CS204 – Advanced ProgrammingCS204 – Advanced Programming

Pointers and Linked ListsPointers and Linked ListsPart 1: BasicsPart 1: Basics

Representation of DataRepresentation of DataAll data in a the computer’s memory is represented as a sequence of bits:

Bit : unit of storage, represents the level of an electrical charge. Can be either 0 or 1. 0 1

Byte (a.k.a Octet): 8 bits make up a byte, and a character is one byte

A byte can represent 256 (0…255) different symbols:

27 26 25 24 23 22 21 20

0 0 0 0 0 0 0 0 0 //binary representation and corresponding integer values

0 0 0 0 0 0 0 1 1

0 0 0 0 0 0 1 0 2

0 0 0 0 0 0 1 1 3

...

1 1 1 1 1 1 1 1 255

Word: typical data size used in computer– 32 bits on some old computers (maybe in use)– 64 bits on most current modern computers

Hex Numbers Hex Numbers

Hexadecimal numbers: - Each `digit` can take values: 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F

corresponding to values in: 0 to 15 (0-9, A = 10, B=11, ... F=15)

- 4 binary digits are one hex digit

e.g. 2435af00

0 0 1 0 | 0 1 0 0 | _ _ _ _ | _ _ _ _| _ _ _ _ | _ _ _ _ | _ _ _ _ | _ _ _ _

Sometimes written as 0x2435af00

Hex Numbers: ctd.Hex Numbers: ctd.

Representation and rules of arithmetic is similar to decimal, but do not forget that the number base is 16.

0x2435AF00 + 1 = 0x2435AF01

0x2435AEFF + 1 = 0x2435AF00

MemoryMemory

What happens when we define variables:char c = 'A'; 

int n = 5;         0x2435af00

0x2435af01

...

c 0x2435af00 n 0x2435af01

.

.

.

Symbol table

Memory

01000001

00000000000000000000000000000101

c

n

Code of 'A'

5 in binary

PointersPointers

A pointer is a variable to store an “address in computer memory”Thus it points to a memory location

Purpose:– Basis for implementing linked data structures linked lists, trees, graphs...– Provide dynamic memory allocation, variable size arrays

...

Memory

01000001

00000101

0x2435af00

c

n

p

7

OverviewOverview

There are a couple of things you can do with pointers:– Declare one: int * ptr;

char * pchar;

– Store a value in it: ptr = &counter;

– Dereference one: cout << *ptr;

(Access the value stored where the pointer points to)

cout << (*ptr).Day(); cout << ptr->Day();

Now we will see them in detail.

Pointers: Pointers: definitiondefinition

A pointer is defined as: type * pointer_variable;

e.g. char * ptr;

ptr_variable does not store a type value, but stores the address of a memory location that

stores a type value

char c = 'A'; 

char *p;

char *q = NULL    

You can never know where an unititialized pointer points (i.e. the address of the memory location that it keeps)

“Null” is defined in standard header files to mean “nowhere” or ”nothing”. It is the initial value for a variable that does not point to anywhere. Normally this value is 0.

?

c A

what happens in memory p

q

Pointers: Pointers: “address of” operator (&)“address of” operator (&)

You can store the address of another variable in a pointer variable:

char c = 'A';  char * p; p = &c;    

Assigning an existing variable’s address is not the main use of pointers; but, this is useful in explaining the concept of address storage.

0x2435af00

c A

pwhat happens in memory

.

.

.

Means address of c

Actually stores the code of 'A'

Pointers: Pointers: dereferencingdereferencing

*pointer-variablederefence (or indirection) operator. It gives the content of memory location pointed by the pointer-variable

char c = 'A';  char * p; p = &c;

cout << *p; prints the value of memory location pointed by p. This is Aand this is the same of the value of regular variable c.

just like “cout << c;” prints the value of the char variable

    

c A

pwhat happens in memory

.

.

.

0x2435af00

PointerPointerss: : assignmentassignment

A pointer can be assigned to another pointer of the same type.

Assume we have done as before: double n;

double * p;

p = &n;

*p = 17.5; // memory location pointed by p contains 17.5

Now you can assign p to another pointer variable:double *q;

q = p; // q and p points to the same location in memory

17.5

p

q

17.5 n

np

Pointers: definitionPointers: definition(what happens behind the scenes)(what happens behind the scenes)

char c = 'A';    

char *p;    

0x2435af00

0x2435af01

.

