33 Execution control library [exec]

stopped_as_optional maps a sender's stopped completion operation into a value completion operation as a disengaged optional.

The sender's value completion operation is also converted into an optional.

The result is a sender that never completes with stopped, reporting cancellation by completing with a disengaged optional.

The name stopped_as_optional denotes a pipeable sender adaptor object.

For a subexpression sndr, let Sndr be decltype((sndr)).

The expression stopped_as_optional(sndr) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(stopped_as_optional, {}, sndr)) except that sndr is only evaluated once.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for stopped_as_optional_t as follows:

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namespace std::execution { template<> struct impls-for<stopped_as_optional_t> : default-impls { template<class Sndr, class... Env> static consteval void check-types() { default-impls::check-types<Sndr, Env...>(); if constexpr (!requires { requires (!same_as<void, single-sender-value-type<child-type<Sndr>, FWD-ENV-T(Env)...>>); }) throw unspecified-exception(); } }; } where unspecified-exception is a type derived from exception.

Let sndr and env be subexpressions such that Sndr is decltype((sndr)) and Env is decltype((env)).

If sender-for<Sndr, stopped_as_optional_t> is false then the expression stopped_as_optional.transform_sender(sndr, env) is ill-formed; otherwise, if sender_in<child-type<Sndr>, FWD-ENV-T(Env)> is false, the expression stopped_as_optional.transform_sender(sndr, env) is equivalent to not-a-sender(); otherwise, it is equivalent to: auto&& [_, _, child] = sndr; using V = single-sender-value-type<child-type<Sndr>, FWD-ENV-T(Env)>; return let_stopped( then(std::forward_like<Sndr>(child), []<class... Ts>(Ts&&... ts) noexcept(is_nothrow_constructible_v<V, Ts...>) { return optional<V>(in_place, std::forward<Ts>(ts)...); }), []() noexcept { return just(optional<V>()); });