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Method \code{result\_wrapper::extract<A>()}: what is \code{A}?

It calls \code{arg->extract<A>()}, with \code{arg} is a shared pointer
to \code{base\_wrapper<T>}, which actually holds a pointer to
\code{derived\_result\_wrapper<T>}. Method
\code{base\_wrapper<T>::extract<A>()} casts \code{*this} to
\code{derived\_wrapper<A>\&} and calls its \code{extract()}.


The
\code{derived\_result\_wrapper<T>} is derived from
\code{derived\_wrapper<T>}. The \code{derived\_wrapper<T>} is derived
from \code{impl::DerivedWrapper<T,\$tag,*>} (where \code{*} denotes
the lower level \code{impl::DerivedWrapper<T,\$tag,*>}), with the
\code{detail::foundation\_wrapper<T>} at the lowest level. Method
\code{derived\_wrapper<T>::extract()} returns a reference to \code{T},
which is \code{this->m\_data}. By the way, \code{m\_data} \emph{has methods}
like \code{print()}, \code{reset()} and \code{merge()} (so it is not a
basic type).

The \code{detail::foundation\_wrapper<T>} has a \code{m\_data} member
of type \code{T}, and is derived from \code{base\_wrapper<V>}, where
\code{V} is \code{value\_type<T>::type}. The type
\code{value\_type<T>::type} is typedef-ed as \code{T::value\_type}.

The \code{base\_wrapper<T>} is derived from
\code{impl::BaseWrapper<T,\$tag,*>}, with
\code{detail::value\_wrapper<T>} at the bottom of the hierarchy. The
latter contains only a \code{typedef T value\_type}.

The classes \code{impl::BaseWrapper<T,\$tag,B>} and
\code{impl::DerivedWrapper<T,\$tag,B>} are defined in the
corresponding feature files, for each \code{\$tag}. 

So, \code{A} should at least have \code{A::value\_type}. Also, a cast
from \code{base\_wrapper<T>} to \code{derived\_wrapper<A>} should exists. 



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