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 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