pointer

Store the address in a pointer variable
Memory address: 1020 1024 10032
…
88
100
a
…
1024
p Result is: 100 1024
…
int a = 100; int* p = &a; cout << a << " " << &a <<endl;
The name of an array points only to the first element not the whole array.
1000
1004 1008 1012 1016
Array name is a pointer constant
#include <iostream> Using namespace std; void main (){ // Demonstrate array name is a pointer constant int a[5]; cout << "Address of a[0]: " << &a[0] << endl << "Name as pointer: " << a << endl; }
char temp = y; y = z; z = temp;
} int main() {
char a = 'y'; char b = 'n'; swap(a, b); cout << a << b << endl; return 0;
}
y, z are „references‟, only names, not like ptr1, ptr2 that are variables
The value of pointer p is the address of variable a
cout << p << " " << &p <<endl; 1024 10032
A pointer is also a variable, so it has its own memory address
Another Pointer Example
int a = 3; char s = „z‟; double d = 1.03; int* pa = &a; char* ps = &s; double* pd = &d; cout << sizeof(pa) << sizeof(*pa) << sizeof(&pa) << endl; cout << sizeof(ps) << sizeof(*ps) << sizeof(&ps) << endl; cout << sizeof(pd) << sizeof(*pd) << sizeof(&pd) << endl;
Pointer to ‘Struct’
struct Date {…}; Date d={1,10,2009}; Date* pd = &d; (*pd).day=2; (*pd).month=11; Or pd->day=2; pd->month=11;
Pointers and arrays
Arrays are pointers!
Getting an address: address operator &
int a=100; “&a” “the address of a”
1024
Memory address: 1020
… …
int a = 100; cout << a; Cout << &a;
100
a 100 1024
int* ptr; ptr = 0; cout << *ptr << endl; // invalid, ptr // does not point to // a valid int
Example
#include <iostream> using namespace std; Result: int main (){ Value1 is 10 int value1 = 5, value2 = 15; Value2 is 20 int* p1; int* p2; p1 = &value1; // p1 = address of value1 p2 = &value2; // p2 = address of value2 *p1 = 10; // value pointed to by p1=10 *p2 = *p1; // value pointed to by p2= value // pointed to by p1 p1 = p2; // p1 = p2 (pointer value copied) *p1 = 20; // value pointed to by p1 = 20 cout << "value1==" << value1 << "/ value2==" << value2; return 0; }
/* result: Address of a[0]: 0x0065FDE4 Name as pointer: 0x0065FDE4 */
Dereference of an array name
a[0] a[1] a[2]
2 4
6
8 22
a[3]
a[4]
#include <iostream> Using namespace std; void main(){ int a[5] = {2,4,6,8,22}; cout << *a << " " << a[0] << " " << *(&a[0]); ..." } //main
*p
gives 100
An asterisk („*‟) has two usages
In a definition, an asterisk indicates that the object is a pointer.
char* s; // s is of type pointer to char (char *s; is possible)
equivalent
int* p;
TypeName *variablename;
Examples of (uninitialized) pointers
int* ip; char* s; int *ip; char *s;
int *p;
// ip is a pointer to an int // s is a pointer to a char
Memory address: 1020 1024 10032
…
88
100
a
…
1024
p
…
int a = 100; int* p = &a; cout << a << endl; cout << &a << endl; cout << p << " " << *p << endl; cout << &p << endl;
In expressions, an asterisk before a pointer indicates the object the pointer pointed to, called dereferencing
int i = 1, j; int* ptr; // ptr is an int pointer ptr = &i; // ptr points to i j = *ptr + 1; // j is assigned 2 cout << *ptr << j << endl; // display "12"
typedef int MyInt; MyInt k; typedef int* IntPt; IntPt a, b;
int k;
Traditional Pointer Usage
void swap(char* ptr1, char* ptr2){
char temp = *ptr1; *ptr1 = *ptr2; *ptr2 = temp;
Null Address
0 is a pointer constant that represents the empty or null address
Indicates that pointer is not pointing to storage of a valid object Cannot dereference a pointer whose value is null

Object whose value represents the location (memory address) of another object
Memory address: