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

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, 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 class DerivedUVW : public Base {...};

And a helper registration macro: #define HELPER_DERIVED_UVW(U, V, W) \ Helper > 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 Ulist; typedef boost::mpl::vector Vlist; typedef boost::mpl::vector 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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