lecture 15

Lecture 15

C++ Multiple Inheritance Operator overloading Templates Pointers Dynamic memory allocation Standard Template Library

Containers Iterators Algorithms

Lecture Schedule

Mon 5/11/01 (Bjoern) VisualWorks/Smalltalk

Mon 12/11/01 (Frank) C++ Game playing (lab4) Course evaluation

Tue 20/11/01 (Bjoern) VisualWorks/Smalltalk

Inheritance

super class

Feature B

Feature A

subclass

Feature B

Feature A

Feature C

Feature D

Inheritance

class Date{ public: // visible outside class scope int Year(); … protected: // visible to sub-classes but hidden from rest of the world int day, month, year;};class DateTime : public Date{ public: int Hours(); … private: // only visible to class DateTime int hours, minutes};

Multiple Inheritance

base class A

Feature B

Feature A

subclass

Feature B

Feature A

Feature C

Feature D

base class B

Feature D

Feature C


class Date{ public: int Year(); private: int day, month, year;

};class Time{ public: int Hours(); private: int hours, minutes;};


class DateTime : public Date, public Time

{

public:

DateTime(int d, int m, int y, int h, int mi);

…

};

DateTime::DateTime(int d, int m, int y, int h, int mi)

: Date(d,m,y), Time(h, mi)

{

}

Ambiguity in Multiple Inheritance

class Date{ void add(int days);};class Time{ void add(int minutes);};Class DateTime : public Date, public Time {};

DateTime dt(13,2,1998,23,10);dt.add(3); // ambiguous -- will not compiledt.Date::add(4); // uses add of class Datedt.Time::add(5); // uses add of class Time

Ambiguity in Multiple Inheritance

class A { public: void F(); };

class B : public A { …};

class C : public A { …};

class D : public B, public C {};

D d;

d.F(); // ambiguous - won´t compile

class A

class B class C

class D

diamond shapedinheritance tree

Overloading Operator

Operator overloading is a useful feature of object oriented programming Operator overloading allows it to give normal C++

operators such as +,-,==,< additional meanings It makes statements more intuitive and readable for example:

Date d1(12,3,1989);

Date d2;

d2.add_days(d1,45);

// can be written with the + operator as

d2=d1+45;

Operator Overloading

The name of an operator function is the keyword operator followed by the operator itself.class complex

{

double re, im; // re and im part of a complex number

public:

complex (double r, double i) : re(r), im(i) {}; //constructor

complex operator+(complex c); // operator function

};

complex c1(2.2,3.0); // instantiate complex c1

complex c2(1.0,-4.5); // instantiate complex c2

complex c3=c1+c2; // shorthand notation for c1 + c2

complex c4=c1.operator+(c2); // explicit call

Overloading Unary Operators

class Date{ Date& operator++(); // prefix increment operator}Date& Date::operator++ (){ if (++day > days_in_month()) { day=1; if (++month > 12) {

month=1; year++; } } return *this;} Date d1(31,12,1999);++d1; // results in 1.1.2000

Overloading Unary Operators

class Date{ Date operator++(int); // postfix increment operator};Date Date::operator++ (int){ Date old(*this); if (++day > days_in_month()) { day=1; if (++month > 12) {

month=1; year++; } } return old;} Date d1(31,12,1999); Date d2;d2=d1++; // assigns 31.12.01 to d2, d1 becomes 1.1.2000

Overloading Binary Operators

class Date{ Date operator+(int days) const; };

Date Date::operator+(int days) const; { Date tmp=*this; // copy object for (int i=0; i < days; i++) ++tmp; return tmp; }

Date d1(1,4,1999);Date d2=d1+25; // results in 26.4.2000


class Date{ Date& operator+=(int days); // must be reference as += modifies // the left hand argument};

Date& Date::operator+=(int days) // return type reference to object { for (int i=0; i < days; i++) *this++; return *this; // return reference to object }

Date d1(1,4,1999);d1+=25; // results in 26.4.2000

Overloading Relational Operators

class Date{ bool operator==(Date d) { return (day==d.day) && (month=d.month) && (year==d.year); }; bool operator<(Date d) { if (year < d.year) return true; else if (year==d.year) && (month < d.month) return true; else return (month==d.month) && (day < d.day); };};


int Date::operator-(Date d) const { int days=0; if (*this > d) while (*this != ++d) days++; else while (*this != --d) days--; return days; } Date d1(24,4,1988);Date d2(13,3,1998);int diff = d1-d2; // diff = 42

