[func.bind]

# 22 General utilities library [[utilities]](./#utilities)

## 22.10 Function objects [[function.objects]](function.objects#func.bind)

### 22.10.15 Function object binders [func.bind]

#### [22.10.15.1](#general) General [[func.bind.general]](func.bind.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13332)

Subclause [func.bind] describes a uniform mechanism for binding
arguments of callable objects[.](#general-1.sentence-1)

#### [22.10.15.2](#isbind) Class template is_bind_expression [[func.bind.isbind]](func.bind.isbind)

[ð](#lib:is_bind_expression)

namespace std {template<class T> struct is_bind_expression; // see below}

[1](#isbind-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13345)

The class template is_bind_expression can be used to detect function objects
generated by bind[.](#isbind-1.sentence-1)

The function template bind uses is_bind_expression to detect subexpressions[.](#isbind-1.sentence-2)

[2](#isbind-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13350)

Specializations of the is_bind_expression template shall meet
the [*Cpp17UnaryTypeTrait*](meta.rqmts#:Cpp17UnaryTypeTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))[.](#isbind-2.sentence-1)

The implementation
provides a definition that has a base characteristic oftrue_type if T is a type returned from bind,
otherwise it has a base characteristic of false_type[.](#isbind-2.sentence-2)

A program may specialize this template for a program-defined type T to have a base characteristic of true_type to indicate thatT should be treated as a subexpression in a bind call[.](#isbind-2.sentence-3)

#### [22.10.15.3](#isplace) Class template is_placeholder [[func.bind.isplace]](func.bind.isplace)

[ð](#lib:is_placeholder)

namespace std {template<class T> struct is_placeholder; // see below}

[1](#isplace-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13369)

The class template is_placeholder can be used to detect the standard placeholders_1, _2, and so on ([[func.bind.place]](#place "22.10.15.5 Placeholders"))[.](#isplace-1.sentence-1)

The function template bind usesis_placeholder to detect placeholders[.](#isplace-1.sentence-2)

[2](#isplace-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13386)

Specializations of the is_placeholder template shall meet
the [*Cpp17UnaryTypeTrait*](meta.rqmts#:Cpp17UnaryTypeTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))[.](#isplace-2.sentence-1)

The implementation
provides a definition that has the base characteristic ofintegral_constant<int, *J*> if T is the type ofstd​::​placeholders​::​_*J*, otherwise it has a
base characteristic of integral_constant<int, 0>[.](#isplace-2.sentence-2)

A program
may specialize this template for a program-defined type T to
have a base characteristic of integral_constant<int, N> with N > 0 to indicate that T should be
treated as a placeholder type[.](#isplace-2.sentence-3)

#### [22.10.15.4](#bind) Function template bind [[func.bind.bind]](func.bind.bind)

[1](#bind-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13401)

In the text that follows:

- [(1.1)](#bind-1.1)

g is a value of the result of a bind invocation,

- [(1.2)](#bind-1.2)

FD is the type decay_t<F>,

- [(1.3)](#bind-1.3)

fd is an lvalue that
 is a target object of g ([[func.def]](func.def "22.10.3 Definitions")) of type FD direct-non-list-initialized with std​::​forward<F>(f),

- [(1.4)](#bind-1.4)

Ti is the ith type in the template parameter pack BoundArgs,

- [(1.5)](#bind-1.5)

TDi is the type decay_t<Ti>,

- [(1.6)](#bind-1.6)

ti is the ith argument in the function parameter pack bound_args,

- [(1.7)](#bind-1.7)

tdi is a bound argument entity
 of g ([[func.def]](func.def "22.10.3 Definitions")) of type TDi direct-non-list-initialized with std​::​forward<​Ti>(ti),

- [(1.8)](#bind-1.8)

Uj is the jth deduced type of the UnBoundArgs&&... parameter
 of the argument forwarding call wrapper, and

- [(1.9)](#bind-1.9)

uj is the jth argument associated with Uj[.](#bind-1.sentence-1)

[ð](#lib:bind_)

`template<class F, class... BoundArgs>
  constexpr unspecified bind(F&& f, BoundArgs&&... bound_args);
template<class R, class F, class... BoundArgs>
  constexpr unspecified bind(F&& f, BoundArgs&&... bound_args);
`

[2](#bind-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13430)

*Mandates*: is_constructible_v<FD, F> is true[.](#bind-2.sentence-1)

