Re: [Boost-users] Type List Iteration?

From: David Abrahams (dave_at_[hidden])
Date: 2005-10-04 23:01:43

"Duane Murphy" <duanemurphy_at_[hidden]> writes:

> How do I iterate over a type list and call a function? MPL?
>
> I have a base class BASE that has several subclasses A, B, C that have
> special capabilities, but no way to identify those capabilities aside
> from the type.
>
> I would like to determine if an object of type BASE is also of one of
> the special subclasses:
>
> ie
> void f( BASE* base)
> {
>     if ( dynamic_cast< A* >( base ) != NULL )
>     {
>         some_function( dynamic_cast< A* >( base ) );
>     }
>     else if ( dynamic_cast< B* >( base ) != NULL )
>     {
>         some_function( dynamic_cast< B* >( base ) );
>     }
>     else if ( dynamic_cast< C* >( base ) != NULL )
>     {
>         some_function( dynamic_cast< C* >( base ) );
>     }
> }
>
> I think you can see why I'd like a type list iterator solution to this. 
>
> I have arranged for the some_function to be a template as well to make
> things easier.
>
> What I am looking for is the equivalent of find_if() but for types and
> done at run time. I looked over MPL, but MPL seems to apply at compile
> time without a door to the run time world. :-)

If you read the chapter 11 of C++ Template Metaprogramming
(http://boost-consulting.com/mplbook/#table-of-contents-and-sample-chapters),
you'll learn about the approach for doing this.  Something like:

   struct do_nothing
   {
       void operator()() const {};
   }

   template <class T, class NextFunctionType>
   struct try_derived_class
   {
       void operator()(base* b) const
       {
           if (T* d = dynamic_cast<T*>(b))
           {
               some_function(d);
           }
           else
           {
               NextFunctionType f;
               f(b);
           }
       }
   };

   mpl::fold< 
       a_sequence_of_derived_classes
     , try_derived_class<_2,_1>
     , do_nothing
   > dispatch;

Now dispatch is a function object that does your thing.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com