Templates

A template is a place-holder for an arbitrary built-in or user-defined data type Templates make is possible to use one function or class to handle many different data types Function templates allow a parameter to assume an

arbitrary data-type Class templates allow a member data to assume an arbitrary data-type Templates are another example for polymorphism

Function Templates

int max(int a, int b) // one max function for int{ if (a>b)

return a; else

return b;}

double max(double a, double b) // max function for double{ if (a>b)

return a; else

return b;}

Function Templates

template <class Type> // class Type placeholder for concrete data type

Type max( Type a, Type b) // substitute templateType for concrete type

{

if (a>b) return a; else return b; // identical code

}

void main(){ int a=3, b=2; Date d1(17,5,1998); // assume operator > is defined for class

Date Date d2(23,6,1997); int c=max(a,b); // template T replaced with int char z=max(’f’,’q’); // template T replaced with char Date d3=max(d1,d2); // template T replaced with date}

Function Templates

template <class T>T max(T a, T b)

{ … };

int max(int a, int b){ … };

int i1,i2,i3;i3=max(i1,i2);

char max(char a, char b)

{ … };

char c1,c2,c3;c3=max(c1,c2);

Date max(Date a, Date b)

{ … };

Date d1,d2,d3;d3=max(d1,d2);

one function templatein source file

argument type determines function instantiation

Class Templates Array.h

template <class Type> // template class Type

class Array // array of arbitrary data type

{

public:

Array(unsigned int sz);

Type& operator[](unsigned int i); // returns a reference to i-th element

Type operator()(unsigned int i); // returns the value of i-th element

private:

static int max_size=100;

unsigned int size;

Type array[100]; // placeholder for concrete data type

};

Class Templates Array.C#include ”Array.h”template class<Type> Array<Type>::Array(unsigned int sz) { if (sz > max_size)

size=max_size; else size=sz;}template class<Type> Type& Array<Type>::operator[](unsigned int i) { if (i<size)

return array[i];} template class<Type> Type Array<Type>::operator()(unsigned int i) { if (i<size)

return array[i];}

Class Templates

Array<double> x(20); // instantiates a double array of size 20

Array<Date> dates(10); // instantiates a Date array of size 10

Date christmas(24,12,2001);

x[10]=5.7; // operator [] returns reference can be used on rhs

dates[3]=christmas;

x[11]=x(10)+3.4; // operator () returns value can only be used on lhs

x[0]=1.2;for (int i=1; i<20;i++) x[i]=x(i-1)*1.2;for (int i=0; i<10;i++) dates[i]++; // increment all dates by one calendar day

Class Templates G++ has two compiler options

-fexternal-templates -fno-external-templates

The later compiler directive is the default one and you need to arrange for all necessary instantiations to appear in the implementation file (.C) for example in Array.C

#include Array.htemplate class<Type> Array<Type>::Array(unsigned int sz) {...}...template class Array<double>; // explictly instantiate Array<double>template class Array<Date>; // explictly instantiate Array<Date>

Pointers

Pointers Pointers and Arrays Pointers and function arguments Dynamic memory management New and delete

Pointers Pointers are used to:

Access array elements Passing arguments to functions when the function needs

to modify the original argument Passing arrays and strings to functions Obtaining memory from the system Creating data structures such as linked lists

Many operations that require pointers in C can be carried out without pointes in C++ using reference arguments instead of pointers, strings instead of char arrays or vectors instead of arrays

Some operations still require pointers, for example creating data structures such as linked lists and binary trees

Pointers Each variable in a program occupies a part of the

computer’s memory, for example an integer variable occupies 4 bytes of memory

The location of the piece of memory used to store a variable is called the address of that variable

An address is some kind of number similar to house numbers in a street that is used to locate the information stored in that particular variable

10101011000011111000100011100011

0x10540x10550x10560x1057

001110110x1058101111000x1059110011000x1060

int i; address of i

char c;address of cshort s; address of s

Pointer Variables

A pointer variable is a variable that holds address values

Each data type has its own pointer variable, pointer to int, pointer to double, pointer to char, …