.

.

A

?

c 0x2435af00 p 0x2435af01

.

.

.

Symbol table

Memory

cp

//a pointer uses 4 bytes of memorywhen you have 32-bit adresses

PointerPointerss: address of a variable: address of a variable (what happens behind the scenes)(what happens behind the scenes)

char c = 'A';    

char *p;    

p = &c; // p now points to c.  

.

.

.

A0x2435af00

c 0x2435af00 p 0x2435af01

.

.

.

Symbol table

Memory

0x2435af00 0x2435af01

c A

p

Alternative and more meaningful view:

PointerPointerss: dereferencing: dereferencing (what happens behind the scenes)(what happens behind the scenes)

char c = 'A';    

char *p;

p = &c; // p now points to c.  

*p = 'B';

.

.

.

A B0x2435af00

c 0x2435af00 p 0x2435af01

.

.

.

Symbol table

Memory

0x2435af00 0x2435af01

//unchanged

c A B

p

Alternative and more meaningful view:

Some RemarksSome Remarks

What happens if you try to assign a string/int/double expression to a pointer variable?– e.g. double *q;

q = 123.45;

:syntax error

What happens if you try to access a memory location pointed by a pointer that is not initialized?– e.g. double *q;

cout << *q << endl;

:a run-time (application) error occurs

What happens if you display the value of a pointer?– e.g. cout << q << endl;

:it displays the value of q, which is the address where it points to.

Dynamic memory allocation with newDynamic memory allocation with new

Dynamic memory allocation using new statementnew type – allocates enough memory from heap (a special part of memory reserved

for dynamic allocation - will discuss later) to store a type value– also returns the address of this memory location– need to assign this address to a pointer variable for processing

Exampledouble *p; //a pointer for double type, but currently points nowhere

p = new double; // memory is allocated to store a double value, but

//currently not initialized. p now points to that location

?p

p ?

Pointers with user-defined types/classesPointers with user-defined types/classes

You can have pointers for any type:– built-in or user-defined types, classes and structs– E.g. int, double, char, string, robot, dice, date, …

You can dynamically allocate memory for any type; note that the class constructor is called automatically to construct the object:

myClass * classPtr;

classPtr = new myClass;

Pointer to Class MembersPointer to Class Members

Date *p_date; //preferred naming - starts with p

Date *p1 = new Date; //p1

Date *p2 = p1; //p2Date tomorrow = *p1 + 1; //tomorrow

int month = (*p1).Month(); //month

int day = p1->Day(); //day

16/2/2015

17/2/2015

2

16

ptr-> is a shorthand for (*ptr). if ptr is a pointer to a struct or a class

Dynamic memory allocation with Dynamic memory allocation with newnew – – Allocating Multiple Variables Allocating Multiple Variables

int *p1, *p2;

new keyword dynamically allocates enough memory for a single int, and returns its address, which is stored in p1

p1 = new int;

In general, the new keyword dynamically allocates enough memory for the following type and count. Pointer p2 points to the first element of the list. This is how we generate DYNAMIC ARRAYs.

p2 = new int[4];

type countp2

Allocation of multiple variables with Allocation of multiple variables with newnew

Assume that we want to hold a dynamic array to hold vacation dates of varying numbers:

cin >> num_vacations;Date * vacations = new Date[num_vacations];

The allocated memory is a contiguous memory than can hold num_vacations dates. Notice that you can access it like an array (it does not change or advance vacations as a pointer; just computes an offset from where vacations points to):

Date today;

vacations[0]= today;

Vacations[1]= today + 3;

21

Allocation of multiple variables with newAllocation of multiple variables with new

Similarly, we can have a pointer to a list of 100 integers:

int * primenumbers = new int[100];

//This offset operation does not mess up the pointer

primenumbers[0]=2;

primenumbers[1]=3;

cout << "First prime is " << primenumbers[0];

Manipulation using pointersManipulation using pointers

You can also manipulate such an array using pointer syntax.

int * pa = new int[100];

pa[0] and *pa are the same things

What about pa[1]? It is the same as *(pa+1)

In general, ptr+x means, the xth memory location (of the type of ptr) in the memory after ptr.

What about pa[500]?Try and see! Also try other values larger than 99!

