1. Pointers

2. Getting the Address of a Variable
Each variable in program is stored at a unique address
Use address operator & to get address of a variable:
int num = -23;
cout << &num; // prints address
// in hexadecimal
Related, sample programs from the text are noted. See pr10-01.cpp

3. Pointer Variables
Pointer variable (pointer): variable that holds an address
Can perform some tasks more easily with an address than by accessing memory via a symbolic name:
Accessing unnamed memory locations
Array manipulation

4. Pointer Variables Definition: Read as:
int *intptr;
Read as:
“intptr can hold the address of an int”
Spacing in definition does not matter:
int * intptr;
int* intptr;

5. int num = 25; Pointer Variables Assignment: int *intptr;
intptr = &num;
Memory layout:
Can access num using intptr and indirection
operator *:
cout << intptr; // prints 0x4a00
cout << *intptr; // prints 25
num
intptr 25
0x4a00
address of num: 0x4a00
See pr10-02.cpp, pr10-03.cpp and pr10-04.cpp

6. Indirection Operator (dereferencing)
Value of a variable pointed to by ptr
Achieved by * operator
int num = 25;
int *intptr;
intptr = &num;
cout<<*intptr; //prints 25

7. Pointers & Arrays

8. Relationship Between Arrays and Pointers
Array name is starting address of array
int a[] = {4, 7, 11};
cout << a; // displays 0x4a00
cout << a[0]; // displays 4 4 7 11
starting address of a: 0x4a00

9. Example
int main () { int a[ ] = {4, 7, 11}; int *ptr = a; cout << *a<<endl; for(int i=0;i<3;i++) cout<<a[i]<<" "; for( i=0;i<3;i++) cout<<*(ptr+i)<<" "; return 0;}

10. Pointers in Expressions
Given:
int a[]={4,7,11};
int *ptr = a;
cout << *a; // displays 4
What is ptr + 1?
cout << *(ptr+1); // displays 7
cout << *(ptr+2); // displays 11
Must use ( ) in expression

11. Array Access Array notation a[i] is equivalent to the pointer notation

12. Pointer Arithmetic

13. Pointer Arithmetic - Integers can be added to or subtracted from
Some arithmetic operators can be used with pointers:
- Increment and decrement operators ++, --
- Integers can be added to or subtracted from
pointers using the operators +, -, +=, and -=
- One pointer can be subtracted from another
by using the subtraction operator -

14. Pointer arithmetic Integer math operations can be used with pointers.
If you increment a pointer, it will be increased by the size of whatever it points to.
int *ptr = a;
*(ptr+2)
*(ptr+4)
*ptr
a[0] a[1] a[2] a[3] a[4]

15. Pointer Arithmetic (++/--)
Assume the variable definitions
int a[]={4,7,11};
int *ptr = a;
Examples of use of ++ and –-
ptr++; // points at 7
ptr--; // now points at 4
See pr10-09.cpp

16. Pointer Arithmetic (+)
Assume the variable definitions:
int a[]={4,7,11};
int *ptr = a;
Example of use of + to add an int to a pointer:
cout << *(ptr + 2)
This statement will print 11

17. Pointer Arithmetic (+=)
Assume the variable definitions:
int a[]={4,7,11};
int *ptr = a;
Example of use of +=:
ptr = a; // points at 4
ptr += 2; // points at 11

18. This statement prints 2: the number of ints between ptr and a
Pointer Arithmetic
Assume the variable definitions
int a[] = {4,7,11};
int *ptr = a;
Example of pointer subtraction
ptr += 2;
cout << ptr - a;
This statement prints 2: the number of ints between ptr and a

19. Comparing Pointers
Relational operators can be used to compare addresses in pointers
Comparing addresses in pointers is not the same as comparing contents pointed at by pointers:
if (ptr1 == ptr2) // compares
// addresses
if (*ptr1 == *ptr2) // compares
// contents
See pr10-10.cpp

20. Pointers & Functions

21. Pointers as Function Parameters
A pointer can be a parameter
Works like a reference parameter to allow change to argument from within function
A pointer parameter must be explicitly dereferenced to access the contents at that address

22. Pointers as Function Parameters
Requires:
1) asterisk * on parameter in prototype and
heading
void getNum(int *ptr);
2) asterisk * in body to dereference the pointer
cin >> *ptr;
3) address as argument to the function
getNum(&num);

23. Pointers as Function Parameters
void swap(int *x, int *y) { int temp; temp = *x; *x = *y; *y = temp; } int num1 = 2, num2 = -3; swap(&num1, &num2);
See pr10-11.cpp and pr10-12.cpp

24. Example (returning search key index)
int find(int *p, int key) { for(int i=0;i<10;i++) if (*p++==key) return i; return (-1); }

25. Contd… int main () { int a[]={1,12,45,63,18,14,6,9,10,7}; int num;
cout<<"Enter number to search\n";
cin>>num;
int *ptr=a;
int k=find(ptr,num);
if (k!=-1)
cout<<"Element is at index "<<k<<endl;
else
cout<<"Element does not exist in array\n";
return 0;}

26. Copying string using pointers
void copystr(char*, const char*); //prototype int main() { char* str1 = "Self-conquest is the greatest victory."; char str2[80]; copystr(str2, str1); cout << str2 << endl; return 0; } void copystr(char* dest, const char* src) while( *src ) *dest++ = *src++; *dest = '\0';

27. Strings as function arguments
void dispstr (char*); int main() { char str[] = "Idle people have the least leisure."; dispstr(str); return 0; } void dispstr(char* ps) while( *ps ) cout << *ps++; cout << endl;