5. What is a void pointer?
A void pointer is a C convention for “a raw address.” The compiler has no idea what type of object a voidpointer “really points to.” If you write
int *ip;
ip points to an int. If you write
void *p;
p doesn’t point to a void!
In C and C++, any time you need a void pointer, you can use another pointer type. For example, if you have achar*, you can pass it to a function that expects a void*. You don’t even need to cast it. In C (but not in C++), you can use a void* any time you need any kind of pointer, without casting. (In C++, you need to cast it.)
6. When is a void pointer used?
A void pointer is used for working with raw memory or for passing a pointer to an unspecified type.
Some C code operates on raw memory. When C was first invented, character pointers (char *) were used for that. Then people started getting confused about when a character pointer was a string, when it was a character array, and when it was raw memory.
For example, strcpy() is used to copy data from one string to another, and strncpy() is used to copy at most a certain length string to another:
char *strcpy( char *str1, const char *str2 );
char *strncpy( char *str1, const char *str2, size_t n );
memcpy() is used to move data from one location to another:
void *memcpy( void *addr1, void *addr2, size_t n );
void pointers are used to mean that this is raw memory being copied. NUL characters (zero bytes) aren’t significant, and just about anything can be copied. Consider the following code:
#include “thingie.h” /* defines struct thingie */
struct thingie *p_src, *p_dest;
/* … */
memcpy( p_dest, p_src, sizeof( struct thingie) * numThingies );
This program is manipulating some sort of object stored in a struct thingie. p1 and p2 point to arrays, or parts of arrays, of struct thingies. The program wants to copy numThingies of these, starting at the one pointed to by p_src, to the part of the array beginning at the element pointed to by p_dest. memcpy() treats p_src andp_dest as pointers to raw memory; sizeof( struct thingie) * numThingies is the number of bytes to be copied.
