Using the Allocator Context

Every allocator in Core Foundation has a context. A context is a structure that defines the operating environment for an object and typically consists of function pointers. The context for allocators is defined by the CFAllocatorContext structure. In addition to function pointers, the structure contains fields for a version number and for user-defined data

Listing 1  The CFAllocatorContext structure

typedef struct {

    CFIndex version;

    void * info;

    const void *(*retain)(const void *info);

    void (*release)(const void *info);

    CFStringRef (*copyDescription)(const void *info);

    void * (*allocate)(CFIndex size, CFOptionFlags hint, void *info);

    void * (*reallocate)(void *ptr, CFIndex newsize, CFOptionFlags hint, void *info);

    void (*deallocate)(void *ptr, void *info);

    CFIndex (*preferredSize)(CFIndex size, CFOptionFlags hint, void *info);

} CFAllocatorContext;

The info field contains any specially defined data for the allocator. For example, an allocator could use the info field to track outstanding allocations.

If you have some user-defined data in the allocator context (the info field), use the CFAllocatorGetContext function to obtain the CFAllocatorContext structure for an allocator. Then evaluate or handle the data as needed. The following code provides an example of this:

Listing 2  Getting the allocator context and user-defined data

static int numOutstandingAllocations(CFAllocatorRef alloc) {

    CFAllocatorContext context;

    context.version = 0;

    CFAllocatorGetContext(alloc, &context);

    return (*(int *)(context.info));

}

Other Core Foundation functions invoke the memory-related callbacks defined in an allocator context and take or return an untyped pointer to a block of memory (void *):

• CFAllocatorAllocate, allocates a block of memory.

• CFAllocatorReallocate reallocates a block of memory.

• CFAllocatorDeallocate deallocates a block of memory.

• CFAllocatorGetPreferredSizeForSize gives the size of memory likely to be allocated, given a certain request.