[exec.adapt.obj]

# 33 Execution control library [[exec]](./#exec)

## 33.9 Senders [[exec.snd]](exec.snd#exec.adapt.obj)

### 33.9.12 Sender adaptors [[exec.adapt]](exec.adapt#obj)

#### 33.9.12.2 Closure objects [exec.adapt.obj]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3055)

A [*pipeable sender adaptor closure object*](#def:sender_adaptor_closure_object,pipeable "33.9.12.2 Closure objects [exec.adapt.obj]") is a function object
that accepts one or more [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") arguments and returns a [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]")[.](#1.sentence-1)

For a pipeable sender adaptor closure object c and
an expression sndr such that decltype((sndr)) models [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]"),
the following expressions are equivalent and yield a [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]"):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]](func.require "22.10.4 Requirements"))
with the following properties:

- [(1.1)](#1.1)

  Its target object is an object d2 of type decltype(auto(d)) direct-non-list-initialized with d[.](#1.1.sentence-1)

- [(1.2)](#1.2)

  It has one bound argument entity,
an object c2 of type decltype(auto(c)) direct-non-list-initialized with c[.](#1.2.sentence-1)

- [(1.3)](#1.3)

  Its call pattern is d2(c2(arg)),
where arg is the argument used in a function call expression of e[.](#1.3.sentence-1)

The expression c | d is well-formed if and only if
the initializations of the state entities ([[func.def]](func.def "22.10.3 Definitions")) of e are all well-formed[.](#1.sentence-5)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3088)

An object t of type T is
a pipeable sender adaptor closure object
if T models [derived_from](concept.derived#concept:derived_from "18.4.3 Concept derived_­from [concept.derived]")<sender_adaptor_closure<T>>,T has no other base classes
of type sender_adaptor_closure<U> for any other type U, andT does not satisfy [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]")[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3096)

The template parameter D for sender_adaptor_closure can be
an incomplete type[.](#3.sentence-1)

Before any expression of type cv D appears as
an operand to the | operator,D shall be complete and
model [derived_from](concept.derived#concept:derived_from "18.4.3 Concept derived_­from [concept.derived]")<sender_adaptor_closure<D>>[.](#3.sentence-2)

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[.](#3.sentence-3)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3107)

A [*pipeable sender adaptor object*](#def:sender_adaptor_object,pipeable "33.9.12.2 Closure objects [exec.adapt.obj]") is a customization point object
that accepts a [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") as its first argument and
returns a [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]")[.](#4.sentence-1)

If a pipeable sender adaptor object accepts only one argument,
then it is a pipeable sender adaptor closure object[.](#4.sentence-2)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3114)

If a pipeable sender adaptor object adaptor accepts more than one argument,
then let sndr be an expression
such that decltype((sndr)) models [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]"),
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))...[.](#5.sentence-1)

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)](#5.1)

  Its target object is a copy of adaptor[.](#5.1.sentence-1)

- [(5.2)](#5.2)

  Its bound argument entities bound_args consist of
objects of types BoundArgs... direct-non-list-initialized withstd​::​forward<decltype((args))>(args)..., respectively[.](#5.2.sentence-1)

- [(5.3)](#5.3)

  Its call pattern is adaptor(rcvr, bound_args...),
where rcvr is
the argument used in a function call expression of f[.](#5.3.sentence-1)

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[.](#5.sentence-3)