Upcasting and Downcasting

• A characteristic feature of inheritance is that an object of a publicly derived class can be used as an object of its corresponding base class. Note, however, that the reverse is not true; objects of the base class cannot be used as objects of its derived class
• This is a powerful feature because it provides a mechanism for handling objects of different types in arrays, or accessing objects of different types through one pointer or reference
• The following are standard conversions for derivation:
• Only apply to publicly derived base classes
• A derived class object will be implicitly converted to a public base class object
• A derived class reference will be implicitly converted to a public base class reference
• A derived class pointer will be implicitly converted to a public base class pointer
• A pointer to any class object will be implicitly converted to a void* pointer
• Consider the following example:
  

  
  class Vehicle{
  	// Vehicle members go here
  };
  
  class Car : public Vehicle{
  	// Car members go here
  };
  
  class Truck : public Vehicle{
  	// Truck members go here
  };
  
  Vehicle v1;
  Vehicle* vPtr;
  Car* cPtr;
  Car c1;
  Truck t1;
  
  v1 = c1; // Valid because a Car is a special type of Vehicle
  vPtr = &t1; // Valid because a Truck is a special type of Vehicle
  cPtr = &v1; // Not Valid because v1 may not be a Car!!
  

• Another Example...