CS212: Object Oriented Analysis and Design

Lecture 17: Virtual Functions

Recap of lecture 16

• Polymorphism

• Pointer to derived type

• Virtual function

• VTABLE, VPTR

• Inheritance and VTABLE

Outline of Lecture 17

• Abstract base class

• Object slicing

• Overloading, overriding

• Constructor and destructors

• Multiple dispatching

• Downcasting

Abstract base class

• Base class to present only an interface for its derived classes

• No object is created of the base class

• This is accomplished by making that class abstract

• pure virtual function.

• It uses the virtual keyword and is followed by = 0

virtual type func-name(parameter-list) = 0;

Pure virtual definitions

• It’s possible to provide a definition for a pure virtual function

• The compiler will not allow objects of that abstract base class

• Pure virtual functions must still be defined in derived classes in order to create objects

• Demonstration

Object slicing

• Passing the addresses of objects and passing objects by value

• In case polymorphism there is a distinct difference

• Upcast to an object instead of a pointer or reference

• The object is “sliced”

• Demonstration

Overloading vs. Overriding

• Redefining an overloaded function in the base class

• Virtual functions: the behaviour is a little different.

• Compiler will not allow you to change the return type

• Demonstration

Virtual functions & constructors

• VPTR must be initialized to point to the proper VTABLE

• The constructor has the job of bringing an object into existence

• The constructor’s job to set up the VPTR

• Compiler inserts code into the beginning of the constructor

• Constructor will include the proper VPTR initialization code

• There are several implications

Implication 1: Efficiency

• Inline functions

• It initialize the VPTR, it must also check the value of this

• Calls base class constructor

• Code size can grow without any benefits in speed

Implication 2: Order of constructor calls

• The order of constructor calls and the way virtual calls are made within constructors

• Base-class constructors are always called in the constructor for an inherited class

• Compiler enforces a constructor call for every portion of a derived class

• Calling virtual function from the constructor

Virtual function & destructors

• Constructor assembles the object parts, while destructor disassembles it

• The disassembling process happens in the reverse order of that of the assembly

• A derived object can be manipulated using base class pointer

• The problem occurs when a pointer of this type is to be deleted for an object that has been created on the heap with new

• Demonstration

Operator overloading

• Operator functions can be made virtual

• Implementing virtual operators often becomes confusing

• Operating on two objects, both with unknown types

• Multiple dispatching

• Demonstration

Downcasting

• Downcasting: to move down a hierarchy

• There are usually several possibilities that you could cast to

• Dilemma is figuring out a way to safely downcast

• Type-safe downcast operation: dynamic_cast

• dynamic_cast uses information stored in the VTABLE to determine the actual type

• Demonstration

Summary

• Polymorphism C++ Virtual Function “Different forms”

• Polymorphism is a feature that cannot be viewed in isolation

• If it isn’t late binding, it isn’t polymorphism

Thank youNext Lecture: Exception Handling