void pointer in C / C++ - GeeksforGeeks
void pointer in C / C++
- Difficulty Level : Easy
- Last Updated : 03 Jan, 2019
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.
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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