Polymorphic Behavior

Polymorphism (poly = many, morphe = form) is the ability to treat many different forms of an object as if they were the same.

Polymorphism is achieved in C++ by using inheritance and virtual functions. Consider the scenario where we have three forms (ExpenseForm, LoanForm, PurchaseForm) that are specializations of a general Form:

Hierarchy of Form classes with parent class Form and children classes ExpenseForm, LoanForm, and PurchaseForm

Each form needs printing at some time. In procedural programming, we would either code a print function to handle the three different forms or we would write three different functions (printExpenseForm, printLoanForm, printPurchaseForm).

In C++ this can be achieved far more elegantly as follows:

   class   Form   {
   public:
      virtual void print();
   };
   class ExpenseForm : public Form {
   public:
      virtual void print();
   };
   class LoanForm : public Form {
   public:
      virtual void print();
   };
   class PurchaseForm : public Form {
   public:
      virtual void print();
   };

Each of these overridden functions is implemented so that each form prints correctly. Now an application using form objects can do this:

   Form* pForm[10]
   //create Expense/Loan/Purchase Forms…
   for (short i=0 ; i < 9 ; i++)
      pForm->print();

Here we create ten objects that might be any combination of Expense, Loan, and Purchase Forms. However, because we are dealing with pointers to the base class, Form, we do not need to know which sort of form object we have; the correct print method is called automatically.

Limited polymorphic behavior is available in the Foundation Classes. Three virtual functions are defined in the base class IccResource:

   virtual void clear();
   virtual const IccBuf& get();
   virtual void put(const IccBuf& buffer);

These methods have been implemented in the subclasses of IccResource wherever possible:

Class	clear	get	put
IccConsole	×	×	✓
IccDataQueue	✓	✓	✓
IccJournal	×	×	✓
IccSession	×	✓	✓
IccTempStore	✓	✓	✓
IccTerminal	✓	✓	✓

These virtual methods are not supported by any subclasses of IccResource except those in the table above.

Note: The default implementations of clear, get, and put in the base class IccResource throw an exception to prevent the user from calling an unsupported method.

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Last updated: Thursday, 27 June 2019

Polymorphic Behavior

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