1. CS31 Discussion 1H Fall18: week 7
TA: Behnam Shahbazi
Credit to former TAs: Sepideh Mazrouee

2. Pointers
Every variable in C++ can be described by five different attributes:
int main(void) {
int bankAccount = ;
... }
  1. every variable has a name
2. every variable has a type
3. every variable has a size
4. every variable has a value
5. every variable has an address!
Acknowledgement: Professor Nachenberg – for the slides
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Variable Addresses
Every byte in the computer‘s memory has its own address (just like each house on a street has an address).
When you define a variable in your program, the computer finds an unused set of address in memory and reserves them for your variable.
int main(void) {
int stdID = ;
char grade = ‘B’; } …
stdID
grade 66 …
1 byte
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Variable Addresses
Important: The address of a variable is defined to be the first address in memory where the variable is stored.
Questions:
1. What is stdID’s address in memory?
2. What about grade’s address?
int main(void) {
int stdID = ;
char grade = ‘B’; } …
stdID
grade 66 …
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5. Finding the Address of a Variable
We can get the address of a variable using the & operator.
int main(void) {
int stdID = ;
char grade= ‘B’;
cout << “Value: “ << stdID << endl;
cout << “Size: “ << sizeof(stdID) << endl;
cout << “StdID’s address: “<< &stdID<<endl;
… cout << “grade’s address: “ << &grade; }
If you place an & before a variable in your program, it means “give me the numerical address of the variable.”
stdID
grade 66
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6. & is for References and Pointers
When we place an & before a function parameter, we make the parameter a reference parameter.
void booboo(int &a) {
... // a is a reference parameter. }
When we place an & before a variable in a program statement, we are asking for that variable’s address.
int main(void)
int a;
  cout << &a; // get a’s address
... } 
Be careful not to confuse the two.
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7. Every Variable Has An Address
...
Question: What is the output for this code?
void change_me(int x);
int main(void) {
int n;
cout << "Enter a #: ";
cin >> n;
cout << n << endl;
change_me(n); }
// definition
void change_me(int x)
x = 12; n 3 x 12 3
...
Output:
Enter a #: 3 3 3
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The Pointer Variable
A pointer variable is a variable that holds an address value rather than a normal int, float, double or string value.
You can set a pointer variable equal to a variable’s address:
int main(void) {
float price = 300;
Pointer ptr; // a pointer variable
ptr = &price; // ptr gets price’s address
...
On PCs, pointer variables require 4 bytes of storage. Why?
(depending on the machine- it’s 4 bytes for 32bit system)
...
price
300
ptr
1000
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9. The sizeof a Pointer Variable
A pointer variable needs to be able to point to any address in the computer’s memory.
Let say a 32 bit system that has 4 gigabytes (4 billion bytes) of memory.
Therefore, each pointer variable needs to be able to hold values between 0 and 4,000,000,000 (4 billion).
In order for a pointer variable to hold values between 0 and 4 billion, it needs 4 bytes of storage (232 values).
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10. Defining a Pointer Variable…
int main(void) {
float price = 300;
Pointer ptr; // WRONG!!! Incorrect syntax
ptr = &price; // ptr gets price’s address
When you define a pointer variable, you must specify what type of variable the pointer will point to…
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11. How to Define a Pointer Variable…
int main(void) {
float price = 300;
float * ptr; // ptr can point to float variables
ptr = &price; // ptr gets price’s address
To define a pointer variable, you use the following syntax:
Type * pointer_name;
e.g int * anIntPointer, *anotherOne;
string *ptr_to_a_string;
When you define your variables the * indicates that the variable is a pointer and not a regular variable.

12. Assigning a Pointer Variable…
The type of the pointer variable must match the type of the variable it’s pointing to.
main() {
float *fptr;
...
 fptr may only point to float variables.
int *pToInt; …
pToInt may only point to integer variables.
double dip; // dip is a double variable
fptr = &dip; // ERROR! fptr can only point to a float variable
int stdID, courseNo; // stdID and courseNo are int variables
pToInt = &stdID; // This is just fine…
pToInt = &courseNo; // This too… Now pToInt points to courseNo
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13. Assigning a Pointer Variable…
int main(void) {
int stdID, courseNo;
stdID = ;
courseNo = ;
int *ptr;
cout << “stdID is at address “
<< &stdID << endl;
ptr = &courseNo;
cout << “ptr points to address “
<< ptr << endl; } …
stdID
courseNo
150012
ptr
1004 ? …
courseNo is at address 1004
ptr points to address 1004
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14. Assigning a Pointer Variable…
int main(void) {
int stdID, courseNo;
stdID = ;
courseNo = ;
int *ptr;
cout << “stdID is at address “
<< & stdID << endl;
ptr = & stdID;
cout << “ptr points to address “
<< ptr << endl;
ptr = & courseNo; } …
stdID
courseNo
150012
ptr
1000
1004 …
stdID is at address 1000
ptr points to address 1000
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ptr points to address 1004

