### Requirements on asynchronous operations

This section uses the names `Alloc1`, `Alloc2`, `alloc1`, `alloc2`, `Args`, `CompletionHandler`, `completion_handler`, `Executor1`, `Executor2`, `ex1`, `ex2`, `f`, `i`, `N`, `Signatures`, `token`, `initiation`, `Initiation`, `Ti`, `ti`, `work1`, and `work2` as placeholders for specifying the requirements below.

##### General asynchronous operation concepts

An initiating function is a function which may be called to start an asynchronous operation. A completion handler is a function object that will be invoked, at most once, with the result of the asynchronous operation.

The lifecycle of an asynchronous operation is comprised of the following events and phases:

— Event 1: The asynchronous operation is started by a call to the initiating function.

— Phase 1: The asynchronous operation is now outstanding.

— Event 2: The externally observable side effects of the asynchronous operation, if any, are fully established. The completion handler is submitted to an executor.

— Phase 2: The asynchronous operation is now completed.

— Event 3: The completion handler is called with the result of the asynchronous operation.

In this library, all functions with the prefix `async_` are initiating functions.

##### Completion tokens and handlers

Initiating functions:

— are function templates with template parameter `CompletionToken`;

— accept, as the final parameter, a completion token object `token` of type `CompletionToken`;

— specify one or more completion signatures, referred to below as a variadic pack of call signatures (C++Std [func.def]) `Signatures`, that determine the possible argument sets to the completion handler.

In this library, initiating functions specify a Completion signature element that defines the call signatures `Signatures`. The Completion signature elements in this library may have named parameters, and the results of an asynchronous operation may be specified in terms of these names.

Completion token behaviour is determined through specialisation of the `async_result` trait. These specialisations must have the form:

    template <class CompletionToken, class... Signatures>
    struct async_result
    {
      template<
          class Initiation,
          class RawCompletionToken,
          class... Args
        >
      static initiating-fn-return-type initiate(
          Initiation&& initiation,
          RawCompletionToken&& token,
          Args&&... args
        );
    };

An `async_result` specialisation’s implementation of the `initiate()` static member function must:

- Transform the completion token into an object `completion_handler` of type `CompletionHandler`. The type `CompletionHandler` must satisfy the requirements of `Destructible` (C++Std [destructible]) and `MoveConstructible` (C++Std [moveconstructible]), and be callable with the specified call signatures.

- Cause the invocation of the function object `initiation` as if by calling:

      std::forward<Initiation>(initiation)(
          std::move(completion_handler),
          std::forward<Args>(args)...);

This invocation of `initiation` may be immediate, or it may be deferred (e.g. to support lazy evaluation). If `initiation` is deferred, the `initiation` and `args...` objects must be decay-copied and moved as required.

The `async_result` trait must be specialised for the decayed type of a `CompletionToken`. A helper function template `async_initiate` is provided to simplify correct invocation of `async_result<>::initiate` for the appropriate specialisation:

    template<
        class CompletionToken,
        completion_signature... Signatures,
        class Initiation,
        class... Args
      >
    DEDUCED async_initiate(
        Initiation&& initiation,
        CompletionToken& token,
        Args&&... args
      );

[Note: No other requirements are placed on the type `CompletionToken`. —end note]

##### Automatic deduction of initiating function return type

An initiating function returns `async_initiate<CompletionToken, Signatures...>(initiation, token, unspecified-args...)`, where `initiation` is a function object of unspecified type `Initiation`, which is defined as:

    class Initiation
    {
    public:
      using executor_type = Executor; // optional
      executor_type get_executor() const noexcept; // optional

      template <class CompletionHandler, unspecified-args...>
        void operator()(CompletionHandler&& completion_handler, unspecified-args...) const;
    };

For the sake of exposition, this library sometimes annotates functions with a return type `DEDUCED`. For every function declaration that returns `DEDUCED`, the meaning is equivalent to an automatically deduced return type, having the type of the expression `async_initiate<CompletionToken, Signatures...>(initiation, token, unspecified-args...)`.

