Mastering Algorithms with C

Generic Pointers and Casts

Recall that pointer variables in C have types just like other variables. The main reason for this is so that when we dereference a pointer, the compiler knows the type of data being pointed to and can access the data accordingly. However, sometimes we are not concerned about the type of data a pointer references. In these cases we use generic pointers, which bypass C’s type system.

Generic Pointers

Normally C allows assignments only between pointers of the same type. For example, given a character pointer sptr (a string) and an integer pointer iptr, we are not permitted to assign sptr to iptr or iptr to sptr. However, generic pointers can be set to pointers of any type, and vice versa. Thus, given a generic pointer gptr, we are permitted to assign sptr to gptr or gptr to sptr. To make a pointer generic in C, we declare it as a void pointer .

There are many situations in which void pointers are useful. For example, consider the standard C library function memcpy, which copies a block of data from one location in memory to another. Because memcpy may be used to copy data of any type, it makes sense that its pointer parameters are void pointers. Void pointers can be used to make other types of functions more generic as well. For example, we might have implemented the swap2 function presented earlier so that it swapped data of any type, as shown in the following code:

#include <stdlib.h> #include <string.h> int swap2(void *x, void *y, int size) { void *tmp; if ...