C++ POINTERS
Pointers are the powerful feature of C++ programming which differs it from other popular programming languages like: Java, Visual Basic etc.
To understand pointers, you should have the knowledge of address in computer memory. Computer memory is broken down into bytes and each byte has its own address. For example: In 1KB memory, there are 1024 bytes and each byte is given an address (0 - 1023).
The Address-of Operator &
The & operator can find address occupied by a variable. If
var is a variable then, &vargives the address of that variable.Example 1: Address-of Operator
#include <iostream>
using namespace std;
int main() {
int var1 = 3;
int var2 = 24;
int var3 = 17;
cout<<&var1<<endl;
cout<<&var2<<endl;
cout<<&var3<<endl;
}
Output
0x7fff5fbff8ac 0x7fff5fbff8a8 0x7fff5fbff8a4
The 0x in the beginning represents the address is in hexadecimal form. (You may not get the same result on your system.). Notice that first address differs from second by 4-bytes and second address differs from third by 4-bytes. It is because the size of integer(variable of type
int) is 4 bytes in 64-bit system.Pointers Variables
Now you know about address in computer memory, it's time to learn about pointers.
Consider a normal variable as in above example, these variables holds data. But pointer variables or simply pointers are the special types of variable that holds memory address instead of data.
How to declare a pointer?
int *p;
OR,
int* p;
The statement above defines a pointer variable p. The pointer p holds the memory address. The asterisk is a dereference operator which means pointer to. Here pointer pis a pointer to
int, that is, it is pointing an integer.
Note: In above statement p is a pointer variable that holds address not
*p. The *pis an expression. The content(value) of the memory address pointer p holds is given by expression *p.Example 2: C++ Pointers
C++ Program to demonstrate the working of pointer.
#include <iostream>
using namespace std;
int main() {
int *pc, c;
c = 5;
cout<< "Address of c (&c): " << &c << endl;
cout<< "Value of c (c): " << c << endl << endl;
pc = &c;// Pointer pc holds the memory address of variable c
cout<< "Address that pointer pc holds (pc): "<< pc << endl;
cout<< "Content of the address pointer pc holds (*pc): " cout<<*pc << endl << endl;
c = 11;//The content inside memory is changed from 5 to 11.
cout << "Address pointer pc holds (pc): " << pc << endl;
cout << "Content of the address pointer pc holds (*pc): " cout<<*pc << endl << endl;
*pc = 2;
cout<< "Address of c (&c): "<< &c <<endl;
cout<<"Value of c (c): "<< c<<endl<< endl;
return 0;
}
Output
Address of c (&c): 0x7fff5fbff80c Value of c (c): 5 Address that pointer pc holds (pc): 0x7fff5fbff80c Content of the address pointer pc holds (*pc): 5 Address pointer pc holds (pc): 0x7fff5fbff80c Content of the address pointer pc holds (*pc): 11 Address of c (&c): 0x7fff5fbff80c Value of c (c): 2
Arithmetic Operations on Pointers
As you understood pointer is an address which is a numeric value; therefore, you can perform arithmetic operations on a pointer just as you can a numeric value. There are four arithmetic operators that can be used on pointers: ++, --, +, and -
To understand pointer arithmetic, let us consider that ptr is an integer pointer which points to the address 1000. Assuming 32-bit integers, let us perform the following arithmatic operation on the pointer:
ptr++
the ptr will point to the location 1004 because each time ptr is incremented, it will point to the next integer. This operation will move the pointer to next memory location without impacting actual value at the memory location. If ptr points to a character whose address is 1000, then above operation will point to the location 1001 because next character will be available at 1001.
Incrementing a Pointer:
We prefer using a pointer in our program instead of an array because the variable pointer can be incremented, unlike the array name which cannot be incremented because it is a constant pointer. The following program increments the variable pointer to access each succeeding element of the array:
#include <iostream> using namespace std; const int MAX = 3; int main () { int var[MAX] = {10, 100, 200}; int *ptr; // let us have array address in pointer. ptr = var; for (int i = 0; i < MAX; i++) { cout << "Address of var[" << i << "] = "; cout << ptr << endl; cout << "Value of var[" << i << "] = "; cout << *ptr << endl; // point to the next location ptr++; } return 0; }
When the above code is compiled and executed, it produces result something as follows:
Address of var[0] = 0xbfa088b0 Value of var[0] = 10 Address of var[1] = 0xbfa088b4 Value of var[1] = 100 Address of var[2] = 0xbfa088b8 Value of var[2] = 200
Decrementing a Pointer:
The same considerations apply to decrementing a pointer, which decreases its value by the number of bytes of its data type as shown below:
#include <iostream> using namespace std; const int MAX = 3; int main () { int var[MAX] = {10, 100, 200}; int *ptr; // let us have address of the last element in pointer. ptr = &var[MAX-1]; for (int i = MAX; i > 0; i--) { cout << "Address of var[" << i << "] = "; cout << ptr << endl; cout << "Value of var[" << i << "] = "; cout << *ptr << endl; // point to the previous location ptr--; } return 0; }
When the above code is compiled and executed, it produces result something as follows:
Address of var[3] = 0xbfdb70f8 Value of var[3] = 200 Address of var[2] = 0xbfdb70f4 Value of var[2] = 100 Address of var[1] = 0xbfdb70f0 Value of var[1] = 10
C++ Pointers and Arrays
Pointers are the variables that hold address. Pointers can point at cells of an array not only single variable. Consider this example:
int* ptr; int a[5]; ptr = &a[2];//&a[2] is the address of third element of a[5]
C++ Program to display address of elements of an array using both array and pointers
#include <iostream>
using namespace std;
int main() {
float a[5];
float *ptr;
cout << "Displaying address using arrays: "<<endl;
for (int i = 0; i < 5; ++i) {
cout << "&a[" << i << "] = " << &a[i] <<endl;
}
ptr = a; // ptr = &a[0]
cout<<"\nDisplaying address using pointers: "<< endl;
for (int i = 0; i < 5; ++i) {
cout << "ptr + " << i << " = "<<ptr+i <<endl;
}
return 0;
}
Output
Displaying address using arrays: &a[0] = 0x7fff5fbff880 &a[1] = 0x7fff5fbff884 &a[2] = 0x7fff5fbff888 &a[3] = 0x7fff5fbff88c &a[4] = 0x7fff5fbff890 Displaying address using pointers: ptr + 0 = 0x7fff5fbff880 ptr + 1 = 0x7fff5fbff884 ptr + 2 = 0x7fff5fbff888 ptr + 3 = 0x7fff5fbff88c ptr + 4 = 0x7fff5fbff890#include <iostream.h>
Pointers to Objects:
A variable that holds an address value is called a pointer variable or simply pointer. Pointer can point to objects as well as to simple data types and arrays. sometimes we dont know, at the time that we write the program , how many objects we want to create. when this is the case we can use new to create objects while the program is running. new returns a pointer to an unnamed objects.Example:#include<iostream.h>class item{int code;float price;public:void getdata(int a, float b){code =a;price=b;}void show(void){cout<<"code:"<<code<<"\n";cout<<"Price:"<<price<<"\n";}};const int size=2;int main(){item *p = new item[size];item *d = p;int x,i;float y;for(i=0;i<size;i++){cout<<"Input code and price for item"<<i+1;cin>>x>>y;p->getdata(x,y);p++;}for(i=0;i<size;i++){cout<<"Item:"<<i+1<<"\n";d->show();d++;}return 0;}Output:
