void pointer in C / C++

A void pointer is a pointer that has no associated data type with it. A void pointer can hold address of any type and can be typecasted to any type. 

C++

// C++ Program to demonstrate that a void pointer
// can hold the address of any type-castable type

#include <iostream>
using namespace std;

int main()
{
    int a = 10;
    char b = 'x';

    void* p = &a;  // void pointer holds address of int 'a'
    p = &b; // void pointer holds address of char 'b'
}

// This code is contributed by sarajadhav12052009

C

int a = 10;
char b = 'x';

void *p = &a;  // void pointer holds address of int 'a'
p = &b; // void pointer holds address of char 'b'

Time Complexity: O(1)
Auxiliary Space: O(1)

Advantages of void pointers: 1) malloc() and calloc() return void * type and this allows these functions to be used to allocate memory of any data type (just because of void *) 

Note that the above program compiles in C, but doesn’t compile in C++. In C++, we must explicitly typecast return value of malloc to (int *). 2) void pointers in C are used to implement generic functions in C. For example compare function which is used in qsort(). Some Interesting Facts: 1) void pointers cannot be dereferenced. For example the following program doesn’t compile. 

C++

// C++ Program to demonstrate that a void pointer
// cannot be dereferenced

#include <iostream>;
using namespace std;

int main()
{
    int a = 10;
    void* ptr = &a;

    cout << *ptr;

    return 0;
}

// This code is contributed by sarajadhav12052009

C

#include <stdio.h>
int main()
{
    int a = 10;
    void *ptr = &a;
    printf("%d", *ptr);

    return 0;
}

Output:

Compiler Error: 'void*' is not a pointer-to-object type 

The following program compiles and runs fine. 

C++

#include <iostream>
using namespace std;

int main()
{
    int a = 10;
    void* ptr = &a;

    cout << *(int *)ptr << endl;

    return 0;
}

// This code is contributed by sarajadhav12052009

C

#include <stdio.h>

int main()
{
    int a = 10;
    void *ptr = &a;
    printf("%d", *(int *)ptr);
    return 0;
}

Output:

10

Time Complexity: O(1)
Auxiliary Space: O(1)

2) The C standard doesn’t allow pointer arithmetic with void pointers. However, in GNU C it is allowed by considering the size of void is 1. For example the following program compiles and runs fine in gcc. 

C

#include<stdio.h>
int main()
{
    int a[2] = {1, 2};
    void *ptr = &a;
    ptr = ptr + sizeof(int);
    printf("%d", *(int *)ptr);
    return 0;
}

Output:

2

Time Complexity: O(1)
Auxiliary Space: O(1)

Note that the above program may not work in other compilers. References: http://stackoverflow.com/questions/20967868/should-the-compiler-warn-on-pointer-arithmetic-with-a-void-pointer http://stackoverflow.com/questions/692564/concept-of-void-pointer-in-c-programming Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above