34 Execution control library [exec]

A pipeable sender adaptor closure object is a function object that accepts one or more sender arguments and returns a sender.

For a pipeable sender adaptor closure object c and an expression sndr such that decltype((sndr)) models sender, the following expressions are equivalent and yield a sender: c(sndr) sndr | c

Given an additional pipeable sender adaptor closure object d, the expression c | d produces another pipeable sender adaptor closure object e:

e is a perfect forwarding call wrapper ([func.require]) with the following properties:

• (1.1)
  Its target object is an object d2 of type decltype(auto(d)) direct-non-list-initialized with d.
• (1.2)
  It has one bound argument entity, an object c2 of type decltype(auto(c)) direct-non-list-initialized with c.
• (1.3)
  Its call pattern is d2(c2(arg)), where arg is the argument used in a function call expression of e.

The expression c | d is well-formed if and only if the initializations of the state entities ([func.def]) of e are all well-formed.

An object t of type T is a pipeable sender adaptor closure object if T models derived_from<sender_adaptor_closure<T>>, T has no other base classes of type sender_adaptor_closure<U> for any other type U, and T does not satisfy sender.

The template parameter D for sender_adaptor_closure can be an incomplete type.

Before any expression of type cv D appears as an operand to the | operator, D shall be complete and model derived_from<sender_adaptor_closure<D>>.

The behavior of an expression involving an object of type cv D as an operand to the | operator is undefined if overload resolution selects a program-defined operator| function.

A pipeable sender adaptor object is a customization point object that accepts a sender as its first argument and returns a sender.

If a pipeable sender adaptor object accepts only one argument, then it is a pipeable sender adaptor closure object.

If a pipeable sender adaptor object adaptor accepts more than one argument, then let sndr be an expression such that decltype((sndr)) models sender, let args... be arguments such that adaptor(sndr, args...) is a well-formed expression as specified below, and let BoundArgs be a pack that denotes decltype(auto(args))....

The expression adaptor(args...) produces a pipeable sender adaptor closure object f that is a perfect forwarding call wrapper with the following properties:

• (5.1)
  Its target object is a copy of adaptor.
• (5.2)
  Its bound argument entities bound_args consist of objects of types BoundArgs... direct-non-list-initialized with std​::​forward<decltype((args))>(args)..., respectively.
• (5.3)
  Its call pattern is adaptor(rcvr, bound_args...), where rcvr is the argument used in a function call expression of f.

The expression adaptor(args...) is well-formed if and only if the initializations of the bound argument entities of the result, as specified above, are all well-formed.