Question

There are two classes A and B, where A is the base class and B is the derived class:

template <typename T>
class A {T a;};

template <typename T>
class B: public A <T> {T b;};

and the following class representing a modified container

template <typename Item>
struct TItems {typedef std::vector <Item> Type;};

template <typename Item>
class ModCont
{
    private:
            typename TItems <Item>::Type items;
};

A function test() has pointer to container of A objects as formal parameter:

template <typename T>
void test ( ModCont <A <T> > *it) {}

I would like to apply polymorphism and pass container of B to the method test:

int main(int argc, char* argv[])
{

  ModCont < A <double> > a_items;
  ModCont < B <double> > b_items;
  test (&a_items); //Works
  test (&b_items); //Does not work
  return 0;
}

The only way I found is to templatize a parameter of the test() method in this way:

template <typename Object>
void test ( ModCont <Object> *it) {}

Is there any way how to use the the “function” polymorphism instead of the compile polymorphism (templates?)

Thanks for your help…

Answer 1

Uhm, first of all, templates are not runtime polymorphism — they’re compile-time polymorphism.

If you want to use runtime polymorphism with this, you have to make sure that ModCont<B<T> > derives from ModCont<A<T> > — the way C++ handles polymorphism does not make this the default. Alternatively, you could have all ModCont<T> derive from some general ModContBase, although it’s unclear how that would work.