Re: [Boost-users] [mpl] Hierarchy Generation

From: Scott Meyers (usenet_at_[hidden])
Date: 2008-02-25 20:12:56

Steven Watanabe wrote:
>     typedef make_features<mpl::vector<int, char, double> >::type f1;
>     typedef make_features<mpl::vector<int, double> >::type f2;
>     typedef make_features<mpl::vector<char> >::type f3;
>     BOOST_MPL_ASSERT((boost::is_base_and_derived<f2, f1>));
>     BOOST_MPL_ASSERT((boost::is_base_and_derived<f3, f1>));

Sorry for taking so long to reply, but TMP always makes my head explode, and it 
takes a while to pick up the pieces and put them back together.  In this case, I 
could have saved myself the explosion, because this interface offers behavior 
that is backwards from what I want.  The conversion I want to allow is from 
fewer features to more, so I don't want f2 and f3 to be bases of f1, I want f1 
to be a base for both f2 and f3.

My current thinking is that this can't be done without knowing all possible 
features, so I'm thinking about an approach where I first define that:

   typedef mpl::vector< ... > AllPossibleFeatures;

What I then want to define is

   template<typename MyFeatures>     // sequence of features
   struct Features...

subject to the following:

   typedef difference<AllFeatures, MyFeatures> TypesToAdd;   // Features in
                                                             // AllFeatures that
                                                             // are not in
                                                             // MyFeatures

   For each type T in TypesToAdd,
   Features<MyFeatures> virtually inherits from
   Features<insert<MyFeatures,T>::type>.

So given

   typedef mpl::vector<int, char, double> AllPossibleFeatures;

Features<mpl::vector<int>> (C++0x says I can omit the space between angle 
brackets :-}) would virtually inherit from Features<mpl::vector<int, char>> and 
Features<mpl::vector<int, double>.

In turn, Features<mpl::vector<int, char>> and Features<mpl::vector<int, double>> 
would both virtually inherit from Features<mpl::vector<int, char, double>.

I can write the difference metafunction above and I can write a rotate 
metafunction to help me iterate through all the types in the difference set. 
What I don't know is a good way to give Features<T> the appropriate number of 
virtual base classes.  Can somebody help?

Thanks,

Scott