std::execution::read_env
| Defined in header <execution>
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| inline constexpr /*unspecified*/ read_env{}; |
(since C++26) (customization point object) |
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| Call signature |
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| execution::sender auto read_env( auto&& query ); |
(since C++26) | |
A sender factory that returns a sender that reaches into a receiver’s environment and pulls out the current value associated with a given query object.
For any query object q, the expression read_env(q) is expression-equivalent to /*make-sender*/(read_env, q).
Customization point objects
The name execution::read_env denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __read_env_fn.
All instances of __read_env_fn are equal. The effects of invoking different instances of type __read_env_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, execution::read_env can be copied freely and its copies can be used interchangeably.
Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to execution::read_env above, __read_env_fn models
- std::invocable<__read_env_fn, Args...>,
- std::invocable<const __read_env_fn, Args...>,
- std::invocable<__read_env_fn&, Args...>, and
- std::invocable<const __read_env_fn&, Args...>.
Otherwise, no function call operator of __read_env_fn participates in overload resolution.
Example
An example usage of this factory is to schedule dependent work on the receiver's scheduler, which can be obtained with read_env(get_scheduler):
std::execution::sender auto task = std::execution::read_env(std::execution::get_scheduler) | std::execution::let_value([](auto sched) { return std::execution::starts_on(sched, /*some nested work here*/); }); std::this_thread::sync_wait( std::move(task) ); // wait for it to finish