34 Execution control library [exec]

into_variant adapts a sender with multiple value completion signatures into a sender with just one value completion signature consisting of a variant of tuples.

The name into_variant denotes a pipeable sender adaptor object.

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

If Sndr does not satisfy sender, into_variant(sndr) is ill-formed.

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

The exposition-only class template impls-for ([exec.snd.general]) is specialized for into_variant as follows: namespace std::execution { template<> struct impls-for<into_variant_t> : default-impls { static constexpr auto get-state = see below; static constexpr auto complete = see below; }; }

The member impls-for<into_variant_t>​::​get-state is initialized with a callable object equivalent to the following lambda: []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept -> type_identity<value_types_of_t<child-type<Sndr>, env_of_t<Rcvr>>> { return {}; }

The member impls-for<into_variant_t>​::​complete is initialized with a callable object equivalent to the following lambda: []<class State, class Rcvr, class Tag, class... Args>( auto, State, Rcvr& rcvr, Tag, Args&&... args) noexcept -> void { if constexpr (same_as<Tag, set_value_t>) { using variant_type = typename State::type; TRY-SET-VALUE(rcvr, variant_type(decayed-tuple<Args...>{std::forward<Args>(args)...})); } else { Tag()(std::move(rcvr), std::forward<Args>(args)...); } }