Constructor.

explicit basic_waitable_timer(
    asio::io_service & io_service);
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Constructor to set a particular expiry time as an absolute time.

basic_waitable_timer(
    asio::io_service & io_service,
    const time_point & expiry_time);
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Constructor to set a particular expiry time relative to now.

basic_waitable_timer(
    asio::io_service & io_service,
    const duration & expiry_time);
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