22

Pointers for Implementing Pointers for Implementing Linked Data StructuresLinked Data Structures

Linked ListsLinked Lists

Built-in Arrays: - too much or too little memory may be allocated+ ease of use- direct access - inserting an element into a sorted array may require shifting

all the elements - deleting an element requires shifting

Vectors : +/- may be resized but inefficient- still some space wasted- inserting an element into a sorted array may require shifting

all the elements (also for deletion)

Note: In CS201 of previous years, Tapestry's Tvector class was being used instead of standart Vector class. Basically they are the same. You can use whatever you want.

Linked ListsLinked Lists

Linked lists: + dynamic memory allocation+ insertion/deletion is cheap (no shifting)- more cumbersome to program with- one extra pointer is needed per element

head

Introduction to linked lists: Introduction to linked lists: definitiondefinition

Consider the following struct definition

struct node //node is a user given name

{

string word;

int num;

node *next; // pointer for the next node

};

node * p = new node;

? ?

num word next

p ?

27

ReminderReminder: structs: as data aggregates: structs: as data aggregates

see Tapestry Chp. 7 pp. 330-

If you need multiple arrays with tied elements (e.g. grades[MAXCLASS_SIZE], names[MAXCLASS_SIZE]...) or in general, any “tied” data, you should consider using a data structure called struct:

struct point

{

double x_coord;

double y_coord;

};

point p1, p2;

p1.x_coord = 10.0; //access members using the dot notation

p1.y_coord = 0.0;

...

Very similar to classes - but no member functions. You should use structs rather than classes only when you want to use them as data aggregates, without any member function (you may have a constructor though, see the next slide).

28

Structs with Structs with constructors constructors If you define one or more constructors:struct point

{double x;double y;

//default constructorpoint::point(){

x = 0;y = 0;

}

//constructorpoint::point(int x_coord, int y_coord)

: x (x_coord), y (y_coord)

{ //nothing more to initialize}

};

Instead of:

point curve[100];

curve[0].x = 0;curve[0].y = 0;curve[1].x = 7;curve[1].y = 12;...

You can use:

point curve[100];

curve[0] = point (0,0);curve[1] = point (7, 12);

General Remarks: If no constructor is used, it is OK.If you want to have a constructor with parameter, also write a default constructorIf there is constructor, creating a struct (dynamic or normal) variable automatically calls default constructor (e.g. curve has 100 elements, all are (0,0) when created)

Introduction to linked lists: Introduction to linked lists: inserting a nodeinserting a node

Without a constructor: With a constructor:

node *p; node * p;p = new node; p=new node(5,"Ali",NULL);

p->num = 5; p->word = "Ali"; p->next = NULL

5 Ali

num word next

p

30

Updated node struct with constructorUpdated node struct with constructor

In order to use the struct as in the previous slide, we need to add a constructor to it:

struct node //node is a user given name{

int num; string word;

node *next; // pointer for the next node

//default constructor; actually does not initialize //anything but should exist node::node() { }

//constructor with 3 parameters node::node(int n, string w, node * p)

: num(n),word(w),next(p) {};};

Introduction to linked lists: Introduction to linked lists: adding a new nodeadding a new node

How can you add another node that is pointed by p->link?

node *p; p = new node(5,"Ali",NULL);

node *q;

5 Ali

num word next

?p

q

Introduction to linked listsIntroduction to linked lists

node *p;

p = new node;

p = new node(5,"Ali",NULL);

node *q;

q = new node;

5 Ali

num word next

? ? ?

num word next

?

q

p

Introduction to linked listsIntroduction to linked lists

node *p, *q;

p = new node;

p = new node(5,"Ali",NULL);

q = new node;

q->num=8; //I can still access fields one by one

q->word = "Veli";

5 Ali

num word next

? 8 Veli

num word next

?p

q

Introduction to linked listsIntroduction to linked lists

node *p, *q;p = new node;p->num = 5; p->word = "Ali";p->next = NULL;

q = new node;q->num=8;q->word = "Veli";q->next = NULL;p->next = q;

5 Ali

num word next

? 8 Veli

num word next

p

q

Linked Lists: Typical FunctionsLinked Lists: Typical Functions

Printing an existing list pointed by head:

struct node {    int info;    node *next;

