I tried looking through source but I cant navigate that much of a template code.
Basically: this is what documentation says (for close()):
One can rely on the socket performing proper cleanup with RAII.
When an IO object, such as socket, is destroyed, its destructor will invoke destroy() on the IO object's service, passing in an instance of the implementation_type on which the IO object's service will operate. The SocketService requirements state that destroy() will implicitly cancel asynchronous operations as-if by calling the close() on the service, which has a post condition that is_open() returns false. Furthermore, the service's close() will cause outstanding asynchronous operations to complete as soon as possible. Handlers for cancelled operations will be passed the error code boost::asio::error::operation_aborted, and scheduled for deferred invocation within the io_service. These handlers are removed from the io_service if they are either invoked from a thread processing the event loop or the io_service is destroyed.