Asynchronous Operations

An asynchronous operation is the basic unit of composition in the Boost.Asio asynchronous model. Asynchronous operations represent work that is launched and performed in the background, while the user's code that initiated the work can continue with other things.

Conceptually, the lifecycle of an asynchronous operation can be described in terms of the following events and phases:

An initiating function is a function which may be called by the user to start an asynchronous operation.

A completion handler is a user-provided, move-only function object that will be invoked, at most once, with the result of the asynchronous operation. The invocation of the completion handler tells the user about something that has already happened: the operation completed, and the side effects of the operation were established.

The initiating function and completion handler are incorporated into the user's code as follows:

Synchronous operations, being embodied as single functions, have several inherent semantic properties as a consequence. Asynchronous operations adopt some of these semantic properties from their synchronous counterparts, in order to facilitate flexible and efficient composition.

Property of synchronous operations	Equivalent property of asynchronous operations
When a synchronous operation is generic (i.e. a template) the return type is deterministically derived from the function and its arguments.	When an asynchronous operation is generic, the completion handler's arguments' types and order are deterministically derived from the initiating function and its arguments.
If a synchronous operation requires a temporary resource (such as memory, a file descriptor, or a thread), this resource is released before returning from the function.	If an asynchronous operation requires a temporary resource (such as memory, a file descriptor, or a thread), this resource is released before calling the completion handler.

The latter is an important property of asynchronous operations, in that it allows a completion handler to initiate further asynchronous operations without overlapping resource usage. Consider the trivial (and relatively common) case of the same operation being repeated over and over in a chain:

By ensuring that resources are released before the completion handler runs, we avoid doubling the peak resource usage of the chain of operations.