For each Ti in BoundArgs, is_constructible_v<TDi, Ti> is true[.](#bind-2.sentence-2)

[3](#bind-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13435)

*Preconditions*: FD and each TDi meet
the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and [*Cpp17Destructible*](utility.arg.requirements#:Cpp17Destructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements[.](#bind-3.sentence-1)

*INVOKE*(fd, w1, w2, …,wN) ([[func.require]](func.require "22.10.4 Requirements")) is a valid expression for some
values w1, w2, …, wN, whereN has the value sizeof...(bound_args)[.](#bind-3.sentence-2)

[4](#bind-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13444)

*Returns*: An argument forwarding call wrapper g ([[func.require]](func.require "22.10.4 Requirements"))[.](#bind-4.sentence-1)

A program that attempts to invoke a volatile-qualified g is ill-formed[.](#bind-4.sentence-2)

When g is not volatile-qualified, invocation ofg(u1, u2, …, uM) is expression-equivalent ([[defns.expression.equivalent]](defns.expression.equivalent "3.22 expression-equivalent")) to*INVOKE*(static_cast<Vfd>(vfd), static_cast<V1>(v1), static_cast<V2>(v2), …, static_cast<VN>(vN)) for the first overload, and*INVOKE*<R>(static_cast<Vfd>(vfd), static_cast<V1>(v1), static_cast<V2>(v2), …, static_cast<VN>(vN)) for the second overload,
where the values and types of the target argument vfd and
of the bound argumentsv1, v2, …, vN are determined as specified below[.](#bind-4.sentence-3)

[5](#bind-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13466)

*Throws*: Any exception thrown by the initialization of
the state entities of g[.](#bind-5.sentence-1)

[6](#bind-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13471)

[*Note [1](#bind-note-1)*:

If all of FD and TDi meet
the requirements of [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]"), then
the return type meets the requirements of [*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]")[.](#bind-6.sentence-1)

â *end note*]

[7](#bind-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13479)

The values of the [*bound arguments*](#def:bound_arguments) v1, v2, …, vN and their
corresponding types V1, V2, …, VN depend on the
types TDi derived from
the call to bind and the
cv-qualifiers cv of the call wrapper g as follows:

- [(7.1)](#bind-7.1)

if TDi is reference_wrapper<T>, the
argument is tdi.get() and its type Vi is T&;

- [(7.2)](#bind-7.2)

if the value of is_bind_expression_v<TDi> is true, the argument isstatic_cast<cv TDi&>(tdi)(std::forward<Uj>(uj)...) and its type Vi isinvoke_result_t<cv TDi&, Uj...>&&;

- [(7.3)](#bind-7.3)

if the value j of is_placeholder_v<TDi> is not zero, the argument is std​::​forward<Uj>(uj) and its type Vi is Uj&&;

- [(7.4)](#bind-7.4)

otherwise, the value is tdi and its type Vi is cv TDi&[.](#bind-7.sentence-1)

[8](#bind-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13507)

The value of the target argument vfd is fd and
its corresponding type Vfd is cv FD&[.](#bind-8.sentence-1)

#### [22.10.15.5](#place) Placeholders [[func.bind.place]](func.bind.place)

namespace std::placeholders {// *M* is the number of placeholders*see below* _1; *see below* _2;
 ⋮ *see below* _*M*;}

[1](#place-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13539)

The number *M* of placeholders isimplementation-defined[.](#place-1.sentence-1)

[2](#place-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13543)

All placeholder types meet the [*Cpp17DefaultConstructible*](utility.arg.requirements#:Cpp17DefaultConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") and[*Cpp17CopyConstructible*](utility.arg.requirements#:Cpp17CopyConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements, and
their default constructors and copy/move
constructors are constexpr functions that
do not throw exceptions[.](#place-2.sentence-1)

It is implementation-defined whether
placeholder types meet the [*Cpp17CopyAssignable*](utility.arg.requirements#:Cpp17CopyAssignable "16.4.4.2 Template argument requirements [utility.arg.requirements]") requirements,
but if so, their copy assignment operators are
constexpr functions that do not throw exceptions[.](#place-2.sentence-2)

[3](#place-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13554)

Placeholders should be defined as:inline constexpr *unspecified* _1{};

If they are not, they are declared as:extern *unspecified* _1;

[4](#place-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L13565)

Placeholders are freestanding items ([[freestanding.item]](freestanding.item "16.3.3.7 Freestanding items"))[.](#place-4.sentence-1)
