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In general, it is safe to make concurrent use of distinct objects, but
unsafe to make concurrent use of a single object. However, types such as
io_context provide a stronger
guarantee that it is safe to use a single object concurrently.
Multiple threads may call io_context::run() to set up a pool of threads from which
completion handlers may be invoked. This approach may also be used with
post()
as a means to perform arbitrary computational tasks across a thread pool.
Note that all threads that have joined an io_context's
pool are considered equivalent, and the io_context
may distribute work across them in an arbitrary fashion.
The implementation of this library for a particular platform may make use of one or more internal threads to emulate asynchronicity. As far as possible, these threads must be invisible to the library user. In particular, the threads:
This approach is complemented by the following guarantee:
io_context::run().
Consequently, it is the library user's responsibility to create and manage all threads to which the notifications will be delivered.
The reasons for this approach include:
io_context::run() from a single thread, the user's
code can avoid the development complexity associated with synchronisation.
For example, a library user can implement scalable servers that are
single-threaded (from the user's point of view).
CoInitializeEx
before any other COM operations can be called from that thread.