33 Execution control library [exec]

33.9 Senders [exec.snd]

33.9.13 Sender consumers [exec.consumers]

33.9.13.2 this_thread​::​sync_wait_with_variant [exec.sync.wait.var]

1
#
The name this_thread​::​sync_wait_with_variant denotes a customization point object.
For a subexpression sndr, let Sndr be decltype(into_variant(sndr)).
If sender_in<Sndr, sync-wait-env> is false, this_thread​::​sync_wait_with_variant(sndr) is ill-formed.
Otherwise, it is expression-equivalent to the following, except sndr is evaluated only once: apply_sender(get-domain-early(sndr), sync_wait_with_variant, sndr) Mandates:
  • (1.1)
    The type sync-wait-with-variant-result-type<Sndr> is well-formed.
  • (1.2)
    same_as<decltype(e), sync-wait-with-variant-result-type<Sndr>> is true, where e is the apply_sender expression above.
2
#
If callable<sync_wait_t, Sndr> is false, the expression sync_wait_with_variant.apply_sender(sndr) is ill-formed.
Otherwise, it is equivalent to: using result_type = sync-wait-with-variant-result-type<Sndr>; if (auto opt_value = sync_wait(into_variant(sndr))) { return result_type(std::move(get<0>(*opt_value))); } return result_type(nullopt);
3
#
The behavior of this_thread​::​sync_wait_with_variant(sndr) is undefined unless:
  • (3.1)
    It blocks the current thread of execution ([defns.block]) with forward progress guarantee delegation ([intro.progress]) until the specified sender completes.
    [Note 1: 
    The default implementation of sync_wait_with_variant achieves forward progress guarantee delegation by relying on the forward progress guarantee delegation provided by sync_wait.
    — end note]
  • (3.2)
    It returns the specified sender's async results as follows:
    • (3.2.1)
      For a value completion, the result datums are returned in an engaged optional object that contains a variant of tuples.
    • (3.2.2)
      For an error completion, an exception is thrown.
    • (3.2.3)
      For a stopped completion, a disengaged optional object is returned.