Applications of Pointers in C/C++
Prerequisite : Pointers in C/C++, Memory Layout of C Programs.
- To pass arguments by reference. Passing by reference serves two purposes
(i) To modify variable of function in other. Example to swap two variables;
C
// C program to demonstrate that we can change// local values of one function in another using pointers.#include <stdio.h>void swap(int* x, int* y){ int temp = *x; *x = *y; *y = temp;}int main(){ int x = 10, y = 20; swap(&x, &y); printf("%d %d\n", x, y); return 0;} |
C++
// C++ program to demonstrate that we can change// local values of one function in another using// pointers.#include <iostream>using namespace std;void swap(int* x, int* y){ int temp = *x; *x = *y; *y = temp;}int main(){ int x = 10, y = 20; swap(&x, &y); cout << x << " " << y << endl; return 0;} |
Output :
20 10
(ii) For efficiency purpose. Example passing large structure without reference would create a copy of the structure (hence wastage of space).
Note : The above two can also be achieved through References in C++.
- For accessing array elements. Compiler internally uses pointers to access array elements.
C
// C program to demonstrate that compiler// internally uses pointer arithmetic to access// array elements.#include <stdio.h>int main(){ int arr[] = { 100, 200, 300, 400 }; // Compiler converts below to *(arr + 2). printf("%d ", arr[2]); // So below also works. printf("%d\n", *(arr + 2)); return 0;} |
C++
// C++ program to demonstrate that compiler// internally uses pointer arithmetic to access// array elements.#include <iostream>using namespace std;int main(){ int arr[] = { 100, 200, 300, 400 }; // Compiler converts below to *(arr + 2). cout << arr[2] << " "; // So below also works. cout << *(arr + 2) << " "; return 0;} |
Output :
300 300
- To return multiple values. Example returning square and square root of numbers.
C
// C program to demonstrate that using a pointer// we can return multiple values.#include <math.h>#include <stdio.h>void fun(int n, int* square, double* sq_root){ *square = n * n; *sq_root = sqrt(n);}int main(){ int n = 100; int sq; double sq_root; fun(n, &sq, &sq_root); printf("%d %f\n", sq, sq_root); return 0;} |
C++
// C++ program to demonstrate that using a pointer// we can return multiple values.#include <bits/stdc++.h>using namespace std;void fun(int n, int* square, double* sq_root){ *square = n * n; *sq_root = sqrt(n);}int main(){ int n = 100; int* sq = new int; double* sq_root = new double; fun(n, sq, sq_root); cout << *sq << " " << *sq_root; return 0;} |
Output :
10000 10
- Dynamic memory allocation : We can use pointers to dynamically allocate memory. The advantage of dynamically allocated memory is, it is not deleted until we explicitly delete it.
C
// C program to dynamically allocate an// array of given size.#include <stdio.h>#include<stdio.h>int* createArr(int n){ int* arr = (int*)(malloc(n * sizeof(int))); return arr;}int main(){ int* pt = createArr(10); return 0;} |
C++
// C++ program to dynamically allocate an// array of given size.#include <iostream>using namespace std;int* createArr(int n){ return new int[n];}int main(){ int* pt = createArr(10); return 0;} |
- To implement data structures.
Example linked list, tree, etc. We cannot use C++ references to implement these data structures because references are fixed to a location (For example, we can not traverse a linked list using references) - To do system level programming where memory addresses are useful. For example shared memory used by multiple threads. For more examples, see IPC through shared memory, Socket Programming in C/C++, etc
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