A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before using it to store any variable address. The general form of a pointer variable declaration is −

type *var-name;

• type – pointer’s base type, it must be a valid C data type.
• var-name – name of the pointer variable.
• * – used to declare var-name is a pointer.

// Examples
int    *ip;    /* pointer to an integer */
double *dp;    /* pointer to a double */
float  *fp;    /* pointer to a float */
char   *ch     /* pointer to a character */

There are a few important operations, which we will do with the help of pointers very frequently. (a) We define a pointer variable, (b) assign the address of a variable to a pointer and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. The following example makes use of these operations −

#include <stdio.h>

// The address formater is %p

int main () {

   int  var = 20;   /* actual variable declaration */

   int  *ip;   /* pointer variable declaration */
   ip = &var;  /* store address of var in pointer variable*/
   /* One line to both declare and initialize: int *ip = &var;  */

   printf("Address of var variable: %p\n", &var  );

   /* address stored in pointer variable */
   printf("Address stored in ip variable: %p\n", ip );

   /* access the value using the pointer */
   printf("Value of *ip variable: %d\n", *ip );

   return 0;
}

When the above code is compiled and executed, it produces the following result −

Address of var variable: bffd8b3c
Address stored in ip variable: bffd8b3c
Value of *ip variable: 20

Another Example:

// C++ compilation
#include <iostream>

using namespace std;

int main()
{

	int var1 = 100;
	int *pVar1 = &var1;

	int var2 = 200;
	int *pVar2;
	pVar2 = &var2;

	cout << "var1 is " << var1 << "\n";
	cout << "*pVar1 is " << *pVar1 << "\n";
	cout << "pVar1 is " << pVar1 << "\n";
	cout << endl;

	cout << "var2 is " << var2 << "\n";
	cout << "*pVar2 is " << *pVar2 << "\n";
	cout << "pVar1 is " << pVar2 << "\n";
	cout << endl;

	cout << "Now update var1 to 300 via the pointer\n";

	*pVar1 = 300;
	
	cout << "var1 is " << var1 << "\n";
	cout << "*pVar1 is " << *pVar1 << "\n";
	cout << "pVar1 is " << pVar1 << "\n";
	cout << endl;

	return 0;
}


// The above produces the below result:

var1 is 100
*pVar1 is 100
pVar1 is 0x7fc419a9b0

var2 is 200
*pVar2 is 200
pVar1 is 0x7fc419a9b4

Now update var1 to 300 via the pointer
var1 is 300
*pVar1 is 300
pVar1 is 0x7fc419a9b0

Pointer to an Array

#include <iostream>

using namespace std;

int main() {
	int arr[3] = {100, 200, 300};
	int *p1_arr = arr;  // assign a pointer to the array.

	// the above has the same effect as:
	int *p2_arr = &arr[0];  // assign a pointer to the same array.

	p1_arr++;  // pointer arithmetic for moving to next element
	p2_arr++;

	cout << endl;
	cout << "second element of arr: " << *p1_arr << endl;
	cout << "second element of arr: " << *p2_arr << endl;
	cout << "So p1_arr and p2_arr are pointing to the same array\n\n";
	return 0;
}

/*** The output of the above:  **/
second element of arr: 200
second element of arr: 200
So p1_arr and p2_arr are pointing to the same array

Passing an argument by reference or by address enable the passed argument to be changed in the calling function by the called function.

The function, which can accept a pointer, can also accept an array.

#include <stdio.h>
#include <time.h>
 
void getSeconds(unsigned long *par);

int main () {

   unsigned long sec;
   getSeconds( &sec );

   /* print the actual value */
   printf("Number of seconds: %ld\n", sec );

   return 0;
}

void getSeconds(unsigned long *par) {
   /* get the current number of seconds */
   *par = time( NULL );
   return;
}

The function has to be declared in the form:

int * myFunction() {
. . .
}

Example:

#include <iostream>
using namespace std;

/* Function to return 5 numbers*/
int *the5Num()
{
	static int r[5] = {15, 14, 13, 12, 11};
	return r;  // r is the address of the first element of array
} // the5Num()

int main()
{
	int *p;
	int i;

	p = the5Num();

	cout << endl;
	cout << "The value of p: " << p << endl;
	cout << "The address of p[0]: " << &p[0] << endl;
	cout << "The value of *p: " << *p << endl;
	cout << "The value of *(p + 3) i.e., the 4th element: " << *(p + 3) << endl;
	return 0;
}

/*  Result:  */
The value of p: 0x558b170010
The address of p[0]: 0x558b170010
The value of *p: 15
The value of *(p + 3) i.e., the 4th element: 12

Common Mistakes

int c, *pc;

// pc is address but c is not
pc = c;  // Error

// &c is address but *pc is not
*pc = &c;  // Error

// both &c and pc are addresses
pc = &c;  // Not an error

// both c and *pc are values 
*pc = c;  // Not an error