1. Learning Objectives Pointers Pointer in function call
* symbol and & symbol
Pointer operations
Pointer in function call
Application: exchange values of two variables
Pointer and array

2. Pointer Introduction Pointer definition: Memory address of a variable
Recall: memory divided
Numbered memory locations
Variable name is used as address
You’ve used memory address already!
Call-by-reference
Address of actual argument was passed
Pass array in a function call

3. Pointer Variables Pointers are "typed" Example: double *p;
Can store addresses in variable
Pointers to int, double, etc.!
Example: double *p;
p is declared a "pointer to double" variable
Can hold pointers to variables of type double
Not other types!

4. Declaring Pointer Variables
Pointers declared like other types
Add "*" before variable name
Produces "pointer to" that type
"*" must be before each variable
int *p1, *p2, v1, v2;
p1, p2 hold pointers to int variables
v1, v2 are ordinary int variables

5. Pointing to … int *p1, *p2, v1, v2; p1 = &v1; Operator, & Read like:
Sets pointer variable p1 to "point to" int variable v1
Operator, &
Determines "address of" variable
Read like:
"p1 equals address of v1"
Or "p1 points to v1"

6. Pointing to … Recall: int *p1, *p2, v1, v2; p1 = &v1;
Two ways to refer to v1 now:
Variable v1 itself: cout << v1;
Via pointer p1: cout << *p1;
Dereference operator, *
Pointer variable "dereferenced"
Means: "Get data that p1 points to"

7. "Pointing to" Example
Consider: v1 = 0; p1 = &v1; *p1 = 42; cout << v1 << endl; cout << *p1 << endl;
Produces output: 42 42
*p1 and v1 refer to same variable

8. & Operator The "address of" operator
Also used to specify call-by-reference parameter
Similar mechanism
Call-by-reference parameters pass "address of" the actual argument

9. Pointer Assignments
Pointer variables can be "assigned": int *p1, *p2; p2 = p1;
Assigns one pointer to another
"Make p2 point to where p1 points"
Do not confuse with: *p1 = *p2;
Assigns "value pointed to" by p1, to "value pointed to" by p2

10. Pointer Assignments Graphic: Display 10
Pointer Assignments Graphic: Display Uses of the Assignment Operator with Pointer Variables

11. Define Pointer Types Can "name" pointer types
To be able to declare pointers like other variables
Eliminate need for "*" in pointer declaration
typedef int* IntPtr;
Defines a "new type" alias
Consider these declarations: IntPtr p; int *p;
The two are equivalent

12. Pointers and Functions
Recall
Pass by value
Pass by reference
Pointers can be function parameters
Behaves like pass by reference
Example: next slide

13. void main() { int m = 77; int
void main() { int m = 77; int * p = &m; cout << "m = " << m << endl; sneaky(p); } void sneaky(int * temp) *temp = 99;

14. Call-by-value Pointers Graphic: The Function Call sneaky(p);

15. Application of Pointer
Exchange values of two variables
Pass by value: won’t work
Pass by reference: will work
Pass by pointer: will work
Pass by pointer to pointer: what is this?

16. Pointer and array
Both array and pointer are pass by reference in a function call
Both array and pointer refer to some physical memory address
So, in C++, pointers can be used to represent arrays
Any type of array (int, double, char, …)

17. Pointer and array (continued)
double a[4] = {1.1, 2.2, 3.3, 4.4}; double f = 5.5; double *p; p = &f; p = a; // or p = & a[0]; // now p can be used same as a to access array for(int i=0; i<4; i++) cout << p[i] << "\t"; cout << endl;

18. Pointer and array (continued)
So, we can use pointer to represent array, even in a function call
double a[4] = {1.1, 2.2, 3.3, 4.4};
double *p = a;
reset(p, 4); //or reset(a, 4);
for(int i=0; i<4; i++) cout << p[i] << "\t";
void reset(double * x, int size) {
for(int i=0; i<size; i++) x[i]=5.5; }

19. Pointer and C-String
C-String is a character array, so pointer can be used to represent C-string
What will be displayed?
char str[] = "cs201";
cout << str << endl;
cout << "str contains " << strlen(str) << " characters" << endl;
char *p;
p = str; // p = &str[0];
cout << p << endl;
cout << "p contains " << strlen(p) << " characters" << endl;

20. Pointer and C-String (continued)
What should be displayed?
char *p = "USA";
char *q = "Indiana";
q = p;
reset(q);
cout << "q=" << q << endl;
void reset(char * x) {
x="IU South Bend"; }

21. Pointer and Array and C-String
Assigment can not be performed in array but can be done in pointers
int m[3] = {1, 2, 3};
int n[3] = {-1, -2, -3};
m = n; // This is NOT allowed
int * p1 = m;
int * p2 = n;
p1 = p2; // This is allowed

22. Pointer and Array and C-String (continued)
Assigment can not be performed in C-string but can be done in pointers
char m[] = “Hello”;
char n[] = “South Bend”;
m = n; // This is NOT allowed
char * p1 = m;
char * p2 = n;
p1 = p2; // This is allowed