34 Execution control library [exec]

Subclause [exec.snd] makes use of the following exposition-only entities.

For a queryable object env, FWD-ENV(env) is an expression whose type satisfies queryable such that for a query object q and a pack of subexpressions as, the expression FWD-ENV(env).query(q, as...) is ill-formed if forwarding_query(q) is false; otherwise, it is expression-equivalent to env.query(q, as...).

For a query object q and a subexpression v, MAKE-ENV(q, v) is an expression env whose type satisfies queryable such that the result of env.query(q) has a value equal to v ([concepts.equality]).

Unless otherwise stated, the object to which env.query(q) refers remains valid while env remains valid.

For two queryable objects env1 and env2, a query object q, and a pack of subexpressions as, JOIN-ENV(env1, env2) is an expression env3 whose type satisfies queryable such that env3.query(q, as...) is expression-equivalent to:

• (4.1)
  env1.query(q, as...) if that expression is well-formed,
• (4.2)
  otherwise, env2.query(q, as...) if that expression is well-formed,
• (4.3)
  otherwise, env3.query(q, as...) is ill-formed.

The results of FWD-ENV, MAKE-ENV, and JOIN-ENV can be context-dependent; i.e., they can evaluate to expressions with different types and value categories in different contexts for the same arguments.

For a scheduler sch, SCHED-ATTRS(sch) is an expression o1 whose type satisfies queryable such that o1.query(get_completion_scheduler<Tag>) is an expression with the same type and value as sch where Tag is one of set_value_t or set_stopped_t, and such that o1.query(get_domain) is expression-equivalent to sch.query(get_domain).

SCHED-ENV(sch) is an expression o2 whose type satisfies queryable such that o1.query(get_scheduler) is a prvalue with the same type and value as sch, and such that o2.query(get_domain) is expression-equivalent to sch.query(get_domain).

For two subexpressions rcvr and expr, SET-VALUE(rcvr, expr) is expression-equivalent to (expr, set_value(std​::​move(rcvr))) if the type of expr is void; otherwise, set_value(std​::​move(rcvr), expr).

TRY-EVAL(rcvr, expr) is equivalent to: try { expr; } catch(...) { set_error(std::move(rcvr), current_exception()); } if expr is potentially-throwing; otherwise, expr.

TRY-SET-VALUE(rcvr, expr) is TRY-EVAL(rcvr, SET-VALUE(rcvr, expr)) except that rcvr is evaluated only once.

The default template argument for the Data template parameter denotes an unspecified empty trivially copyable class type that models semiregular.

It is unspecified whether a specialization of basic-sender is an aggregate.

An expression of type basic-sender is usable as the initializer of a structured binding declaration ([dcl.struct.bind]).

The expression in the noexcept clause of the constructor of basic-state is: is_nothrow_move_constructible_v<Rcvr> && nothrow-callable<decltype(impls-for<tag_of_t<Sndr>>::get-state), Sndr, Rcvr&>

The object connect-all is initialized with a callable object equivalent to the following lambda:

The expression in the noexcept clause of the constructor of basic-operation is: is_nothrow_constructible_v<basic-state<Self, Rcvr>, Self, Rcvr> && noexcept(connect-all(this, std::forward<Sndr>(sndr), indices-for<Sndr>()))

The expression in the noexcept clause of the connect member function of basic-sender is: is_nothrow_constructible_v<basic-operation<Self, Rcvr>, Self, Rcvr>

The member default-impls​::​get-attrs is initialized with a callable object equivalent to the following lambda: [](const auto&, const auto&... child) noexcept -> decltype(auto) { if constexpr (sizeof...(child) == 1) return (FWD-ENV(get_env(child)), ...); else return empty_env(); }

The member default-impls​::​get-env is initialized with a callable object equivalent to the following lambda: [](auto, auto&, const auto& rcvr) noexcept -> decltype(auto) { return FWD-ENV(get_env(rcvr)); }

The member default-impls​::​get-state is initialized with a callable object equivalent to the following lambda: []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept -> decltype(auto) { auto& [_, data, ...child] = sndr; return std::forward_like<Sndr>(data); }

The member default-impls​::​start is initialized with a callable object equivalent to the following lambda: [](auto&, auto&, auto&... ops) noexcept -> void { (execution::start(ops), ...); }

The member default-impls​::​complete is initialized with a callable object equivalent to the following lambda: []<class Index, class Rcvr, class Tag, class... Args>( Index, auto& state, Rcvr& rcvr, Tag, Args&&... args) noexcept -> void requires callable<Tag, Rcvr, Args...> { static_assert(Index::value == 0); Tag()(std::move(rcvr), std::forward<Args>(args)...); }

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

Let rcvr be a receiver with an associated environment of type Env such that sender_in<Sndr, Env> is true.

completion-signatures-for<Sndr, Env> denotes a specialization of completion_signatures, the set of whose template arguments correspond to the set of completion operations that are potentially evaluated as a result of starting ([exec.async.ops]) the operation state that results from connecting sndr and rcvr.

When sender_in<Sndr, Env> is false, the type denoted by completion-signatures-for<Sndr, Env>, if any, is not a specialization of completion_signatures.

Recommended practice: When sender_in<Sndr, Env> is false, implementations are encouraged to use the type denoted by completion-signatures-for<Sndr, Env> to communicate to users why.