. . . //constructors};

node *head, *ptr;//list is filled here...//head points to first node

head

End of a linked list should point to NULL

ptr = head;while (ptr != NULL) { cout << ptr ->info << endl; ptr = ptr->next;}

Linked Lists: Typical FunctionsLinked Lists: Typical Functions

Adding a node to the end of the list:

void Add2End(node * tail, int id){ node *nn = new node(id,NULL); tail->next = nn; //This added the new id to the end of the list, //but now tail also needs updating – how? //we could return the new tail from the function: node * Add2End(node * tail, int id) //and let the caller do the update //or we could make tail a reference parameter and update it here… //but we left it as it is right now}

head

tail

Linked Lists: buildingLinked Lists: building//Modified from strlink.cpp in Tapestry code//TASK: Put contents of storage into a linked list,struct node {   

int info;    node *next;

};

int storage[] = {1,2,3,4};node *head = NULL;node *temp = NULL;

for (int k=0; k < 4; k++) {

temp = new node();temp->info = storage[k];temp->next = head;head = temp;

}

What happens as a result of thiscode’s execution? Let's trace on the board

Linked Lists: buildingLinked Lists: building

You better use a constructor to insert data more compactly and in a less error-prone fashion – like this.

// Modified from strlink.cpp in Tapestry codestruct node {   

int info;    node *next;

node::node () {}node (const int & s, node * link)

: info(s), next (link){}

}; int storage[] = {1,2,3,4};node *head = NULL, *temp = NULL; for (int k=0; k < 4; k++) {

temp = new node (storage[k], head);temp = new node();temp->info = storage[k];temp->next = head;head = temp;

}

The codes in the last 3 slides are in ptrfunc.cpp, but this file also contains some features that we

have not seen as of now. Please do not get confused.

39

Stack and HeapStack and HeapScope and LifetimeScope and LifetimeExtern and Static VariablesExtern and Static VariablesFreeing Memory Allocated by NewFreeing Memory Allocated by New

40

Static vs. Dynamic Memory AllocationStatic vs. Dynamic Memory AllocationAutomatic (ordinary) variables: Normal declaration of variables within a function

(including main): e.g. ints, chars that you define by “int n;” , “char c;” etc..

– C++ allocates memory on the stack (a pool of memory cells) for automatic variables when a function begins, releases the space when the function completes.

– The lifetime of an automatic variable is defined by its scope (the compound block in which the variable is defined). After the block finishes, the variable’s location is returned to memory.

• A block is any code fragment enclosed in an left curly brace, {, and a right curly brace, }.

Dynamic variables: Allocated by the new operator, on the heap (a storage pool of

available memory cells for dynamic allocation). E.g. p = new int[100];

– new returns the address of the first element allocated (this is generally assigned to a pointer variable)

– System will return NULL if there is no more space in heap to be allocated. – Programmer should use delete to release space when it is no longer needed. – The lifetime of a dynamic variable is until they are explicitly deleted

41

Local Variables vs Global VariablesLocal Variables vs Global Variables• A block is any code fragment enclosed in an left curly brace,

{, and a right curly brace, }.

• Variables are categorized as either local or global solely based on there they are declared: inside a block or outside of all blocks. – Local variables are declared in a block. – Global variables are declared outside of all blocks.

• The scope of a local variable is the block in which the variable is declared.

• A global variable is not limited in scope. This type of variable is visible to every module in the project. – A commonly complained shortcoming of C++ is the exposure created

by using global variables. – This situation is usually avoided by prohibiting the use of global

variables and instead passing information between modules in the form of function/method parameters.

Extern variablesExtern variables• We said that a global variable is visible to every module in the

project– But this is not automatic; you cannot simply use a global variable defined

in say student.cpp in another cpp (say classes.cpp) of the same project

• There is a mechanism to reach a global variable declared in another cpp file; using extern variable.

• The actual definition of the global variable remains as is– This is where the memory is allocated for that variable

• In the cpp file that you will reach the global variable defined outside of this cpp– You have to redefine the same global variable with preceeding extern

keyword.– In this way, you do not allocate a new memory location, but inform the

compiler that the actual definition of the variable is outside of that cpp file.

