Re: class composition

From:
Larry Evans <cppljevans@suddenlink.net>
Newsgroups:
comp.lang.c++
Date:
Mon, 24 Jan 2011 19:21:44 -0600
Message-ID:
<4d3e25a9$0$9342$bbae4d71@news.suddenlink.net>
On 01/24/11 16:31, er wrote:

Hi,

Is there a way to define A such that if B is defined like this

    struct tag1{};
    struct tag2{};
    struct B : A<tag1, int>, A<tag2, double>{};

then

    typedef B::F<tag1>::type v1_ // v1_ == int;
    typedef B::F<tag2>::type v2_; // v2_ == double

By analogy, the above is feasible for a member function, say f,

    template<typename Tag, typename T> struct A{
        T f(Tag){ return T(); }
    };

    struct B : A<tag1, int>, A<tag2, double>
    {
        typedef A<tag1, int> a1_; using a1_::f;
        typedef A<tag2, double> a2_; using a2_::f;

    };


Yes. This technigue of overloading a function within
a class with a tag in order to select a the function in an
inheritance hierarchy was used here:

http://svn.boost.org/svn/boost/sandbox/variadic_templates/boost/composite_storage/layout/operators_one_of_maybe.hpp

The overloaded function was the static:

  boost::composite_storage::layout::operators<one_of_maybe,,,,>::
  push_back<HeadLayout,TailComponent>::inject(index_part,...)

Although inject is static instead of a member function, I think
it would work for member functions also. The tag in the
above call is the index_part.

HTH.

  Larry

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