[exec.then]

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

## 33.9 Senders [[exec.snd]](exec.snd#exec.then)

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

#### 33.9.12.9 execution​::​then, execution​::​upon_error, execution​::​upon_stopped [exec.then]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3757)

then attaches an invocable as a continuation
for an input sender's value completion operation[.](#1.sentence-1)

upon_error and upon_stopped do the same
for the error and stopped completion operations, respectively,
sending the result of the invocable as a value completion[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3764)

The names then, upon_error, and upon_stopped denote pipeable sender adaptor objects[.](#2.sentence-1)

Let the expression *then-cpo* be one ofthen, upon_error, or upon_stopped[.](#2.sentence-2)

For subexpressions sndr and f,
if decltype((sndr)) does not satisfy [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]"), ordecltype((f)) does not satisfy [*movable-value*](exec.general#concept:movable-value "33.1 General [exec.general]"),*then-cpo*(sndr, f) is ill-formed[.](#2.sentence-3)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3774)

Otherwise,
the expression *then-cpo*(sndr, f) is expression-equivalent to:transform_sender(*get-domain-early*(sndr), *make-sender*(*then-cpo*, f, sndr)) except that sndr is evaluated only once[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3782)

For then, upon_error, and upon_stopped,
let *set-cpo* beset_value, set_error, and set_stopped, respectively[.](#4.sentence-1)

The exposition-only class template *impls-for* ([[exec.snd.expos]](exec.snd.expos "33.9.2 Exposition-only entities"))
is specialized for *then-cpo* as follows:

[🔗](#lib:impls-for%3cdecayed-typeof%3cthen-cpo%3e%3e)

namespace std::execution {template<>struct *impls-for*<*decayed-typeof*<*then-cpo*>> : *default-impls* {static constexpr auto *complete* =[]<class Tag, class... Args>(auto, auto& fn, auto& rcvr, Tag, Args&&... args) noexcept -> void {if constexpr ([same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<Tag, *decayed-typeof*<set-cpo>>) {*TRY-SET-VALUE*(rcvr,
 invoke(std::move(fn), std::forward<Args>(args)...)); } else { Tag()(std::move(rcvr), std::forward<Args>(args)...); }}; template<class Sndr, class... Env>static consteval void *check-types*(); };}

[🔗](#lib:check-types,impls-for%3cdecayed-typeof%3cthen-cpo%3e%3e)

`template<class Sndr, class... Env>
  static consteval void check-types();
`

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3817)

*Effects*: Equivalent to:auto cs = get_completion_signatures<*child-type*<Sndr>, *FWD-ENV-T*(Env)...>();auto fn = []<class... Ts>(set_value_t(*)(Ts...)) {if constexpr (![invocable](concept.invocable#concept:invocable "18.7.2 Concept invocable [concept.invocable]")<remove_cvref_t<*data-type*<Sndr>>, Ts...>)throw *unspecified-exception*();};
cs.*for-each*(*overload-set*{fn, [](auto){}}); where *unspecified-exception* is
a type derived from exception[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3832)

The expression *then-cpo*(sndr, f) has undefined behavior
unless it returns a sender out_sndr that

- [(6.1)](#6.1)

invokes f or a copy of such
with the value, error, or stopped result datums of sndr for then, upon_error, and upon_stopped, respectively,
using the result value of f as out_sndr's value completion, and

- [(6.2)](#6.2)

forwards all other completion operations unchanged[.](#6.sentence-1)