In Boost.Asio, many objects, such as sockets and timers, support object-wide cancellation of outstanding asynchronous operations via their close or cancel member functions. However, certain asynchronous operations also support individual, targeted cancellation. This per-operation cancellation is enabled by specifying that every asynchronous agent has an associated cancellation slot.
To support cancellation, an asynchronous operation installs a cancellation handler into the agent's slot. The cancellation handler is a function object that will be invoked when a cancellation signal is emitted by the user into the slot. Since a cancellation slot is associated with a single agent, the slot holds at most one handler at a time, and installing a new handler will overwrite any previously installed handler. Thus, the same slot is reused for subsequent asynchronous operations within the agent.
Cancellation is particularly useful when an asynchronous operation contains multiple child agents. For example, one child agent may be complete and the other is then cancelled, as its side effects are no longer required.