C/C++ uses the address-of operator & to get the address of an variable

C/C++ uses the indirection or contents-of operator * to access the value of the variable pointed by

int i=17;

int* ptr; // defines a pointer to an integer variable

ptr= &i; // assign the address of x to pointer

cout << *ptr << endl; // prints contents of variable i

Pointer Variables

0x1054int i;

17int *ptr;

ptr=&i;

address of

cout << *ptr << endl;

contents

of

Pointer Variables

int v; // defines variable v of type int

int w; // defines variable w of type int

int *p; // defines variable p of type pointer to int

p=&v; // assigns address of v to pointer p

v=3; // assigns value 3 to v

*p=7; // assigns value 7 to v

p=&w; // assigns address of w to pointer p

*p=12; // assigns value 12 to w

Using the indirection operator *p to access the contents of a variable is called indirect addressing

or dereferencing the pointer

Pointers and Arrays

There is a close association between pointers and arrays Arrays can be accessed using pointers The name of an array is also a constant pointer to the data type

of the elements stored in the array

int array[5] = { 23, 5, 12, 34, 17 }; // array of 5 ints

for (int i=0; i< 5; i++)

cout << array[i] << endl; // using index to access elements

for (int i=0; i< 5; i++)

cout << *(array+i) << endl; // using pointer to access elements

// array is of type pointer to integer

Pointers as Function Arguments

C/C++ offers three different ways to pass arguments to a function

by value : void f(int x); by reference : void f(int& x); by pointer : void f(int* x);

In passing by value the function obtains only a local copy of the variable, so that changes to the local variable have no impact on the argument with which the function was invoked

In passing by reference and passing by pointer the function manipulates the original variable rather than only a copy of it

Pointers as Function Arguments

void swap( double& x, double& y){ double tmp=x; x=y; // access variable by its alias name y=tmp;}void swap( double* ptr1, double* ptr2){ double tmp=*ptr1; *ptr1=*ptr2; // de-referencing pointer *ptr2=tmp;}double a=3.0;double b=5.0swap(a,b); // call by reference to variables a andswap(&a, &b); // call by pointer using the addresses of a and b

BubbleSort

void bsort (double *ptr, int n) // pass pointer to array and

// size of array as arguments to bsort

{

int j,k; // indices to array

for (j=0; j<n-1; j++) // outer loop

for(k=j+1; k<n; k++) // inner loop starts at outer

if(*(ptr+j) > *(ptr+k))

swap(ptr+j,ptr+k);

}

double array[6] = { 2.3, 4.5, 1.2, 6.8, 0.8, 4.9 };

bsort(array,n); // sort the array

Const Modifiers and Pointers

The use of the const modifier with pointers is confusing as it can mean two things

const int* cptrInt; // cptrInt is a pointer to a const int

You can not the change the value of the integer that cptrInt points to but you can change the pointer itself

int* const ptrcInt; // ptrcInt is a constant pointer to int

You can change the value of the integer that ptrcInt points to but you can not change the pointer itself

Memory Management

In order to create an array in C/C++ you have to know its size in advance during compile time, in other words it has to be a constant

int size;

cout << ”Enter size of array : ”;

cin >> size;

int array[size]; // ERROR size has to be a constant Solution in C++, use vector class from the STL which is

expandable

Memory Management

Date* CreateDate() // allows the user to create a date object{

int day, month, year; char dummy; cout << ”Enter dd/mm/yyyy :”; cin >> day >> dummy >> month >> dummy >> year; Date date(day, month, year); return &date; // ERROR!! Scope of date ends with end of

function}Date *ptr;ptr=CreateDate(); // call CreateDate() to generate a new datecout << ”You entered ” << *ptr << endl; // variable to which ptr points no longer exist , segmentation

fault !!!

