Inheritance

Junaed SattarOctober 22, 2008

Lecture 8

OOP and Inheritance

OOP : the combination of Abstract Data Types (ADTs) with Inheritance and Dynamic Binding

ADTs decompose systems into two-dimensional grids of modules Each module has public and private

interfaces Inheritance decomposes systems into

three-dimensional hierarchies of modules

Formally...

mechanism of reusing and extending existing classes without modification produces hierarchical relationships almost like embedding one class into another

include the names and definitions of another class's members as part of a new class

Why inheritance?

write code to handle certain cases and allows others to write code that handles more specialized cases

partitions a system architecture into semi-disjoint components that are related hierarchically

ability to modify and/or reuse sections of the inheritance hierarchy without disturbing existing code

Overview

A type (called a subclass or derived type) can inherit the characteristics of another type(s) (called a superclass or base type)

A derived type acts just like the base type, except for an explicit list of: Specializations

Change implementations without changing the base class interface

Most useful when combined with dynamic binding Generalizations/Extensions

Add new operations or data to derived classes

Composition

class A {int data;

public:void f(int arg) { data = arg; }int g() { return data; }

};

class B { public: A x;};

int main() {B obj;obj.x.f(20);cout << obj.x.g() << endl;// cout << obj.g() << endl;

}

The Non-OOP way

How can we write obj.g(() legally? Rewrite another data structure:

Basically, copy-paste base code and add the new bits

What if the base code changes? Have to rewrite all the new classes

Inherited version

class A { // Base classint data;

public:void f(int arg) { data = arg; }int g() { return data; }

};

class B : public A{ // Inherited class};

int main() {B obj;obj.f(20);cout << obj.g() << endl;

}

Inherited class...

can add new methods and data members can modify implementation of existing

member functions and data, too technique known as overriding, used in

dynamic binding can be inherited again

Types of inheritance

Two forms based on number of parents:

1.Single Inheritance (SI) only one parent per derived class requires a small amount of run-time overhead

when used with dynamic binding

2.Multiple inheritance More than one parent per derived class Compared with SI, MI adds additional run-time

overhead (also involving dynamic binding)

Production code example

Flow Schematic

Image SourceWebcam

Image SourceFireWire

Image SourceDisk Files

Image SourceMovie Files

Processing

Color Tracking Gesture Recognition Diver Detection Visual Localization

Output to Robot(Image/Faces/Objects)

“Real-world” example #1

The catch?

May create deep and/or wide hierarchies that are hard to understand and navigate without class browser tools

May decrease performance slightly when combined with multiple inheritance and

dynamic binding Without dynamic binding, inheritance has

limited utility And dynamic binding is essentially pointless

without inheritance

“Real-world” example #2

See the class inheritance diagram for RoboDevel

In C++...

The class head is modified to allow a derivation list consisting of base classes class Foo { /* . . . */ }; class Bar : public Foo { /* . . . */ }; class FooBar : public Foo, public Bar { /* . . . */ };

Base and derived class pointers

A pointer to a derived class can be assigned to a pointer to any of its public base classes without requiring an explicit cast: Menu m; Window *w = &m; Screen *ps1 = w;Screen *ps2 = &m;

Casting

How about the other way? pointer casting from base to derived class is invalid (guess why?)

but it can be forced class CBase { };class CDerived: public CBase { };CBase b; CBase* pb; CDerived d; CDerived* pd;

int main(){ pb = &d; // ok: derived-to-base // valid syntax but based-to-derived

pd = (CDerived*)(&b); }

dynamic cast

to stop such disasters from happening, use dynamic_cast

pb = dynamic_cast<CBase*>(&d); // ok: derived-to-basepd = dynamic_cast<CDerived*>(&b);// not ok, and will generate // compiler error!

static cast

the old-fashioned C-way:int k = 10;double d = 10.11;

k = (int)d;//or k = int(d);

The new C++ way:k = static_cast<int>(d);

More static_cast-ing

class CBase { };

class CDerived: public CBase { };

CBase b; CBase* pb; CDerived d; CDerived* pd;

int main(){ pb = &d; // ok: derived-to-base // valid syntax but based-to-derived

pd = static_cast<CDerived*>(&b); }

Practical C++

Common practice to use header files declare class in header (.hh/.hpp/.h) define in source (.cpp/.cc/.cxx)

Benefits? hide implementation detail in source cpp file provide header file as an API to other

developers separate implementation from interface

How?

// this is ImageReader.hh

#ifndef IMAGEREADER_H_

#define IMAGEREADER_H_

class ImageReader{protected:

unsigned char *imageBuffer;std::string *pathString;unsigned iWidth, iHeight, iDepth;static const int FIRST_FRAME;

public:ImageReader();virtual ~ImageReader();bool OpenDirectory( const char *dir );bool GetFrame();bool RewindSequence();int GetFrameNumber();

};

#endif /*IMAGEREADER_H_*/

The source

// this is ImageReader.cc

#include <iostream>

#include "ImageReader.hh" // here's where we include the // declaration

const int ImageReader::FIRST_FRAME = 0; // constant definition

ImageReader::ImageReader(){std::cout << "Image reader loading..\n";}

... // the rest of the class

using the class

// this is main.cc

#include "ImageReader.hh" // here's where we include // the declaration

int main() {ImageReader reader;...

// do our stuff with reader here

}

“Building” the program

Two source files main.cc and ImageReader.cc

manual compilation with g++:

g++ main.cc ImageReader.cc -o mainProgram

Makefile

a simpler way to combine multiple source files with build rules, to produce one binary

very useful, when project contains many files, and many build rules debugging options, optimizations, libraries,

etc etc links to tutorials to makefiles on the

course website