• See externdemo1.cpp and externdemo2.cpp42

43

Static variablesStatic variablesStatic Local Variables• A variant of the 'normal' local variable is the static local. When the

keyword static preceeds to the variable declaration, the lifetime of the variable becomes the entire execution of the program. – The compiler to preserve the value of the variable even when it goes out

of scope.– When program execution reenters the block in which the variable is

declared, the variable still has the value it had when execution last left that block.

Static Global Variables• A variant of the 'normal' global variable is the static global. Static

global variables are visible to all methods/functions in the module (i.e. .cpp or .h files) where the variable is declared, but not visible to any other modules in the project. This strategy greatly reduces the opportunities for logic errors in larger programs. By doing this sharing information within a module is still possible. As long as modules are kept small and manageable, this strategy is useful.

44

Heap/Stack: usage overviewHeap/Stack: usage overview

Stack (also called as runtime stack)• All automatic (ordinary) variables use a special part of memory called the runtime

stack• The stack is useful for storing context during function calls, which is performed

automatically and transparent to the programmer.  – When a function, say dothis, is called in another function, say dothat, the function

dothat simply pushes all its local variables (its context) onto the stack before passing the control to dothis. When the function dothis is completed, control returns to dothat but beforehand, dothat pops the context off the stack.

 Heap• The heap is basically rest of memory that the program has.  In that sense, it is

often the largest segment in a program.    • Dynamic variables are allocated on the heap and memory allocated for them

stays until explicitly freed (deleted).

• The memory area used for static and global variable allocation is basically part of heap, but mostly the area used for dynamically allocated variables is separated from static/global

– This is compiler dependant See stackheapaddress.cpp

45

Heap/Stack Heap/Stack

Where are pointer variables stored, stack or heap?

Pointer variables (themselves), just like the normal built-in variables and objects (int, double, string, vector, etc.) also use up memory, but not from the heap– they use the run-time stack

46

Pointers:Pointers: Delete Delete

The statement to de-allocate a memory location and return to the heap is:delete PointerVariable;

– the memory location pointed by PointerVariable is now returned back to the heap;

– this area now may be reallocated with a new statement

– careful: PointerVariable still points to the same location, but that location is no longer used. This may cause confusion, so it is a good idea to reset the pointer to NULL (zero) after deleting.

e.g. p = NULL;

47

Freeing allocated memory with Freeing allocated memory with deletedelete

Date *p1 = new Date(); p1

Date *p2 = p1; p2

...

delete p1; //We need to delete memory allocated with new

delete p2;

//Deleting (freeing) previously freed memory possibly

//causes a crash or a corrupt program depending on the

// compiler and platform

25/02/2013

48

Pointers to variables on the Pointers to variables on the stackstack

Can we have a pointer to point a variable that is not dynamically allocated?- using the & (address of) operator

- such variables are not allocated from heap!

• that is why their addresses are not close to dynamically allocated variables

int num;

int *ptr;

num=5;

ptr = &num; //ptr contains the address of num;

cout << *ptr << endl;

What is output?

49

QuestionQuestion

int n;

int * p_temp = &n;

Do we need to delete p_temp?

No. It points to a stack variable.

What happens if we delete?

Most likely a crash or corrupt program, depending on the compiler.

50

Memory allocation with the Memory allocation with the newnew operator operatorPoints to be careful forPoints to be careful for

Dice * MakeDie(int n)

//return pointer to n sided object

{

Dice nSided(n);

return &nSided;

}

Dice * cube = MakeDie (4);

Dice * tetra = MakeDie (6);

cout << cube->NumSides();

Warning message: address of local variable returnedWhat is the problem here?

51

Memory allocation with the Memory allocation with the newnew operator operator

What is the problem?

Dice * MakeDie(int n)

//return pointer to n sided object

{

Dice nSided(n); nsided

return &nSided;

}

Dice * cube = MakeDie (4); cube

Dice * tetra = MakeDie (6);

cout << cube->NumSides();

Returning a pointer to a variable on the stack (with the & operator),

is wrong!

no longer exists after the

function returns

52

Memory allocation with the Memory allocation with the newnew operator operator

Solution:Dice * MakeDie(int n)

//return pointer to n sided object

{

Dice * dice_ptr = new Dice (n); dice_ptr

return dice_ptr;

}

Dice * cube = MakeDie (4); cube

Dice * tetra = MakeDie (6);

cout << cube->NumSides();

//Is there any problem left?