[namespace.constraints]

# 16 Library introduction [[library]](./#library)

## 16.4 Library-wide requirements [[requirements]](requirements#namespace.constraints)

### 16.4.5 Constraints on programs [[constraints]](constraints#namespace.constraints)

#### 16.4.5.2 Namespace use [namespace.constraints]

#### [16.4.5.2.1](#namespace.std) Namespace std [[namespace.std]](namespace.std)

[1](#namespace.std-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L2991)

Unless otherwise specified,
the behavior of a C++ program is undefined if it adds declarations or definitions to namespacestd or to a namespace within namespacestd[.](#namespace.std-1.sentence-1)

[2](#namespace.std-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L2998)

Unless explicitly prohibited,
a program may add a template specialization for
any standard library class template
to namespacestd provided that

- [(2.1)](#namespace.std-2.1)

the added declaration
depends on at least one program-defined type, and

- [(2.2)](#namespace.std-2.2)

the specialization meets the standard library requirements
for the original template[.](#namespace.std-2.sentence-1)[151](#footnote-151 "Any library code that instantiates other library templates must be prepared to work adequately with any user-supplied specialization that meets the minimum requirements of this document.")

[3](#namespace.std-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3018)

The behavior of a C++ program is undefined
if it declares an explicit or partial specialization
of any standard library variable template,
except where explicitly permitted by the specification of that variable template[.](#namespace.std-3.sentence-1)

[*Note [1](#namespace.std-note-1)*:

The requirements on an explicit or partial specialization
are stated by each variable template that grants such permission[.](#namespace.std-3.sentence-2)

— *end note*]

[4](#namespace.std-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3028)

The behavior of a C++ program is undefined if it declares

- [(4.1)](#namespace.std-4.1)

an explicit specialization of any member function of a standard
library class template, or

- [(4.2)](#namespace.std-4.2)

an explicit specialization of any member function template of a
standard library class or class template, or

- [(4.3)](#namespace.std-4.3)

an explicit or partial specialization of any member class template
of a standard library class or class template, or

- [(4.4)](#namespace.std-4.4)

a deduction guide for any standard library class template[.](#namespace.std-4.sentence-1)

[5](#namespace.std-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3043)

A program may explicitly instantiate
a class template defined in the standard library
only if the declaration

- [(5.1)](#namespace.std-5.1)

depends on the name of at least one program-defined type, and

- [(5.2)](#namespace.std-5.2)

the instantiation meets the standard library requirements for the
original template[.](#namespace.std-5.sentence-1)

[6](#namespace.std-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3054)

Let *F* denote
a standard library function ([[global.functions]](global.functions "16.4.6.4 Non-member functions")),
a standard library static member function,
or an instantiation
of a standard library function template[.](#namespace.std-6.sentence-1)

Unless *F* is designated
an [*addressable function*](#def:function,addressable "16.4.5.2.1 Namespace std [namespace.std]"),
the behavior of a C++ program is unspecified (possibly ill-formed)
if it explicitly or implicitly attempts
to form a pointer
to *F*[.](#namespace.std-6.sentence-2)

[*Note [2](#namespace.std-note-2)*:

Possible means of forming such pointers include
application of the unary & operator ([[expr.unary.op]](expr.unary.op "7.6.2.2 Unary operators")),addressof ([[specialized.addressof]](specialized.addressof "20.2.11 addressof")),
or
a function-to-pointer standard conversion ([[conv.func]](conv.func "7.3.4 Function-to-pointer conversion"))[.](#namespace.std-6.sentence-3)

— *end note*]

Moreover,
the behavior of a C++ program is unspecified (possibly ill-formed)
if it attempts to form a reference
to *F* or
if it attempts to form a pointer-to-member designating
either a standard library non-static member function ([[member.functions]](member.functions "16.4.6.5 Member functions"))
or an instantiation of a standard library member function template[.](#namespace.std-6.sentence-4)

[7](#namespace.std-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3082)

Let *F* denote
a standard library function or function template[.](#namespace.std-7.sentence-1)

Unless *F* is designated an addressable function,
it is unspecified if or how
a reflection value designating the associated entity can be formed[.](#namespace.std-7.sentence-2)

[*Note [3](#namespace.std-note-3)*:

For example, it is possible that std​::​meta​::​members_of will not return reflections of standard library functions
that an implementation handles through an extra-linguistic mechanism[.](#namespace.std-7.sentence-3)

— *end note*]

[8](#namespace.std-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3095)

Let *C* denote
a standard library class or class template specialization[.](#namespace.std-8.sentence-1)

It is unspecified if or how
a reflection value can be formed to any private member of *C*,
or what the names of such members may be[.](#namespace.std-8.sentence-2)

[9](#namespace.std-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3102)

A translation unit shall not declare namespace std to be an inline namespace ([[namespace.def]](namespace.def "9.9.2 Namespace definition"))[.](#namespace.std-9.sentence-1)

[151)](#footnote-151)[151)](#footnoteref-151)

Any
library code that instantiates other library templates
must be prepared to work adequately with any user-supplied specialization
that meets the minimum requirements of this document[.](#footnote-151.sentence-1)

#### [16.4.5.2.2](#namespace.posix) Namespace posix [[namespace.posix]](namespace.posix)

[1](#namespace.posix-1)

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

The behavior of a C++ program is undefined if it adds declarations or definitions to namespaceposix or to a namespace within namespaceposix unless otherwise specified[.](#namespace.posix-1.sentence-1)

The namespace posix is reserved for use by
ISO/IEC/IEEE 9945 and other POSIX standards[.](#namespace.posix-1.sentence-2)

#### [16.4.5.2.3](#namespace.future) Namespaces for future standardization [[namespace.future]](namespace.future)

[1](#namespace.future-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3117)

Top-level namespaces whose [*namespace-name*](namespace.def.general#nt:namespace-name "9.9.2.1 General [namespace.def.general]") consists of std followed by one or more [*digit*](lex.name#nt:digit "5.11 Identifiers [lex.name]")*s* ([[lex.name]](lex.name "5.11 Identifiers"))
are reserved for future standardization[.](#namespace.future-1.sentence-1)

The behavior of a C++ program is undefined if
it adds declarations or definitions to such a namespace[.](#namespace.future-1.sentence-2)

[*Example [1](#namespace.future-example-1)*:

The top-level namespace std2 is reserved
for use by future revisions of this International Standard[.](#namespace.future-1.sentence-3)

— *end example*]