[/
 / Copyright (c) 2003-2022 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:associators Associated Characteristics and Associators]

An asynchronous agent has ['associated characteristics] that specify how
asynchronous operations should behave when composed as part of that agent, such
as:

* An allocator, which determines how the agent's asynchronous operations obtain
  memory resources.

* A cancellation slot, which determines how the agent's asynchronous operations
  support cancellation.

* An executor, which determines how the agent's completion handlers will be
  queued and run.

When an asynchronous operation is run within an asynchronous agent, its
implementation may query these associated characteristics and use them to
satisfy the requirements or preferences they represent. The asynchronous
operation performs these queries by applying ['associator] traits to the
completion handler. Each characteristic has a corresponding associator trait.

An associator trait may be specialised for concrete completion handler types to:

* accept the default characteristic supplied by the asynchronous operation,
  returning this default as-is

* return an unrelated implementation of the characteristic, or

* adapt the supplied default to introduce additional behaviour required by the
  completion handler.

[heading Specification of an Associator]

Given an associator trait named[footnote The associator traits are named
[link boost_asio.reference.associated_allocator `associated_allocator`],
[link boost_asio.reference.associated_executor `associated_executor`], and
[link boost_asio.reference.associated_cancellation_slot
`associated_cancellation_slot`].] `associated_R`, having:

* a source value `s` of type `S`, in this case the completion handler and its
  type,

* a set of type requirements (or a concept) `R` that define the syntactic and
  semantic requirements of the associated characteristic, and

* a candidate value `c` of type `C` that meets the type requirements `R`, which
  represents a default implementation of the associated characteristic,
  supplied by the asynchronous operation

the asynchronous operation uses the associator trait to compute:

* the type `associated_R<S, C>::type`, and

* the value `associated_R<S, C>::get(s, c)`

that meet the requirements defined in `R`. For convenience, these are also
accessible via type alias `associated_R_t<S, C>` and free function
`get_associated_R(s, c)`, respectively.

The trait's primary template is specified such that:

* if `S::R_type` is well-formed, defines a nested type alias type as
  `S::R_type`, and a static member function get that returns `s.get_R()`

* otherwise, if `associator<associated_R, S, C>::type` is well-formed and
  denotes a type, inherits from `associator<associated_R, S, C>`

* otherwise, defines a nested type alias type as `C`, and a static member
  function get that returns `c`.

[endsect]
