When to use a void pointer?

One good scenario void* use is when you want to implement any generic ADT's, in case you just don't know what datatype it going to keep and deal with. For example, a linked list like the following:

typedef struct node_t node;
struct
{
    void* data;
    node* prev, next;
} node_t;

typedef struct list_t list;
typedef void* (func)(void*) cpy_func;
typedef void (func)(void*) del_func;
struct
{
   node* head, tail, curr;
   cpy_func copy;
   del_func delete;
} list_t;

initializeLinkedList(cpy_func cpy, del_func del);
//here you keep going defining an API

Here for example you will pass in initialization function pointers to other functions which will be capable of copying your datatype to your list and freeing it afterwards. So by using void* you make your list more generic.

I think void* remained in C++ only because of backward compatibility, since in C++ you have more safe and sophisticated ways to achieve the same result like templates, functors, etc., and you don't need to use malloc while programming C++.

Regarding C++, I don't have any specific useful examples.

If you are interfacing with C code and need to pass through a C++ object, but a C library will only take a generic pointer, then when you retrieve the pointer you need to re-cast it to the proper type.

Void pointers probably shouldn't be used very often, but they can help when you're trying to use a library function that works with arbitrary pointers, and doesn't really care what data is represented by that memory.

void pointers should be used any time the contents of a block of data is not important. For example when copying data the contents of a memory area is copied but the format of the data is not important.

For functions that operate on blocks of memory without needing to understand the contents using void pointers clarifies the design to users so that they know the function does not care for any data format. Often functions a coded to take a char * to handle blocks of memory when the function is actually content agnostic.

A GREAT way to learn all about void * and other C topics is to watch the first half of the fantastic Stanford "Programming Paradigms" on iTunes-U. It really explains void * (C generics) and pointers in general fantastically! It definately helped me learn C better...

One of the biggest uses is to use void * if you want to be able to accept different types of data in a function. (heres an example: http://142.132.30.225/programming/node87.html)

Here's a further example of what you can use them for:

  int i;
  char c;
  void *the_data;

  i = 6;
  c = 'a';

  the_data = &i;
  printf("the_data points to the integer value %d\n", *(int*) the_data);

  the_data = &c;
  printf("the_data now points to the character %c\n", *(char*) the_data);

If you don't want to watch the free stanford classes, i'd recommend googling void pointer and reading all the material there.

It is commonly used in numerical code, for example a C root solver function might look like that:

double find_root(double x0, double (*f)(double, void*), void* params)
{
/* stuff */
y = f(x, params);
/* other stuff */
}

params is cast by f to some structure it knows about, but find_root doesn't.

Another example of such C "generics", implemented with void *, is a standard qsort function:

void qsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));

You can sort an array of any type: int, long, double, char * or some struct pointers...

Void pointers are useful when you write code that needs to run on multiple operating systems and needs to be fairly agnostic of underlying framework APIs.

For example, OS X, Windows and Linux all have the basic concept of a window object, but they're all very different. So I have common code that passes them around as void*'s and then platform specific implementations that cast the void* to the native type (HWND, etc).

But, yeah, as others have said in this thread, this sort of thing is certainly to be avoided except where necessary.

Next to interfacing with C, I find myself only using void pointers when I need to debug / trace some code and like to know the address of a certain pointer.

SomeClass * myInstance;
// ...
std::clog << std::hex << static_cast< void* >(myInstance) << std::endl;

Will print something like

0x42A8C410

And, in my opinion, nicely documents what I'm trying to do (know the pointer address, not anything about the instance)

Look at sqlite3_exec(). You start an SQL query and want to process the results in some way (store them into a container). You call the sqlite3_exec() and pass a callback pointer and a void* pointer to whatever object you want (container included). When sqlite3_exec() runs it calls the callback for each retrieved row and passed that void* pointer into it so the callback can cast the pointer and do whatever you intended.

The important thing is that sqlite3_exec() doesn't care what the callback does and what pointer you pass. void* is exactly for such pointers.