Saturday, January 10, 2009

void ptr - thread library and other usages

There's an ivory tower view that "malloc, char arrays (string) and other arrays are largely unused in c++, since we have delete/new, std::string, vector". Now i see one more -- void pointers. despite the ivory tower, void pointers are widely used and indispensable

* global op-new returns a pointer of type ... can't be any specific type; must be void
** even per-class op-new (implicitly static) returns void ptr. See P282 ARM
* allocators have an op-new. ditto
* other usages --- See boost::any.

A lot of, if not most, thread libraries are written in C to be usable to C and C++. A single void pointer is a flexible and versatile vehicle to "transport" several arguments into a functor, which is then handed to a thread. Here's an example.

thread (void (FVVP*)(void *), void* vptr) // --------creates and starts a new thread. FVVP means a functor returning Void, accepting a Void Pointer

Above is a thread api with 2 arguments -- a functor and a void ptr. The functor points to a function accepting a void ptr, so the 2nd argument feeds into the functor.

Now Suppose you realize the actual function you intend for the thread needs access to multiple objects, but you have only the 2nd argument to transport them. Thanks to void pointer, you can create a struct containing those multiple objects, and pass a single pointer as 2nd arg to the thread API. This usage of void pointer resembles
* the Object pointer in java.
* the raw HashMap argument Piroz suggested.
* the std::vector holding heterogeneous objects.

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