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
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