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C++ Overview (I). What is Object Orientated Programming? Approach: Break problem into subgroups of related parts that take into account code and data; organise subgroups into hierarchy; translate to objects…. Objects Encapsulation: combine together code and data: data-hiding

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C overview i

C++ Overview (I)

What is Object Orientated Programming?

Approach: Break problem into subgroups of related parts that takeinto account code and data; organise subgroups into hierarchy;translate to objects…

  • Objects

  • Encapsulation: combine together code and data: data-hiding

  • Polymorphism: same interface, different ways of calling

  • Inheritance: derive specific objects from more general ones

An object is defined by a class; the class combines data and methods;a class defines a new data type… think in objects!

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C++ Overview (II)

Function overloading

A function can represent a concept (e.g. set_value) but it may becalled with different types of data – the compiler sorts out whichvariant of set_value to call

Operator overloading

Any of the operators in C++ (e.g. ‘+’) can be redefined; a classcan define how ‘+’ operates on objects defined by the class


For a group of related concepts (e.g. types of buildings) extractcommon functionality and data into a base class (e.g. building)and derive more specific classes (e.g. house, garage, hotel)

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Classes (I)

In general, a class is declared in a header file and defined in asource file; #include the header file wherever the class is usedin file “andy.h”class Andy {private: int age;public: Andy(); ~Andy(); void setAge(int); int getAge();};

Name of class and new data type

Until otherwise specified, declarations canonly be accessed by methods of class ‘Andy’

Specify that declarations can be accessedby any part of the program

A function that returns an integer:a method of class ‘Andy’

Note the semi-colon

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Classes (II)

In file “andy.cpp”

Constructor function: calledwhenever an object of type‘Andy’ is initialised

Andy::Andy(){ age = 0;}Andy::~Andy()


void Andy::setAge(int _age){ age = _age;}

int Andy::getAge(){ return age;}

Destructor function: calledwhenever an ‘Andy’ objectgoes out of scope (or is deleted)

‘age’ is a private member of class ‘Andy’so must be accessed by a class method

‘::’ is the scope operator; need to tell thecompiler we are defining ‘getAge’ in class‘Andy’ – no ‘::’ means function is not aclass method

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Classes (III)

in file ‘main.cpp’#include <iostream.h>

#include “andy.h”.. Andy andy1, andy2; Andy* andyPtr = 0;


andy2.setAge(99); andyPtr = &andy2; cout << “Andy is aged “ << andy1.getAge() << “ or “ << andyPtr->getAge() << “\n”;..

Variables are declared in just the sameway as if they were, say, integers

Call methods on objects using the‘.’ operator (just like structs)

Call methods through a pointer toan object by using ‘->’ operator;just like with structs

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Classes (IV)

Construction and DestructionCan have many different forms of constructorConstructor and destructor do NOT return a valueGood practice to supply an empty constructor



Andy(char*, int);

Object Assignment and passing to functionsThe compiler automatically does a bitwise copy if an object isassigned (‘=‘) to another object or used as an argument to afunction – can override the automatic behaviour…

Andy a, b;a.setAge(10);b = a;

cout << b.getAge();

void func(Andy aaa);Andy a;func(a);

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Arrays of objects (I)

in file ‘element.h’

class Element {private:

double u, v;

public: double result; Element() : u(0.0), v(0.0) {} Element(double _u, double _v) : u(_u), v(_v) {} double length() { return sqrt(u*u + v*v); }};

Two constructors: one is the ‘empty’ or‘default’ and the other takes two valuesused to initialise private members

Function (aka method) defined in headerfile: called an ‘inline’ function

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Arrays of objects (II)

int j;Element* elem = new Element[100];for (j=0; j<100; j++) { elem[j] = Element(1.0, 0.5);

}for (j=0; j<100; j++) { double len = elem[j].length(); double res = elem[j].result;}Element* ptr = elem;for (j=0; j<100; j++) { double len = ptr->length(); elem++;}

Make new copy of elem[j] byassignment from Elementconstructed with initial values

Call ‘length’ methodAccess public ‘result’ variable

Call ‘length’ method thru pointer

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Overloading (I)

  • One function/method can take many forms

  • Overloaded functions must all return same type of value

  • Compiler decides which form of function to call

class Andy {private: double dVal; float fVal; int iVal;public: void setData(double val) { dVal = val; } void setData(float val) { fVal = val; } void setData(int val) { iVal = val; }


Andy a;a.setData(1.0f);a.setData(42);

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Overloading (II)

  • Overloading operators can change their meaning

  • Can overload any operator

  • Makes for easy to read code

Class Vector {private: double a, b;public: Vector() : a(0.0), b(0.0) {} Vector(double _a, double _b) : a(_a), b(_b) {} Vector operator+(Vector x) { return Vector(a+x.a, b+x.b); }


