1. CS204 – Advanced Programming Pointers and Linked Lists Part 1: Basics

2. Representation of Data
All 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:
 0 //binary representation and corresponding integer values
 1
 2
 3
...
 128
 129
 254
 255
Word: typical data size used in computer
32 bits on some old computers (maybe in use)
64 bits on most current modern computers

3. Hex Numbers Hexadecimal numbers: e.g. 2435af00 
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, C=12, D=13, E=14, F=15)
4 binary digits are one hex digit
e.g. 2435af 
| | _ _ _ _ | _ _ _ _| _ _ _ _ | _ _ _ _ | _ _ _ _ | _ _ _ _
Sometimes written as 0x2435af00

4. Hex Numbers: ctd.
Representation and rules of arithmetic is similar to decimal, but do not forget that the number base is 16.
0x2435AF = 0x2435AF01
0x2435AEFF + 1 = 0x2435AF00

5. Memory: variables are stored here
What happens when we define variables:
char c = 'A'; 
int n = 5;   
    
0x2435af00
0x2435af01
c x2435af00
n x2435af01
Code of 'A' . c n
Symbol table: variables and
their memory addresses are listed here; Symbol table is not something that you can directly manipulate in program
5 in binary
...
Memory: variables are stored here

6. Pointers
A pointer is a variable to store an “address in computer memory”
Thus it points to a memory location
A pointer is something you can
manipulate in program
char c = 'A'; 
char * p;
p = &c;
Purpose:
Basis for implementing linked data structures linked lists, trees, graphs...
Provide dynamic memory allocation, variable-size arrays
0x2435af00 c n p
...
Memory

7. Overview cout << (*ptr).Day();
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.

8. Pointers: definition 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    
c A
what happens in memory p ? q
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 can be used as the initial value for a variable that does not point to anywhere. Normally this value is 0.

9. Pointers: “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.
Actually stores the code of 'A'
0x2435af00
c A p
what happens in memory .
Means "address of c"

10. Pointers: dereferencing
*pointer-variable
derefence (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
and this is the same of the value of regular variable c.
just like “cout << c;” prints the value of the char variable
    
c A p
what happens in memory .
0x2435af00 A

11. Pointers: assignment
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 n q
17.5 p n

12. Pointers: definition (what happens behind the scenes)
char c = 'A';    
char *p;    
0x2435af00
0x2435af01
c x2435af00
p x2435af01 . A ? c p .
Symbol table
//a pointer uses 4 bytes of memory
when you have 32-bit adresses
Memory

13. Pointers: address of a variable (what happens behind the scenes)
char c = 'A';    
char *p;    
p = &c; // p now points to c.  
c x2435af00
p x2435af01 . A
0x2435af00
Symbol table
0x2435af00
0x2435af01
Alternative and more meaningful view: .
c A p
Memory

14. Pointers: dereferencing (what happens behind the scenes)
char c = 'A';    
char *p;
p = &c; // p now points to c.  
*p = 'B';
c x2435af00
p x2435af01
//unchanged .
A B
0x2435af00
0x2435af00
0x2435af01
Symbol table .
Alternative and more meaningful view:
c A B p
Memory

15. Some Remarks
What happens if you try to assign a string/int/double expression to a pointer variable?
e.g double *q;
q = ;
:syntax error
What happens if you try to access (dereference) 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.
Is it possible to assign an address directly to a pointer?
:yes but we need typecasting, and mostly causes a crash or an unexpected behavior
- e.g double *q;
q = (double *) 0x00EFF8A8;

16. Dynamic memory allocation with new
Dynamic memory allocation using new statement
new 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
Example
double *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

17. Pointers and Dynamic Allocation for 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;

18. Pointer to Class Members
Date *p_date; //preferred naming - starts with p, but not a rule
Date *p1 = new Date; //p1
Date *p2 = p1; //p2
Date tomorrow = *p1 + 1; //tomorrow
int month = (*p1).Month(); //month
int day = p1->Day(); //day
19/2/2019
20/2/2019 2 16
ptr-> is a shorthand for (*ptr). if ptr is a pointer to a struct or a class

19. Dynamic memory allocation with new – 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
count p2

20. Allocation of multiple variables with new
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 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];

22. Manipulation 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!

23. Pointers for Implementing Linked Data Structures

24. Linked Lists
Built-in Arrays: - too much or too little memory may be allocated
+ ease of use
- 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)

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

26. Introduction to linked lists: definition
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; p ? ? ?
num
word
next

27. Reminder: structs: as data aggregates see Tapestry Chp. 7 pp. 330-
If you need to create a data structure in order to combine different attributes of a concept, you can struct. These attributes are also called as "tied" data. For example, you can create a struct for a student and store a student's id, grade, address in this struct. Another example follows:
struct point // a struct for a point in 2D space {
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).
Well, it is possible to have member functions for structs, but we will not use structs like that.

28. Structs with constructors
If you define one or more constructors:
struct point {
double x;
double y;
//default constructor
point::point()
x = 0;
y = 0; }
//constructor
point::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;
...
With the help of constructors,
You can use:
point curve[100];
// all values are 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 constructor
If 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)

29. Introduction to linked lists: inserting 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 p
num
word
next

30. Updated 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)
{}; };

31. Introduction to linked lists: adding 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; q 5
Ali ? p
num
word
next

32. Introduction to linked lists
node *p;
p = new node;
p = new node(5,"Ali",NULL);
node *q;
q = new node; q 5
Ali ? ? ? p
num
word
next
num
word
next

33. Introduction to linked lists
node *p, *q;
p = new node;
p = new node(5,"Ali",NULL);
q = new node;
q->num=8; //I can access fields one by one
q->word = "Veli";
q->next = NULL; q 5
Ali 8
Veli ? p
num
word
next
num
word
next

34. Introduction to linked lists
node *p, *q;
p = new node;
p = new node(5,"Ali",NULL);
q = new node(8,"Veli",NULL);
p->next = q; q 5
Ali 8
Veli ? p
num
word
next
num
word
next

35. Linked 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
ptr = head;
while (ptr != NULL) {
cout << ptr ->info << endl;
ptr = ptr->next; }
End of a linked list should point to NULL

36. Linked 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

37. Linked Lists: building
//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 this
code’s execution? Let's trace on the board

38. Linked Lists: building
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.
struct 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; }
You better use a constructor to insert data more compactly and in a less error-prone fashion – like this.

39. Stack and Heap Scope and Lifetime Extern and Static Variables Freeing Memory Allocated by New

40. Static vs. Dynamic Memory Allocation
Automatic (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 (automatically).
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 defined as 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. 
If you do not do so, you cause so-called "memory leak"; This is one of deadly sins of programming.
The lifetime of a dynamic variable is until they are explicitly deleted

41. Local 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.

42. Extern 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.cpp

43. Static variables Static 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 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 file or the files that include it) 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 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 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: 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 in use. 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 delete
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
25/02/2017

48. Pointers to variables on the stack
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. Question 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 new operator Points to be careful for
Warning message: address of local variable returned
What is the problem here?
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();
DiceDynAlloc.cpp

51. Memory allocation with the new 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 new 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?