Garbage value while incrementing a void*

Question

This code:

#include <stdio.h>
int main(void)
{
   void *ptr;
   int arr[] = {1,2,3,4,5};
   ptr = arr;
   ptr++;
   printf("%d",*(int*)ptr);
}

Prints some garbage value but I was expecting it to print 2. Why doesn't it print 2?

Answer 1

You can't perform pointer arithmetic on a void pointer because the compiler doesn't have any idea about the size of the pointed to objects.

Your code doesn't get compiled on comeau online. Its another evil gcc extension I guess.

Answer 2

Some C compilers treat void pointer arithmetic as they do char*. It's invalid in C++.

No matter, you really should only be incrementing non void pointers since pointer arithmetic relies on knowledge of the size and alignment of the data type.