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CS300: Object Oriented Analysis and Design

Run Time Polymorphism

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Polymorphism

Polymorphism in C++

Compile Time

Function overloading

Operator overloading

Runtime

Virtual Function

The provision of a single interface to entities of different types.

Methods with same name but different implementations.

Operators have different implementations depending on their arguments

A member function that you expect to be redefined in derived classes.

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Introduction

Encapsulation

Access control

Inheritance

Virtual function

Creates new data types

Separates the interface from the implementation

An object as its own type or its base type

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Paradigm shift in programming

Better C

Enhanced version of C
Structure 🡪 Class
Variable declaration

Object Based

Grouping a data structure together (Encapsulation)
Constructor and destructors
Inheritance

Object oriented

Virtual function
Intimately bound with the concept of type
Understood only from a design viewpoint

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Polymorphism

Early

Binding occurs at compile time

Early binding polymorphism

Process of overloading members

Late

Binding occurs at runtime

Late binding polymorphism

The code to implement the method is chosen at runtime

Appropriate code chosen sending a message to

the object …. Not to the pointer to the object

Implemented through virtual functions

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Pointers to Derived Types

A pointer of one type cannot point to an object of a different type

An important exception to this rule that relates only to derived classes

B

D

A base class pointer can point to any derived classes object

Demonstration

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Virtual Function

A virtual function is a member function

Declared within a base class

Redefined by a derived class

“one interface,

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Binding

Connecting a function call to a function body is called binding.

Types of Bindings

Early

Late

Before the program is run (by the compiler and linker)

Binding occurs at runtime, based on the type of the object

Dynamic binding

or

Runtime binding

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Virtual functions

To cause late binding to occur for a particular function

Use the virtual keyword when declaring the function

When a virtual function is redefined, all aspects of its prototype must be the same.

Constructor functions cannot be virtual, but destructor functions can.

Overriding : virtual function redefinition by a derived class.

Demonstration

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Polymorphism

Virtual Functions

A virtual function must be declared in a parent class

syntax

virtual function

virtual returnType functionName ( args_i) { function body ;}

pure virtual function

virtual returnType functionName ( args_i) = 0;

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Polymorphism

Virtual Functions

Declaration

A function name is preceded by the keyword virtual

Function name can only be used once in the parent class
Cannot overload virtual functions
Only class member functions can be declared virtual

A function is virtual…..

If it is declared virtual
There is a base class function with the same signature

declared virtual

Any or all class member functions (except constructors)

can be declared virtual

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Polymorphism

Virtual Functions

Implementation

The body of the virtual function must be supplied in the parent

class unless declared to be a pure virtual function

A derived class can override the definition by providing its own

implementation

If the re-declaration does not match exactly…...

The function not considered virtual for that class

A virtual function still permitted in a subsequently derived class

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Polymorphism

Virtual Functions

Such a capability permits multiple functions to be called through a

common interface.

Can be overridden by explicit qualification with the scope operator.

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Gives Uniform Function

Call Interface

Base

Derived1

Derived2

Derived3

……..

Derivedn

Public Interface

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Polymorphism

Virtual Functions

When function in a class is declared virtual

Keyword virtual tells compiler

Don’t perform early binding
Install mechanisms to perform late binding

Compiler responds by creating

Table of function pointers
Installing a data member to the class to point to the table

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Polymorphism

Virtual Functions

The compiler created table is called the vtable (vtbl)

Contains pointers to all functions declared virtual within the class

and derived classes.

Each class gets its own vtable

A data member called the vpointer (vPtr)

Usually placed as the first element in object in memory.

Initialized to the starting address of the vtable.

The function call through a base class pointer

Indexes into the vtable calls the function located at the address.

