[/
 / 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:Executor1 Executor requirements]

The library describes a standard set of requirements for ['executors]. A type
meeting the `Executor` requirements embodies a set of rules for determining how
submitted function objects are to be executed.

A type `X` meets the `Executor` requirements if it satisfies the requirements of
`CopyConstructible` (C++Std [copyconstructible]) and `Destructible` (C++Std
[destructible]), as well as the additional requirements listed below.

No constructor, comparison operator, copy operation, move operation, swap
operation, or member functions `context`, `on_work_started`, and
`on_work_finished` on these types shall exit via an exception.

The executor copy constructor, comparison operators, and other member functions
defined in these requirements shall not introduce data races as a result of
concurrent calls to those functions from different threads.

Let `ctx` be the execution context returned by the executor's `context()`
member function. An executor becomes ['invalid] when the first call to
`ctx.shutdown()` returns. The effect of calling `on_work_started`,
`on_work_finished`, `dispatch`, `post`, or `defer` on an invalid executor is
undefined. [inline_note The copy constructor, comparison operators, and
`context()` member function continue to remain valid until `ctx` is destroyed.]

In the table below, `x1` and `x2` denote (possibly const) values of type `X`,
`mx1` denotes an xvalue of type `X`, `f` denotes a `MoveConstructible` (C++Std
[moveconstructible]) function object callable with zero arguments, `a` denotes
a (possibly const) value of type `A` meeting the `Allocator` requirements
(C++Std [allocator.requirements]), and `u` denotes an identifier.

[table Executor requirements
  [[expression] [type] [assertion/note\npre/post-conditions]]
  [
    [`X u(x1);`]
    []
    [Shall not exit via an exception.\n
     \n
     post: `u == x1` and
     `std::addressof(u.context()) == std::addressof(x1.context()).`]
  ]
  [
    [`X u(mx1);`]
    []
    [Shall not exit via an exception.\n
     \n
     post: `u` equals the prior value of `mx1` and
     `std::addressof(u.context())` equals the prior value of
     `std::addressof(mx1.context())`.]
  ]
  [
    [`x1 == x2`]
    [`bool`]
    [ Returns `true` only if `x1` and `x2` can be interchanged with identical
     effects in any of the expressions defined in these type requirements.
     [inline_note Returning `false` does not necessarily imply that the effects
     are not identical.]\n
     \n
     `operator==` shall be reflexive, symmetric, and transitive, and shall not
     exit via an exception.]
  ]
  [
    [`x1 != x2`]
    [`bool`]
    [Same as `!(x1 == x2)`.]
  ]
  [
    [`x1.context()`]
    [`execution_context&`, or `E&` where `E` is a type that satifisfies the
     [link boost_asio.reference.ExecutionContext `ExecutionContext`] requirements.]
    [Shall not exit via an exception.\n
     \n
     The comparison operators and member functions defined in these
     requirements shall not alter the reference returned by this function.]
  ]
  [
    [`x1.on_work_started()`]
    []
    [Shall not exit via an exception.]
  ]
  [
    [`x1.on_work_finished()`]
    []
    [Shall not exit via an exception.\n
     \n
     Precondition: A preceding call `x2.on_work_started()` where `x1 == x2`.]
  ]
  [
    [`x1.dispatch(std::move(f),a)`]
    []
    [Effects: Creates an object `f1` initialized with
     [^['DECAY_COPY]]`(forward<Func>(f))` (C++Std \[thread.decaycopy\]) in the
     current thread of execution . Calls `f1()` at most once. The executor may
     block forward progress of the caller until `f1()` finishes execution.\n
     \n
     Executor implementations should use the supplied allocator to allocate any
     memory required to store the function object. Prior to invoking the
     function object, the executor shall deallocate any memory allocated.
     [inline_note Executors defined in this Technical Specification always use
     the supplied allocator unless otherwise specified.]\n
     \n
     Synchronization: The invocation of `dispatch` synchronizes with (C++Std
     \[intro.multithread\]) the invocation of `f1`.]
  ]
  [
    [`x1.post(std::move(f),a)`\n
     `x1.defer(std::move(f),a)`]
    []
    [Effects: Creates an object `f1` initialized with
     [^['DECAY_COPY]]`(forward<Func>(f))` in the current thread of execution.
     Calls `f1()` at most once. The executor shall not block forward progress
     of the caller pending completion of `f1()`.\n
     \n
     Executor implementations should use the supplied allocator to allocate any
     memory required to store the function object. Prior to invoking the
     function object, the executor shall deallocate any memory allocated.
     [inline_note Executors defined in this Technical Specification always use
     the supplied allocator unless otherwise specified.]\n
     \n
     Synchronization: The invocation of `post` or `defer` synchronizes with
     (C++Std \[intro.multithread\]) the invocation of `f1`.\n
     \n
     [inline_note Although the requirements placed on `defer` are identical to
     `post`, the use of `post` conveys a preference that the caller ['does not]
     block the first step of [^f1]'s progress, whereas `defer` conveys a
     preference that the caller ['does] block the first step of [^f1]. One use
     of `defer` is to convey the intention of the caller that [^f1] is a
     continuation of the current call context. The executor may use this
     information to optimize or otherwise adjust the way in which `f1` is
     invoked.]]
  ]
]

[endsect]
