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

AMDG Scott Meyers wrote:

I have a set of types representing features code might have, e.g.,

typedef mpl::vector Features;

These can occur in any combination as sets of features, e.g.,

tyepdef FeatureSet<ExceptionSafe> F1; typedef FeatureSet F2; typedef FeatureSet F3; <snip>

I'm looking for a way to automatically generate an inheritance hierarchy that will give me the behavior I've described. For the above example, the hierarchy has four levels. The root is all the features, because any subset of features is permitted to be implicitly convertible to the set of all features.

Root: FeatureSet

The leaves are the individual features:

Leaves: FeatureSet<ExceptionSafe> FeatureSet<ThreadSafe> FeatureSet<ReallyFast> FeatureSet<GPLed> <snip>

Is there some way for me to use the MPL to automatically generate the hierarchy I need?

Thanks,

Scott

There's nothing that works out of the box. Here's a quick hack: #include #include #include #include #include #include #include #include #include #include #include #include #include namespace mpl = boost::mpl; template struct virtual_inherit : virtual T1, virtual T2 {}; template<class T1> struct virtual_inherit : virtual T1 {}; template<class S> struct make_features; template struct make_features_base { typedef typename make_features::type>::type item; typedef typename make_features_base::type next; typedef virtual_inherit type; }; template<class S> struct make_features_base { typedef virtual_inheritmpl::empty_base type; }; template<class S> struct features : virtual mpl::eval_if >, make_features_base<S>, mpl::identitympl::empty_base > >::type {}; template<class S> struct make_features { typedef features > >::type> type; }; int main() { typedef make_features >::type f1; typedef make_features >::type f2; typedef make_features::type f3; BOOST_MPL_ASSERT((boost::is_base_and_derived)); BOOST_MPL_ASSERT((boost::is_base_and_derived)); } In Christ, Steven Watanabe