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Polymorphism

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class A

{

public:

int i;

virtual void f ( );

virtual void g( );

};

vptr

i

&f ( )

&g ( )

vtable[0]

vtable[1]

class A

vtable

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Polymorphism

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class A

{

public:

int i;

virtual void f ( );

};

class B : public A

{

public:

virtual void f ( ); // override f( )

virtual void g ( ); // define g ()

};

class A

vtable

&f ( )

vtable[0]

&f ( )

&g ( )

vtable[0]

vtable[1]

class B

vtable

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Polymorphism

Virtual Functions - vtable

Contains pointers to all virtual functions.
Each class gets its own vtable.
Abstract classes have no vtable.
Vtable size is proportional to the number of virtual functions.

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Polymorphism

Virtual Functions - Invocation

A virtual function is invoked through a public base class pointer or reference.

Runtime Binding

Typically polymorphic binding is done dynamically at runtime
Virtual functions are not inlined

Compile Time Binding

Occasionally have compile time polymorphic binding
Invoked through an object of the class type
Invoked using the scope operator
Invoked through a constructor or destructor

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Polymorphism

Virtual Functions - Access Protection

The access level of a virtual function is determined by

Access level in class of the pointer through which it’s invoked
Not by the class in which it’s defined.

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Polymorphism

A

B : public A

C : public B

D : public C

E : public D

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Declare a and public virtual function f() public virtual function f() in B

protected virtual function f() in C

private virtual function f() in D

Write:

1. A* a = new B;

a-> f(); // f() accessible through *a (as a *B - *a in public area)

2. a = new C;

a-> f(); // f() accessible through *a (as a *C - a* in public area)

3. C* c = new D;

c-> f(); // f() not accessible through *c (as a *D - c* in protected area)

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Polymorphism

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Virtual Destructors

When a base class pointer is used to refer to a derived class object and the object is deleted…..

Only the base class destructor will be invoked leaving behind the derived class parts of the object.

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Polymorphism

Virtual Destructors

syntax

virtual ~ class ClassName ( ) { destructor body }

Specifying a destructor as virtual ensures all appropriate destructors

are invoked.

Rule of thumb…..If a class is abstract then declare the destructor as

virtual.

Don’t declare the destructor if there are no other virtual functions.

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Polymorphism

Virtual Destructors

Invocation order….

The derived type destructor

The destructor for each base class…..

…...Invoked in turn in normal fashion

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Polymorphism

Polymorphism and Object Slicing

One must exercise caution when treating objects polymorphically

There is a distinct difference between passing objects by value and by reference.

When a derived class object is passed by value to a function expecting a base class value….

The derived class portion is sliced off.

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Polymorphism

Pure Virtual Functions

….A virtual function must be defined when it is declared

Abstract base class may be defined that is not intended to be instantiated
If virtual function is declared pure …. an implementation

may still be supplied

Derived class may use implementation

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Polymorphism

Pure Virtual Functions

When a function is declared pure…..

There is no address to put into the vtable

The 0 keys the compiler that no instances of this class can be created

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Polymorphism

Pure Virtual Functions

Restrictions

A class with one or more pure virtual functions.

1. Can only be used as a base class

2. Cannot have instances

3. Cannot be used as

An argument type
Return type
Type for explicit conversion

4. Can be used as a

Pointer
Reference type

A class cannot define a pure virtual destructor

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Polymorphism

Pure Virtual Definitions

There may be occasions when it’s desirable to share code with

derived classes but not duplicate in each class.

Can prevent base class instantiation yet provide a definition for a pure

virtual function.

syntax

virtual returnType functionName ( argsi ) = 0 { function body }

access

BaseClassName :: functionName ( );

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Polymorphism

Virtual Functions

Rules for Virtual Functions

1. Virtual functions called from within a constructor use the local

version.

2. The first class in a derivation hierarchy that declares a virtual function it must provide a definition or it must declare the virtual function to be pure

3. If a definition is provided, the definition serves as the default instance in subsequent derivations

4. If pure, a subsequently derived class must provide a definition - to have instances or inherit the pure virtual function - have no instances

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Polymorphism

Virtual Functions

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vf1₁( )

vf1₂( )

must define vf1₁( ) and vf1₂( )

vf1₂( )

can have instances

pvf1₃( )

no instances

pvf1₃( ) =

defines pvf1₃( )

can have instances

Class 1

Class 2

Class 3

Class 4

Class 5

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Polymorphism

Virtual Functions - Access Level

The access level of a virtual function is…..

