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.
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:
10
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:
2
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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