Using Allocators in Creation Functions

Each Core Foundation opaque type has one or more creation functions, functions that create and return an object of that type initialized in a particular way. All creation functions take as their first parameter a reference to an allocator object (CFAllocatorRef). Some functions may also have allocator parameters for specialized allocation and deallocation purposes.

You have several options for the allocator-reference parameters:

• You can pass the constant kCFAllocatorSystemDefault; this specifies the generic system allocator (which is the initial default allocator).

• You can pass NULL to specify the current default allocator (which might be a custom allocator or the generic system allocator). This is the same as passing kCFAllocatorDefault.

• You can pass the constant kCFAllocatorNull which indicates an allocator that does not allocate—it is an error to attempt to use it. Some creation functions have a parameter for a special allocator used to reallocate or free a backing store; by specifying kCFAllocatorNull for the parameter, you prevent automatic reallocation or deallocation.

• You can get a reference to the allocator used by another Core Foundation object with the CFGetAllocator function and pass that reference in. This technique allows you to put related objects in a memory “zone” by using the same allocator for allocating them.

• You can pass a reference to a custom allocator (see Creating Custom Allocators).

If you are to use a custom allocator and you want to make it the default allocator, it is advisable to first get a reference to the current default allocator using the CFAllocatorGetDefault function and store that in a local variable. When you are finished using your custom allocator, use the CFAllocatorSetDefault function to reset the stored allocator as the default allocator.