More on inheritance Abstract classes Inheritance of constructorsdestructors Types of inheritance - PowerPoint PPT Presentation

Title: More on inheritance Abstract classes Inheritance of constructorsdestructors Types of inheritance


1
More on inheritanceAbstract classesInheritance
of constructors/destructorsTypes of inheritance
2
Inheritance
Inheritance - mechanism of deriving new classes
from old ones. The keywords public, private, and
protected are used as visibility modifiers for
class members.
The keyword virtual is a function specifier that
provides a mechanism to dynamically select at
runtime the appropriate member function from
among base and derived class functions (polymorphi
smmany forms ). There are two types of virtual
functions virtual double area() // normal
virtual virtual double area() 0 // pure virtual
3
Abstract Base Classes
A pure virtual function is one whose body is
undefined. virtual void printName() 0 A pure
virtual function is used to defer the
implementation decision of the function. A class
that has at least one pure virtual function is
called an abstract class (normal classes are
sometimes called concrete classes). Abstraction
is just a design mechanism - a way of
deferring design decisions.
4
Abstract and concrete classes

• An abstract class is any class with one or more
  pure virtual functions - it cannot be
  instantiated
• A class that can be instantiated is called a
  concrete class
• Sub-classes of an abstract class will inherit the
  pure virtual functions in the base class and be
  abstract classes themselves
• If you want to make a sub-class of an abstract
  class into a concrete class, you must over-ride
  the pure virtual function and provide a concrete
  implementation

5
Shape - virtual area()0
ThreeD - virtual volume()0
TwoD
Rectangle
Circle
Sphere
If you do not supply implementations (bodies)
for all the inherited pure virtual function then
this class will remain abstract and
un-instantiable.
6
Abstract Base Classes
Typically the base class in a hierarchy of
classes is an abstract class. (It is only there
to help the designer start the hierarchy - an
abstract class might often contain some data
declarations and maybe some methods, but it is
never intended to actually be instantiated) An
abstract class cannot be used to declare objects
(cannot be instantiated). An abstract class can
be used to declare pointers that can access
objects derived from the abstract base class.
7
Inheritance rules

• A derived class inherits everything from its base
  class except
• Its constructors
• Its destructors
• Its assignment operators
• Its friends

8
Virtual - constructors and destructors Constructo
rs are always dynamic (dont need
virtual.) Destructors should be declared virtual
(if you delete a derived class through a base
pointer.) There can be significant problems if
you dont declare destructors virtual and you try
to delete through the base pointer. (The wrong
memory can be freed up if your classes use
malloc or new etc).
9
Consider a time keeper class hierarchy
class TimeKeeper public TimeKeeper() cout
ltlt "Base ctor ltlt endl TimeKeeper() cout ltlt
"Base dtor" ltlt endl class AtomicClock
public TimeKeeper public AtomicClock()
cout ltlt "AT ctor"ltltendl AtomicClock() cout
ltlt "AT dtor"ltltendl class WaterClock
public TimeKeeper class WristWatch
public TimeKeeper
10
and a piece of code that dynamically allocates
an object of some class derived from
TimeKeeper // Get option from user input at run
time TimeKeeper ptk if (option1) ptk
new AtomicClock() else if (option2) ptk
new WaterClock() else if (option3) ptk new
WristWatch() delete ptk
What destructors are called?
11
Virtual Destructors

• Deleting an object via its base class pointer
  will only call the base class destructor.
• We want to destroy the entire object, ie call the
  derived class destructor, then the base class
  destructor
• How?

12
Make the base class destructor virtual class
TimeKeeper public TimeKeeper() cout ltlt
"Base ctor"ltltendl virtual TimeKeeper() cout
ltlt "Base dtor" ltlt endl Now the destructors
of the base and derived classes are called in the
right order This becomes very important if
memory allocations are done by these classes.
13
When to use Virtual Destruction

• Even if your objects dont have any memory
  cleanup to do and therefore dont need
  anything other than the default destructor, it is
  still important to have virtual destructors

• When storage is freed up (or deleted) the
  address and size of the block of storage is
  needed before it can be returned to the heap.
  The destructor normally supplies this to the
  operating system. Things can crash if this is
  not properly passed.

14
When?

• Generally it is safest always to use a virtual
  destructor - however
• class bit public char c
• Is very tiny - never designed to inherit from this

• In a case like this putting a virtual destructor
  in the class will add four extra bytes - ie
  going from one to five bytes - not really what
  is wanted for something you might want to declare
  a lot of.

15
Base classes as parameter arguments

• Given a base class and a number of sub-classes
  (and sub-subclasses), consider this function
  definition
• void doSomethingWithIt(BaseClass b)
• Any object of any sub-class of base class can be
  passed through - in the same way that base class
  pointers can point to any subclass
• Example suppose we had developed a sophisticated
  algorithm that we want to apply to any
  mathematical function

16
Simple example an algorithm to find the minimum
value of any given function of one variable that
may be declared as double myfunction(double x)
void findMinimum( )
What goes in here?
We could use a pointer-to-function and pass the
pointer as a parameter argument Void
findMinimum(double ()(double) A better way is
to pass through function objects
17
class MathematicalFunction public virtual
double operator() (double x) 0 void
findMinimum(MathematicalFunction fun) //
Implement algorithm here class
MySpecialFunction public MathematicalFunction
// Provide a concrete implementation of ()
You can apply findMinimum to any object of any
subclass of MathematicalFunction
18
MySpecialFunction f findMinimum(f)
f is a function object or functor
See lecture demo functor.cpp See also C FAQ
Lite for discussion on functionoids
19
Polymorphic Function Call

• Consider also a member function for the Shape
  class
• virtual void draw() 0
• And a utility standalone function that places any
  shape on the screen
• void placeShape(Shape s)
• // decide how to place the shape on the
  screen, then invoke the draw method
• s -gt draw()
• The placeShape function sends a message (function
  call) to the whichever shape object is pointed
  to, the correct draw method is then selected
  polymorphically.

20
Inheritance of friends
Friends of base classes are not friends of
subclasses class Base public friend
friendly_function() Class Derived public
Base private
The friends of Base do not have access to private
or protected members in Derived
21
Public, protected, and private inheritance
When deriving a class Derived from another class
Base then
Base
Derived
22
Summary A pure virtual function is a function
whose body is undefined. A class having a pure
virtual member function is an abstract class. An
abstract base class defines the interface for its
derived classes but cannot itself be used to
declare objects. Polymorphism - the ability for
objects of different classes related by
inheritance to respond differently to the same
message (function call). Constructors,
destructors, assignment, and friends are not
inherited. Virtual destructors should be used in
base classes. We can use more elaborate
combinations of the access modifiers when
deriving classes from base classes.