15. Pointer Variables are Variables Too!
A pointer variable, like a regular variable has the same 5 attributes.
1. name: pfv
2. type: pointer to a float variable
3. size: 4 bytes (= 32bits)
4. value: the value of a pointer variable is the address of another variable.
5. an address: Pointer variables have their OWN address too!
main() {
float fv = 3.14;
float *pfv;
pfv = &fv;
cout << “value: “ << pfv << endl;
cout << “size: "<<sizeof(pfv)<<endl;
cout << "address: " << &pfv << endl; }
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16. Pointer Variable Attributes
main() {
float fv = 3.14;
float *pfv;
pfv = &fv;
cout << “value: “ << pfv << endl;
cout << “size: "<<sizeof(pfv)<<endl;
cout << "address: " << &pfv << endl; } … fv
3.14
pfv
9241
value: 9241
size: 4 …
address: 9245
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Pointer Challenge
Question: what is the size of the following variables?
main() {
char c = ‘A’;
short s = 42;
char *pc = &c;
short *ps = &s;
bool *pbool;
... } … c 65 s 42 pc
9240 ps
9241
All pointer variables are 4 bytes long, regardless of what type of variable they point to, since they have to hold an address.
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* Operator
The * operator is used to “de-reference” a pointer.
It lets you access what the pointer points to.
main() {
short hands = 2;
short *ptr;
ptr = &hands;
*ptr = 3;
cout << *ptr << endl;
cout << hands << endl; } …
hands 2 3
ptr
9240 3 3
“Store a value of 3 where ptr points to.”
“Print the value stored where ptr points.”
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19. Question: What Happens Here?
main() {
int *iptr;
*iptr = ; } …
iptr
989699
Where does iptr point to?
It points randomly in memory!
You must always point a pointer variable at another variable before using the * operator on it!
123456
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Operating system??

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Another Example…
main() {
int a=1, b=2;
int *p1, *p2; 
p1 = &a;
p2 = &b;
*p1 = *p2; } … a 1 b 2 2 p1 p2
6420
“Get the value pointed to by p2.
Store this value in the variable pointed to by p1.”
6424
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Another Example…
main() {
int a=1, b=2;
int *p1, *p2; // how to define 2 ptrs
p1 = &a;
p2 = &b;
p1 = p2; } … a 2 b p1 p2
6420
6424
6424
“Store the value of variable p2 into variable p1.”
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Question: What do you think about this example?
main() {
int a=1, b=2;
int *p1, *p2; // how to define 2 ptrs
p1 = &a;
p2 = &b;
if (*p1 == *p2) { do something; } } … a 1 b 2 p1 p2
6420
6424
“Compare the value pointed to by p1 to the value pointed to by p2.”
if (1 == 2) …
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Question: How about this?
main() {
int a=1, b=2;
int *p1, *p2; // how to define 2 ptrs
p1 = &a;
p2 = &b;
if (p1 == p2) { do something; } } … a 1 b 2 p1 p2
6420
6424
“Compare the value of variable p1 to the value of variable p2.”
if (6420 == 6424) …
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24. Using Pointers Instead of References
void set(int *pa) // like a reference! {
pa = 5; }
main()
int x = 1;
set(&x);
cout << &x; … x 1 5 pa
9240
Store a value of 5 in the variable where pa points (at location 9240).
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Question: what is the output of this code?
void set(int *pa) // like a reference! {
*pa = 5; }
main()
int x = 1;
set(&x);
cout << x; … x 5 5
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26. References to Pointers
short arr[3] = {2,-3, 1}; // global
void dosth(short * & r2p) {
r2p = &arr[1]; }
main()
short *ptr = NULL;
dosth(ptr);
cout << *ptr; …
arr
// ptr = &arr[1]; 2 -3 1
ptr
9242
NULL
r2p is a reference to a short pointer variable.
r2p
So any time we refer to r2p, we’re really referring to ptr!
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27. What If We Drop the Reference?
short arr[3] = {2,-3, 1}; // global
void dosth(short * & r2p) {
r2p = &arr[1]; }
main()
short *ptr = NULL;
dosth(ptr);
cout << *ptr; …
arr 2 -3 1
NULL
ptr
NULL
r2p
NULL
9242
r2p is a regular short pointer var. It gets a copy of ptr’s value.
So when we refer to r2p, we’re referring to a copy of ptr!

28. CS31 - Spring13 - Section1J TA: Sepid Maz
Question: What does it print?
main() {
char ch = 'A';
char *pch,*pch2;
pch = &ch;
pch2 = pch;
(*pch2)++;
cout << ch; } … x 65 66
pch
pch2
2200
2200
“Increment the value pointed to by pch2.” B
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29. CS31 - Spring13 - Section1J TA: Sepid Maz
Review
A pointer is a memory address of a variable.
We have already used pointer concepts through the use of
Call-by-reference
Passing an array identifiers through functions
QUESTION: Using x as an integer and p as an int pointer, you should be able to know what's being accessed in each of these cases: x:
&x:
*p: p:
&p:
value of x
address of x
the value stored at the address that p refers to
address that p refers to (value of p)
address of p (p is a variable, and all variables have addresses.
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int *p1; *p1 = 100; cout << *p1 << endl; Output?
int *p1; int v1; v1 = -7; p1 = &v1; *p1 = 100; cout << v1 << endl; cout << *p1 << endl; Output?
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Additional Materials
Check my webpage for additional materials.
Behnam
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