### Custom Memory Allocation

Many asynchronous operations need to allocate an object to store state associated with the operation. For example, a Win32 implementation needs `OVERLAPPED`-derived objects to pass to Win32 API functions.

Furthermore, programs typically contain easily identifiable chains of asynchronous operations. A half duplex protocol implementation (e.g. an HTTP server) would have a single chain of operations per client (receives followed by sends). A full duplex protocol implementation would have two chains executing in parallel. Programs should be able to leverage this knowledge to reuse memory for all asynchronous operations in a chain.

Given a copy of a user-defined `Handler` object `h`, if the implementation needs to allocate memory associated with that handler it will execute the code:

    void* pointer = asio_handler_allocate(size, &h);

Similarly, to deallocate the memory it will execute:

    asio_handler_deallocate(pointer, size, &h);

These functions are located using argument-dependent lookup. The implementation provides default implementations of the above functions in the `asio` namespace:

    void* asio_handler_allocate(size_t, ...);
    void asio_handler_deallocate(void*, size_t, ...);

which are implemented in terms of `::operator new()` and `::operator delete()` respectively.

The implementation guarantees that the deallocation will occur before the associated handler is invoked, which means the memory is ready to be reused for any new asynchronous operations started by the handler.

The custom memory allocation functions may be called from any user-created thread that is calling a library function. The implementation guarantees that, for the asynchronous operations included the library, the implementation will not make concurrent calls to the memory allocation functions for that handler. The implementation will insert appropriate memory barriers to ensure correct memory visibility should allocation functions need to be called from different threads.