std::execution::read_env

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

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...>
• 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