[Example: Given an asynchronous operation with a single Completion signature `void(R1 r1, R2 r2)`, an initiating function meeting these requirements may be implemented as follows:

    template<class CompletionToken>
    auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
    {
      return async_initiate<CompletionToken, void(R1, R2)>(
          [](auto completion_handler, T1 t1, T2 t2)
          {
            // initiate the operation and cause completion_handler to be invoked
            // with the result
          }, token, std::move(t1), std::move(t2));
    }

The concepts `completion_token_for` and `completion_handler_for` may also be used to improve compile-time diagnostics:

    template<completion_token_for<void(R1, R2)> CompletionToken>
    auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
    {
      return async_initiate<CompletionToken, void(R1, R2)>(
          [](completion_handler_for<void(R1, R2)> auto completion_handler, T1 t1, T2 t2)
          {
            // initiate the operation and cause completion_handler to be invoked
            // with the result
          }, token, std::move(t1), std::move(t2));
    }

Initiation functions may also be implemented using the `async_result` trait directly:

    template<class CompletionToken>
    auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
    {
      return async_result<decay_t<CompletionToken>, void(R1, R2)>::initiate(
          [](auto completion_handler, T1 t1, T2 t2)
          {
            // initiate the operation and cause completion_handler to be invoked
            // with the result
          }, forward<CompletionToken>(token), std::move(t1), std::move(t2));
    }

Note the use of `decay_t` and `forward` applied to the `CompletionToken` type. However, the first form is preferred as it preserves compatibility with legacy completion token requirements.

—end example]

##### Lifetime of initiating function arguments

Unless otherwise specified, the lifetime of arguments to initiating functions shall be treated as follows:

— If the parameter has a pointer type or has a type of lvalue reference to non-const, the implementation may assume the validity of the pointee or referent, respectively, until the completion handler is invoked. [Note: In other words, the program must guarantee the validity of the argument until the completion handler is invoked. —end note]

— Otherwise, the implementation must not assume the validity of the argument after the initiating function completes. [Note: In other words, the program is not required to guarantee the validity of the argument after the initiating function completes. —end note] The implementation may make copies of the argument, and all copies shall be destroyed no later than immediately after invocation of the completion handler.

##### Non-blocking requirements on initiating functions

An initiating function shall not block (C++Std [defns.block]) the calling thread pending completion of the outstanding operation.

[Note: Initiating functions may still block the calling thread for other reasons. For example, an initiating function may lock a mutex in order to synchronize access to shared data. —end note]

##### Associated executor

Certain objects that participate in asynchronous operations have an associated executor. These are obtained as specified in the sections below.

##### Associated I/O executor

An asynchronous operation has an associated executor satisfying the `Executor` requirements. If not otherwise specified by the asynchronous operation, this associated executor is an object of type `system_executor`.

All asynchronous operations in this library have an associated I/O executor object that is determined as follows:

— If the initiating function is a member function, the associated executor is that returned by the `get_executor` member function on the same object.

— If the initiating function is not a member function, the associated executor is that returned by the `get_executor` member function of the first argument to the initiating function.

The operation's associated I/O executor may be exposed via the `Initiation` function object's `executor_type` type alias and `get_executor()` member function.

Let `Executor1` be the type of the associated executor. Let `ex1` be a value of type `Executor1`, representing the associated executor object obtained as described above.

##### Associated completion handler executor

A completion handler object of type `CompletionHandler` has an associated executor of type `Executor2` satisfying the Executor requirements. The type `Executor2` is `associated_executor_t<CompletionHandler, Executor1>`. Let `ex2` be a value of type `Executor2` obtained by performing `get_associated_executor(completion_handler, ex1)`.

##### Associated immediate completion executor

A completion handler object of type `CompletionHandler` has an associated immediate executor of type `Executor3` satisfying the Executor requirements. The type `Executor3` is `associated_immediate_executor_t<CompletionHandler, Executor1>`. Let `ex3` be a value of type `Executor3` obtained by performing `get_associated_immediate_executor(completion_handler, ex1)`.

##### Outstanding work

If the operation does not complete immediately (that is, the operation does not complete within the thread of execution calling the initiating function, before the initiating function returns) then, until the asynchronous operation has completed, the asynchronous operation shall maintain:

— an object `work1` of type `executor_work_guard<Executor1>`, initialized as `work1(ex1)`, and where `work1.owns_work() == true`; and

— an object `work2` of type `executor_work_guard<Executor2>`, initialized as `work2(ex2)`, and where `work2.owns_work() == true`.

