory pattern (full compilable
> source code is attached):
>
> class Base {...};
> class Factory {...};
> template <typename T> struct Helper {...}; // constructor registers creation
> function for template parameter
>
> Now we have a template defining a family of derived classes:
> template <typename U, typename V, typename W> class DerivedUVW : public Base
> {...};
>
> And a helper registration macro:
> #define HELPER_DERIVED_UVW(U, V, W) \
>     Helper<DerivedUVW<U, V, W> > BOOST_PP_CAT(h, __LINE__) (#U #V #W);
>
> And all possible U V and W classes in the combination game:
> struct A1 {}; struct A2 {}; // for U
> struct B1 {}; struct B2 {}; // for V
> struct C1 {}; struct C2 {}; // for W
>
> Now with manual registration its just a matter of patience:
> //    Manual registration:
>  //    must type all possible combinations -> ugly
> HELPER_DERIVED_UVW(A1, B1, C1);
> HELPER_DERIVED_UVW(A1, B1, C2);
> // etc...
>
> To create the desired object just use
> Factory::instance().create("A1B1C1")->doInterestingStuff()
>
> And now the six million dollar question:
> Is there a way in which MPL and fusion can automatically do the
> registration?
>
> Here registration means the instantiation of template Helper with all
> possible combinations of DerivedUVW with elements from lists like:
> typedef boost::mpl::vector<A1,A2> Ulist;
> typedef boost::mpl::vector<B1,B2> Vlist;
> typedef boost::mpl::vector<C1,C2> Wlist;
> and registered with an appropiate name like "A1B1C1"
>
>  Of course the objective is to easily add A3, A4, B3, C4, etc... to allow
> more combinations, without having to manually update all the registration
> code, ideally just the typelists. And, let's not forget variants DerivedUV
> or DerivedUVWZ with a different number of template parameters.
>
> I'm trying to think how I would do combinations for values in runtime and
> translate it to the MPL domain. Still no success.
>
> Any help, suggestions or comments will be appreciated.
>
> PS: the context of this problem is being able to expose a heavily
> templatized library to a user selecting the combination of template classes
> at runtime with a text string. I'm sure there's a suitable solution to this
> problem somewhere out there, or at least some hints to build my own
> solution...
>
>
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