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Long running I/O operations will often have a deadline by which they must have completed. These deadlines may be expressed as absolute times, but are often calculated relative to the current time.
As a simple example, to perform a synchronous wait operation on a timer using a relative time one may write:
io_context i; ... deadline_timer t(i); t.expires_from_now(boost::posix_time::seconds(5)); t.wait();
More commonly, a program will perform an asynchronous wait operation on a timer:
void handler(asio::error_code ec) { ... }
...
io_context i;
...
deadline_timer t(i);
t.expires_from_now(boost::posix_time::milliseconds(400));
t.async_wait(handler);
...
i.run();
The deadline associated with a timer may also be obtained as a relative time:
boost::posix_time::time_duration time_until_expiry = t.expires_from_now();
or as an absolute time to allow composition of timers:
deadline_timer t2(i); t2.expires_at(t.expires_at() + boost::posix_time::seconds(30));