template and container, looking for advice

From: Istvan Buki (istvan.buki_at_euronet.be)
Date: 10/08/03 

Date: Wed, 08 Oct 2003 13:55:39 +0200

Dear C++ gurus,

I'm having some problems inserting some template classes into containers.
I'm looking for ideas, suggestions,... on how to achieve this but first let
me explain my problem. Everything starts with a simple template class like
the one below. In this example class A has two template parameters but that
number can be different.

    template < typename T, typename U >
    class A
    {
    public:
      void a_method( T *t ) ;

      // Other methods...

    private:
      // private stuff...
    } ;

Now let's say that in the application using class A, I instantiate A with 10
different values for the T template parameter and I create one object of
each type.

To keep track of all these objects I would like to put them in a stl
container but I can't because all the objects have a different type.

I thought about creating a base class and make A derive from it but it
doesn't work because of the "a_method" that depends on one of the template
parameter and I cannot create a pure virtual a_method in the base class.

Then I thought about changing the signature of the "a_method" to something
like this:

          void a_method( BaseT *t );

and make sure that template parameter T is derived from BaseT but it is not
what I need because if I reuse class A in another application I would be
forced to derive all classes used as T template parameter from BaseT which
is probably not meaninful in the context of the new application.

Finally I thought about doing the following:

    template < typename BaseT >
    class ParentA
    {
    public:
      typedef BaseT BaseType ;

      virtual void a_method( BaseT *t ) = 0 ;

      // Other pure virtual methods...
    } ;

    template < typename MyParent, typename T, typename U >
    class A : public MyParent
    {
    public:
      void a_method( MyParent::BaseType *t ) ;

      // ...
    } ;

Then using it like this for example:

    struct SomeBaseClass
    {
      // ...
    } ;

    struct SomeDerivedClass : public SomeBaseClass
    {
      // ...
    } ;

    typedef ParentA< SomeBaseClass > ParentExample ;
    typedef A< ParentExample, SomeDerivedClass, SomeUType > ;

This works but it is not very nice and it forces the user of the class to
code a lot of stuff he should not worry about.
And here I'm stuck. Does anybody have some ideas about how to achieve the
same goal but in cleaner (possibly simpler) way ?

Thank you for your help,
Istvan

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