Abstract Classes

• In class hierarchies base classes, often, only define an interface for all derived classes in the hierarchy. Each derived class in the hierarchy is free to implement the interface to suit its own purpose.
• Such base classes are called Abstract Classes.
• Abstract base classes specify virtual functions without actually implementing them.
• An abstract class defines one or more pure virtual functions. Pure virtual functions are virtual function declarations without the function definitions.
• Pure virtual functions are specified by appending = 0 to the function declaration in the abstract class definition.
• Because pure virtual functions have no definitions, an abstract class (which contains pure virtual functions) cannot be instantiated.
• The following code illustrates how abstract classes are defined:
  

  
  class BankAccount{
  public:
  	virtual CalcBalance() = 0;
  	// Other public members go here
  protected:
  	// Protected members go here
  private:
  	// Private members go here
  };
  
  class CheckingAccount : public BankAccount{
  public:
  	virtual CalcBalance() { // implement member function here }
  	// Other public members go here
  protected:
  	// Protected members go here
  private:
  	// Private members go here
  };
  
  class SavingsAccount : public BankAccount{
  public:
  	virtual CalcBalance() { // implement member function here }
  	// Other public members go here
  protected:
  	// Protected members go here
  private:
  	// Private members go here
  };
  

• In the above code, CheckingAccount and SavingsAccount can have different implementations for CalcBalance() depending on the manner in which balances are calculated for each type of account. However, specifying an implementation of CalcBalance() for the BankAccount class is meaningless because the class does not represent any particular type of bank account. It would be meaningless to instantiate such a class. The BankAccount class serves only to define an interface, i.e., the CalcBalance() member function, which must be implemented by derived classes
• NOTE: Even though an abstract base class cannot be instantiated, it is perfectly legal to declare base class pointers and references, which could be used to reference objects derived from the abstract base class.