C++ for Scientists - Technische Universität Dresden

214 CHAPTER 7. EFFECTIVE PROGRAMMING: THE POLYMORPHIC WAY
7.5.2 Universal Polymorphism
The universal in the title means, that the different kinds of expression for polymorphic behavior
in this section are the most useful techniques to accomplish the desired behavior and should be
used preferably:
• Dynamic polymorphism (subtyping)
• Static polymorphism (parametric)
Subtyping Polymorphism
In C++ the object-oriented paradigm implements subtyping polymorphism 3 using sub-classing.
The term dynamic polymorphism is often found for this type of polymorphism.
To introduce the applicability of this kind of polymorphism an example from the topological
area is given. Classes for different kinds of points are used, which should be comparable in their
own set. Traversing through containers or data structures is a quite common task in generic
programming. The next code snippet presents the base class for all kind of vertices.
#include
class topology { };
class vertex
{
public:
virtual bool equal(const vertex∗ ve) const = 0;
};
If these vertex types have to be extended, only the new class with the according equal method
should be implemented. The next code snippet presents two possible implementations for a
vertex, which can be used in different topologies.
class structured vertex : public vertex
{
public:
structured vertex(int id, topology∗ topo) : id(id), topo(topo) {}
virtual bool equal(const vertex∗ ve) const
{
const structured vertex∗ sv = dynamic cast(ve);
return ((id == sv→ id) && (topo == sv→ topo));
}
protected:
int id;
topology∗ topo;
};
3 Also called inclusion polymorphism.