##### Allocation of intermediate storage

Asynchronous operations may allocate memory. [Note: Such as a data structure to store copies of the `completion_handler` object and the initiating function's arguments. —end note]

Let `Alloc1` be a type, satisfying the `ProtoAllocator` requirements, that represents the asynchronous operation's default allocation strategy. [Note: Typically `std::allocator<void>`. —end note] Let `alloc1` be a value of type `Alloc1`.

A completion handler object of type `CompletionHandler` has an associated allocator object `alloc2` of type `Alloc2` satisfying the `ProtoAllocator` requirements. The type `Alloc2` is `associated_allocator_t<CompletionHandler, Alloc1>`. Let `alloc2` be a value of type `Alloc2` obtained by performing `get_associated_allocator(completion_handler, alloc1)`.

The asynchronous operations defined in this library:

— If required, allocate memory using only the completion handler's associated allocator.

— Prior to completion handler execution, deallocate any memory allocated using the completion handler's associated allocator.

[Note: The implementation may perform operating system or underlying API calls that perform memory allocations not using the associated allocator. Invocations of the allocator functions may not introduce data races (See C++Std [res.on.data.races]). —end note]

##### Execution of completion handler on completion of asynchronous operation

Let `Args...` be the argument types of a completion signature in `Signatures...` and let `N` be `sizeof...(Args)`. Let `i` be in the range [`0`,`N`). Let `Ti` be the `i`th type in `Args...` and let `ti` be the `i`th completion handler argument associated with `Ti`.

Let `f` be a function object that:

— is callable as `f()`, and when so called invokes `completion_handler` as if by `std::move(completion_handler)(forward<T0>(t0), ..., forward<TN-1>(tN-1))`;

— has an associated executor such that `get_associated_executor(f, ex1)` returns an executor that is equal to `ex2`; and

— has an associated allocator such that `get_associated_allocator(f, alloc1)` returns an allocator that is equal to `alloc2`.

[Note: These associated executor and allocator requirements on `f` are typically implemented by specialising the associator traits for `f` so that they forward to the associator traits for `completion_handler`. —end note]

If an asynchonous operation completes immediately (that is, the operation completes within the thread of execution calling the initiating function, and before the initiating function returns), the completion handler shall be submitted for execution as if by performing `post(ex1, std::move(f))`.

Otherwise, when the operation completes, the completion handler shall be submitted for execution as if by performing `dispatch(ex2, std::move(f))`.

##### Optimisation of immediate completion

If an asynchronous operation completes immediately then, as an optimisation, the operation may either:

— obtain the associated immediate completion executor `ex3` by performing `get_associated_immediate_executor(completion_handler, ex1)`, and then submit the completion handler for execution as if by performing `dispatch(ex3, std::move(f))`; or

— submit the completion handler for execution by performing the expression `post(ex2, std::move(f))`, if that expression is well-formed.

[Note: If `completion_handler` does not customise the associated immediate executor, the behaviour of the first optimisation is equivalent to `post(ex1, std::move(f))`. —end note]

##### Completion handlers and exceptions

Completion handlers are permitted to throw exceptions. The effect of any exception propagated from the execution of a completion handler is determined by the executor which is executing the completion handler.

##### Default completion tokens

Every I/O executor type has an associated default completion token type. This is specified via the `default_completion_token` trait. This trait may be used in asynchronous operation declarations as follows:

    template <
        typename IoObject,
        typename CompletionToken =
          typename default_completion_token<
            typename IoObject::executor_type
          >::type
      >
    auto async_xyz(
        IoObject& io_object,
        CompletionToken&& token =
          typename default_completion_token<
            typename IoObject::executor_type
          >::type{}
      );

If not specialised, this trait type is `void`, meaning no default completion token type is available for the given I/O executor.

[Example: The `default_completion_token` trait is specialised for the `use_awaitable` completion token so that it may be used as shown in the following example:

    auto socket = use_awaitable.as_default_on(tcp::socket(my_context));
    // ...
    co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable.

In this example, the type of the `socket` object is transformed from `tcp::socket` to have an I/O executor with the default completion token set to `use_awaitable`.

Alternatively, the socket type may be computed directly:

    using tcp_socket = use_awaitable_t<>::as_default_on_t<tcp::socket>;
    tcp_socket socket(my_context);
    // ...
    co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable.

—end example]