Re: Alternative to virtual template function

"Greg Herlihy" <>
26 Sep 2006 09:12:30 -0400
<> wrote:

Since some of my target platforms don't provide an STL implementation,
I decided to create interfaces mimicking nice modern container classes
as well as factory methods in IPlatform that create the objects. For
example (pardon my simplified examples):

template<class T> class IList
     virtual int getCount() = 0;
     // etc

class IPlatform
     template<class T> virtual IList<T>* createList() = 0;
     // etc

However, this is not allowed, since template members cannot be virtual!

More precisely, a class method that is a function template cannot be
declared virtual. And that restriction should not be all that
surprising when one considers that a virtual method and a function
template are, as programming techniques, pretty much polar opposites. A
function template can create multiple routines from just one
implementation, whereas a virtual method can create multiple
implementations of just one routine.

  So I ditched the member function idea and went the route of
regular-old functions instead:

In a platform-independent header:

   template<class T> IList<T>* createList();

I'm not clear why the getList method itself has to be a template,
instead of its class, IPlatform.

In a platform-specific header:

   template<class T> StlListAdapter : public IList<T>
       // blah, blah

It seems to me that making the class a template would at least make it
possible to declare a virtual method. There is no restriction against a
class template from declaring virtual, non-template function methods:

      template <class T>
      class IPlatform
            virtual IList<T> * createList()

At this point, I don't have enough of an understanding of the problem
domain to suggest other solutions, or even to able to form an opinion
one way or the other whether a class template with a virtual method is
a reasonable solution.


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