void pointer in C / C++ - GeeksforGeeks

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 typcasted to any type.

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' 

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 *)

int main(void) 
{ 
    // Note that malloc() returns void * which can be  
    // typecasted to any type like int *, char *, .. 
    int *x = malloc(sizeof(int) * n); 
} 

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.

#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.

#include<stdio.h> 
int main() 
{ 
    int a = 10; 
    void *ptr = &a; 
    printf("%d", *(int *)ptr); 
    return 0; 
} 

Output:

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.

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

Output:

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

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