C++ tips - Mode "has-a" or "is-implemented-in-terms-of" through layering

[ben2ong2]

(This example from "Effective C++" by Scott Meyers, p142 - 145.)


Layering is the process of building one class on top of another class by having
the layering class contain an object of the layered class as a data member.
For example:

   class String      { ... };
   class Address     { ... };
   class PhoneNumber { ... };

   class Person {
     private:
      String name;              // layered object
      Address address;          // ditto
      PhoneNumber voiceNumber;  // ditto
      PhoneNumber faxNumber;    // ditto
     public:
      ...
   };

In this example, the Person class is said to be layered on top of the String,
Address, and PhoneNumber classes, because it contains data members of those
types. Layering is also known as containment or embedding.

The Person class above demonstrates the has-a relationship. A Person object
has a name, an address, and telephone numbers for both voice and FAX
communication. You wouldn't say that a person is a string or that a person is
an address, you would say that a person has a string ...

Somewhat more troublesome is the difference between isa and is-implemented-
in-terms-of. For example, suppose that you have a class that implements linked
lists of objects ...

   template<class T>
   class LinkedList {
     private:
      ...
     public:
      LinkedList ();
      LinkedList (const LinkedList& rhs);
      virtual ~LinkedList();

      LinkedList& operator=(const LinkedList& rhs);

      virtual void insert (T& item, int position);
      virtual T& remove (int position);

      virtual T& operator[] (int position);
      virtual const T& operator[] (int position) const;

      int length () const;
   };

...

Now suppose that your real goal is to develop a template class to represent
sets of items of type T, where a set is, as usual, an unordered collection
without duplicates. Being the data structure maven that you are, you know
that of the nearly limitless choices for implementing sets, one particularly
simple way is to employ linked lists. Because you have a linked list class
already written, you decide to engage in a little code reuse by having your
nascent Set class inherit from LinkedList. After all, in your implementation,
a Set object will in fact be a LinkedList object. Your decision is further
buttressed by the fact that the LinkedList class is clearly designed to be a
base class: notice the virtual functions, including, significantly, the
virtual destructor...

Hence you declare your Set class like this:

   // the wrong way to associate sets and linked lists

   template<class T>
   class Set : public LinkedList<T> { ... };

Everything may seem fine and dandy at this point, but in fact there is
something quite wrong. ...if B isa A, then everything that is true of A is
also true of B. However, a LinkedList object is an ordered collection, a fact
made plain by the presence of the operator[] functions that take a position
argument in the LinkedList class. In contrast, a Set is an unordered collection,
and the operator[] functions it inherits from LinkedList make no sense for it.
It is not true that a Set isa LinkedList, because some of the things that are
true for LinkedList objects are not true for Set objects.

Because the relationship between these two classes isn't isa, public
inheritance is the wrong way to model that relationship. The right way to
model that relationship is to realize that a Set object can be implemented
in terms of a LinkedList object...