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 you Next Lecture: Exception Handling