Vector u(2.0, -2.0), v(-1.0, 1.0), w;w = u + v;

‘+’ operator method of u is calledwith v as the function argument

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Inheritance (I)

  • One class can inherit the public data and methods of another

  • Can build complexity thru using a hierarchy

  • Can access specialised classes thru a pointer to the base class

For instance, a square, a cross and a triangle are all shapes;they can all be represented using differing numbers of verticesCreate a base class called shape and derive different types

class Shape {public: int nvertex; Vertex* vertices; Shape(int, Vertex*); virtual draw();};

Data to represent any of thedifferent shape types

Base class constructor

‘virtual’ function is inherited

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Inheritance (II)

class Square : public Shape {public: Square(Vertex* _v) : Shape(4, _v) {} void draw();};void Square::draw(){ // draw a line loop thru all vertices}

class Cross : public Shape {public: Triangle(Vertex* _v) : Shape(4, _v) {} void draw();};void Cross::draw(){ // draw 2 lines, connecting opposite vertices}

Call the base class constructorbefore constructing ‘Square’

Explicit drawing methods:different from each other

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Inheritance (III)



// declare the Vertex data for each type of shape



Shape* shapes[4];shapes[0] = new Cross(crossData);shapes[1] = new Square(squareData);shapes[2] = new Triangle(triangleData);shapes[3] = new Hexagon(hexagonData);for (int j=0; j<4; j++) { Shape* ptr = shapes[j]; ptr->draw();}



Array of pointers to base class

‘new’ operator allocatesmemory for object andreturns pointer to it;pointer to ‘Cross’ is alsoa pointer to ‘Shape’

The compiler works out which‘draw’ function to call

Example: shape

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Template classes

  • Templates are generic classes that offer particular functionality

  • They ‘wrap’ around other classes

  • Standard template classes are available

Header file from standard libraries

#include <list.h>..list<int> listOfInts;listOfInts.push_back(4);..

listOfInts.sort();list<int>::iterator iter;for (iter=listOfInts.begin(); iter!=listOfInts.end(); iter++) { if (*iter == 4) { cout << “found it!\n”; }


Specify what we want a list of

Add something to the list

‘list’ class uses ‘<‘ and ‘>’ operatorsof ‘int’ to order the list

Processing thru the list

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Array based I/O

Frequently need to write variables into a character string

#include <strstrea.h>#include <fstream.h>char cstring[80];

int filenumber = 11;

ostrstream ostr(cstring, 80);ostr << “datafile” << filenumber << “.dat” << ends;// now can use cstring to open a fileofstream ofile;ofile.open(cstring); // opens a file named “datafile11.dat”

ostrstream object usescstring as a buffer

Write into ‘ostr’ in thesame way as to thescreen or to a file

‘ends’ adds a ‘\0’ to theend of the C string

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C++ Standard Libraries

Some slight confusion, courtesy of Microsoft (?)

#include <fstream.h>

#include <fstream>

using namespace std;

You are allowed to use thistype of header (*.h) sometimes

You should use this type,but need to add the ‘using’line after one or more includes

<cmath> = <math.h>

Main standard header files are:

<algorithm>, <climits>, <cmath>, <cstdlib>,<fstream>, <iostream>, <list>, <string>, <strstream>

There are lots more: see “Library, Standard C++” in MSDN index

You can include C headers (and use C functions) in C++ programs

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The ‘string’ class

  • The standard string class is very useful; saves a lot of work

  • Many (library) functions require C strings (char*)

  • The string class can be used to manipulate and create C strings

#include <string>using namespace std;..string s1(“C++ is my friend”);string s2 = “Andy”;string s3;s3 = s2 + “, “ + s1;char* cs1 = s3.c_str();..

Other methods of class stringinclude finding sub-strings,getting string length, ==, <,>, != and lots more…See “string” and “basic_string”in the MSDN documentation

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Worked example: ‘finite’ (I)

  • Create grid of elements dymanically

  • Initialise elements – random values

  • For each timestep: Each element interacts mathematically with its 4 neighbours to find a result Output result from each element

  • Destroy memory used

‘this’ element


  • Classes required:

  • Grid (to contain and process elements)

  • Element (to store result and data)

  • Index (to help find neighbours)

This element stores the addressesof its neighbouring elements

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Worked example: ‘finite’ (II)

class Element {


double u, v;

Element* leftElemPtr;

Element* rightElemPtr;

Element* aboveElemPtr;

Element* belowElemPtr;


double result;


~Element() {}

void addNeighbours(Element*, Element*, Element*, Element*);

void init(double, double);

void update();

void normalise();

double length();

double sumOfParts();


Some data

Pointer to neighbour

Result can be accessed directly

Set ‘u’ and ‘v’ values

This method does the(daft) computation

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The End



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