Chapter 11: Inheritance II

Chapter Goals

• To understand the difference between static and dynamic binding
• To be able to implement dynamic binding with virtual functions

Polymorphism

• A base-class pointer can point to a derived class object. (The reverse is an error).
• By creating a vector of pointers, we can hold a collection of both base-class and derived-class objects.

  vector<Clock*> clocks(3);
  /* populate clocks */
  clocks[0] = new Clock(true);
  clocks[1] = new TravelClock(true, "Rome", 9);
  clocks[2] = new TravelClock(false, "Tokyo", -7);

• Pointers to the various clock objects all have the same size - the size of a memory address - even though the objects themselves may have different sizes.
• Since every TravelClock is a special case of a Clock, the starting address of a TravelClock object is the starting address of a Clock object.

• Unfortunately, the following code does still not work!

  cout << clocks[i]->get_location() << " time: "
     << clocks[i]->get_hours() << ":"
     << setw(2) << setfill('0')
     << clocks[i]->get_minutes()
     << setfill(' ') << "\n";

• The output is still

  Local time is 21:15
  Local time is 21:15
  Local time is 9:15

• Recall that both Clock and TravelClock have a get_location and a get_hours member function.
• The compiler calls the Clock member function because the pointer is of type Clock*.

• You must alert the compiler that the function call needs to be preceded by the appropriate function selection, which can be a different one for every iteration in the loop.
• Such a selection/call combination is called dynamic binding.
• The traditional call, which always invokes the same function, is called static binding.
• To tell the C++ compiler that a particular function needs to be bound dynamically, the function must be tagged as virtual.

  class Clock
  {
  public:
     Clock(bool use_military);
     virtual string get_location() const;
     virtual int get_hours() const;
     int get_minutes() const;
     bool is_military() const;
  private:
     . . .
  };

Polymorphism (Syntax 11.3 Virtual Function Definition)

class Class_name
{
   virtual return_type function_name(parameter1, parameter2, ..., parametern);
   . . . 
};

class Employee
{
public:
   virtual double get_salary();
   . . . 
};

Polymorphism (clocks3.cpp)