Memory Management The new operator in C++ can be used to create objects on the heap that are

”alive” after returning from a function Objects allocated in dynamic memory are called heap objects or to be ”on

free store” and have a permament existence

Date* CreateDate() // allows the user to create a date object{

int day, month, year; char dummy; cout << ”Enter dd/mm/yyyy :”; cin >> day >> dummy >> month >> dummy >> year; Date *tmpptr = new Date(day, month, year); return tmpptr; // returns pointer to heap object} Date *ptr;ptr=CreateDate(); // call CreateDate() to generate a new datecout << ”You entered ” << *ptr << endl; // ok, ptr refers to heap object

Memory Management

New can also be used to allocate blocks of memory The delete operator is used to release the memory

allocated with new once it is no longer needed#include <cstring>

char *str =”This is an old C-style string”;

int len=strlen(str); // computes the length of str

char *ptr; // create a pointer to char

ptr = new char[len+1]; // set aside memory string + ’\0’

strcpy(ptr,str); // copy str to new memory

cout << ”ptr=” << ptr << endl;

delete [] ptr; // release ptr’s memory

New Operator in Constructors

class String // user-defined string class{ private: char* str; // pointer to block of characters public: String(char* s) // one-argument constructor { int length=strlen(s); // length of string argument str = new char[length+1]; // allocate memory strcpy(str,s); // copy argument to it } ~String() // destructor { delete [] str; } void Display() { cout << str << endl; }};String mystring=”This is my string of Type String”; mystring.Display();

Pointers to Objects

Pointers can point to objects as well as to built-in data types

Date date; // define a named Date object

date.Set(12,3,1996); // set the date

date.Display(); // display the date

Date *dateptr; // define a pointer to a Date object

dateptr=new Date; // points to new Date object

dateptr->Set(9,12,1999); // set date using -> operator

dateptr->Display(); // display date

(*dateptr).Display(); // works as well but less elegant

Linked List Example A linked list is composed of a chain of elements (links).

Each element contains some data and a pointer to the next element in the list.

In a double linked list, each element also contains a pointer to its predecessor.

nextdata

Element Element Element

nextdata

nextdata

Element Element Elementnextprevdata

nextprevdata

nextprevdata

Linked List Examplestruct link // one element of list{ int data; // data item link *next; // pointer to next element};class linklist{ private:

link* first; // pointer to first link public:

linklist() { first = NULL;} // no argument constructor void push_back(int d); // add new element to the end of list int pop_back(); // delete last element and return data value void push_front(int d); // add new element to the front of list int pop(); // delete first element and return data value

void display(); // display all elements in list}

Linked List Example

void linklist::push(int d) // add data item at the front

{

link* newlink = new link; // create a new link

newlink->data = d; // assign new data d

newlink->next=first; // it points to the next link

first = newlink; // now first points to this link

}

Linked List Exampleint linklist::pop() // remove element at the front{ int tmp_data = first->data; link* tmp=first; first=first->next; delete tmp; // free storage return tmp_data;}int linklist::pop_back() // remove element at the end{ link* tmp=first; while (tmp->next != NULL) tmp=tmp->next; int tmp_data = tmp->data; delete tmp; // free storage return tmp_data;}

Linked List Example

void linklist::push_back(int d) // add data item at the end{ link* newlink = new link; // create a new link newlink->data = d; // assign new data d newlink->next = 0; // points to nil link* tmp=first; if (tmp == NULL) // empty list

first=newlink; else { while (tmp->next!=NULL) tmp=tmp->next; tmp->next=newlink; // it points to the next link }}

Linked List Example

void linklist::display() // display all links

{

link* tmp=first; // set ptr to first link

while(tmp != NULL) // until ptr points beyond last link

{

cout << current->data << ” ”; // print data

current=current->next; // move to next link

}

}

Linked List Exampletemplate <class T>struct link // one element of list{ T data; // data item link *next; // pointer to next element};template <class T>class linklist{ private:

link* first; // pointer to first link public:

linklist() { first = NULL;} // no argument constructor void push(T t); // add data item (one link) T pop(); void display(); // display all links}

Linked List Exampletemplate <class T>void linklist<T>::push(T t) // add element at the front{ link* newlink = new link; // create a new link newlink->data = t; // give it data d newlink->next=first; // it points to the next link first = newlink; // now first points to this link}template <class T>void linklist<T>::display() // display all links{ link* current=first; // set ptr to first link while(current != NULL) // until ptr points beyond last link { cout << current->data << ” ”; // print data current=current->next; // move to next link } }

lecture 15

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