[/
 / Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:WaitTraits Wait traits requirements]

The `basic_waitable_timer` template uses wait traits to allow programs to
customize `wait` and `async_wait` behavior.
[inline_note Possible uses of wait traits include:\n
[mdash] To enable timers based on non-realtime clocks.\n
[mdash] Determining how quickly wallclock-based timers respond to system time
changes.\n
[mdash] Correcting for errors or rounding timeouts to boundaries.\n
[mdash] Preventing duration overflow. That is, a program may set a timer's
expiry `e` to be `Clock::max()` (meaning never reached) or `Clock::min()`
(meaning always in the past). As a result, computing the duration until timer
expiry as `e - Clock::now()` may cause overflow.]

For a type `Clock` meeting the `Clock` requirements (C++Std
[time.clock.req]), a type `X` meets the `WaitTraits` requirements if it
satisfies the requirements listed below.

In the table below, `t` denotes a (possibly const) value of type
`Clock::time_point`; and `d` denotes a (possibly const) value of type
`Clock::duration`.

[table WaitTraits requirements
  [[expression] [return type] [assertion/note\npre/post-condition]]
  [
    [`X::to_wait_duration(d)`]
    [`Clock::duration`]
    [
      Returns a `Clock::duration` value to be used in a `wait` or `async_wait`
      operation. [inline_note The return value is typically representative of
      the duration `d`.]
    ]
  ]
  [
    [`X::to_wait_duration(t)`]
    [`Clock::duration`]
    [
      Returns a `Clock::duration` value to be used in a `wait` or `async_wait`
      operation. [inline_note The return value is typically representative of
      the duration from `Clock::now()` until the time point `t`.]
    ]
  ]
]

[endsect]