Specified in the class where it is defined
Not by initial definition

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Polymorphism

Virtual Base Classes

Parent classes may have a common base class

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Fruit

Peach

Fruit

Plum

Peach

Nectarine

Stem

Plum

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Polymorphism

Virtual Base Classes

Problem:

Fruit has a stem data member
Peach and plum each inherit a stem member from Fruit
Nectarine inherits a stem member from each
Could resolve using the scope operator

Plum::stem

Peach::stem

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Polymorphism

Virtual Base Classes

Solution:

Declare Fruit as a virtual base class

Result:

Only a single copy of the base class in the derivation hierarchy.

Only a single copy of all inherited data members.

Subsequent derivations point to shared members.

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Polymorphism

Virtual Base Classes - Specification

Syntax

class DerivedClass : virtual accessSpec BaseClass

DerivedClass - The class being derived

BaseClass - The parent class

Specification - Specify base class member access

public

protected

private

The keyword virtual identifies BaseClass as a virtual base class of

DerivedClass.

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Polymorphism

Virtual Base Classes - Implementation

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B Data Members

A Data Members

B Data Members

Virtual Derivation

Non-Virtual Derivation

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Polymorphism

Virtual Base Classes - Access Protection

When there are multiple paths from a common root …

….the most public path dominates.

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Fruit

Plum

Peach

Nectarine

virtual public

virtual private

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Polymorphism

Virtual Base Classes - Initialization

A virtual base class is initialized by the most derived class.

Initialization Order:

1. Constructors for any virtual base class(es).

2. Constructors for any non-virtual base class.

3. The most derived class must provide initialization values.

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Polymorphism

Virtual Base Classes - Initialization

Specify class E…

class E : public D, public C, public virtual F

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A

virtual

B

C

D

E

F

virtual

virtual base class

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How C++ implements late binding

Compiler takes care of the virtual function mechanism

Run-time polymorphism is achieved when access is through a base-class pointer (or reference)

Compiler creates a single table (called the VTABLE) for each class

Compiler places the addresses of the virtual functions

Pointer to the VTABLE for an object: VPTR (vpointer

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Storing type information

No explicit type information stored in any of the classes.

How the type is determined at run time?

The type information is hidden.

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An Example

Instrument; play(), what(), adjust()

Wind

Percussion

Stringed

Brass

Woodwind

Demonstration

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Visualizing virtual functions

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Visualizing virtual functions

Begins with the Instrument pointer

Compiler can pick the VPTR of that object

VPTR points to the starting address of the VTABLE

Compiler knows adjust( ) function is at the location VPTR+2

“Call the function at VPTR+2”

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The Virtual Attribute Is Inherited

When a virtual function is inherited, its virtual nature is also inherited.

Base class

Virtual function

Derived class 1

Virtual function

Derived class 2

Virtual function

No matter how many times a virtual function is inherited, it remains virtual

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Inheritance and the VTABLE

Compiler creates a new VTABLE for your new class

For non-overridden virtual functions, inserts the addresses using the base-class function addresses

What happens when you inherit and add new virtual functions in the derived class? (Demonstration)

RTTI: Run Time Type Identification

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Hierarchical Virtual Functions

Base class

Virtual function

Derived class 1

Virtual function

Derived class 2

Virtual function

Case 1

Base class

Virtual function

Derived class 1

Virtual function

Derived class 2

Case 2

Base class

Virtual function

Derived class 1

Virtual function

Derived class 2

Case 3

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Overheads of virtual functions

https://www.cs.ucsb.edu/~urs/oocsb/papers/oopsla96.pdf