Hi Rob, --- Rob Stewart <stewart@sig.com> wrote:
Should that interface take an extra argument (void *, for example) that permits the client to get back some context? The client may need to provide a this pointer or allocate a structure on the free store and get back data needed to correctly react.
The context is the pointer to the win_iocp_operation object. The way it works is like this: - The class to perform the asynchronous handling is derived from win_iocp_operation, which is in turn derived from OVERLAPPED. Any context information (e.g. the application's handler for the operation) is stored as a data member in this class: OVERLAPPED ^ | win_iocp_operation ^ | send_operation<HANDLER> - When an async operation is started, a new instance of the derived class is created, and it is passed as the OVERLAPPED* argument of the asynchronous operation (e.g. ::WSASend). - In win_iocp_demuxer_service::run(), the call to ::GetQueuedCompletionStatus retrieves the OVERLAPPED* value, which it then casts to win_iocp_operation and then calls the associated function pointer. - The class derived from win_iocp_operation invokes the application's handler.
(If there are no other arguments to the win_iocp_operation invocation, then the client needn't be bothered with creating such a structure, but the implementation might still want to.)
Please remember that all this stuff is right down in the bowels of asio. A user of asio never needs to deal with it. Cheers, Chris