Receiver concepts

A receiver represents the continuation of an asynchronous operation. An asynchronous operation may complete with a (possibly empty) set of values, an error, or it may be cancelled. A receiver has three principal operations corresponding to the three ways an asynchronous operation may complete: set_value, set_error, and set_done. These are collectively known as a receiver’s completion-signal operations.

template<class T, class E = exception_ptr>
concept receiver =
  move_constructible<remove_cvref_t<T>> &&
  constructible_from<remove_cvref_t<T>, T> &&
  requires(remove_cvref_t<T>&& t, E&& e) {
    { execution::set_done(std::move(t)) } noexcept;
    { execution::set_error(std::move(t), (E&&) e) } noexcept;
  };

template<class T, class... An>
concept receiver_of =
  receiver<T> &&
  requires(remove_cvref_t<T>&& t, An&&... an) {
    execution::set_value(std::move(t), (An&&) an...);
  };

The receiver’s completion-signal operations have semantic requirements that are collectively known as the receiver contract, described below:

None of a receiver’s completion-signal operations shall be invoked before execution::start has been called on the operation state object that was returned by execution::connect to connect that receiver to a sender.
Once execution::start has been called on the operation state object, exactly one of the receiver’s completion-signal operations shall complete non-exceptionally before the receiver is destroyed.
If execution::set_value exits with an exception, it is still valid to call execution::set_error or execution::set_done on the receiver.

Once one of a receiver’s completion-signal operations has completed non-exceptionally, the receiver contract has been satisfied.

Boost C++ Libraries

Receiver concepts