13 Asynchronous model [async]

An initiating function produces its return type as follows:

• (1.1)

  constructing an object result of type async_result<decay_t<CompletionToken>, Signature>, initialized as result(completion_handler); and

• (1.2)

  using result.get() as the operand of the return statement.

[ Example: Given an asynchronous operation with 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){
  typename async_result<decay_t<CompletionToken>, void(R1, R2)>::completion_handler_type
    completion_handler(forward<CompletionToken>(token));

  async_result<decay_t<CompletionToken>, void(R1, R2)> result(completion_handler);

  // initiate the operation and cause completion_handler to be invoked with
  // the result

  return result.get();
}

For convenience, initiating functions may be implemented using the async_completion template:

template<class CompletionToken>
auto async_xyz(T1 t1, T2 t2, CompletionToken&& token){
  async_completion<CompletionToken, void(R1, R2)> init(token);

  // initiate the operation and cause init.completion_handler to be invoked
  // with the result

  return init.result.get();
}

 — end example ]