16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.5 Constraints on programs [constraints]

16.4.5.2 Namespace use [namespace.constraints]

16.4.5.2.1 Namespace std [namespace.std]

Unless otherwise specified, the behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std.
Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std provided that
  • the added declaration depends on at least one program-defined type, and
  • the specialization meets the standard library requirements for the original template.155
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.
[Note 1: 
The requirements on an explicit or partial specialization are stated by each variable template that grants such permission.
— end note]
The behavior of a C++ program is undefined if it declares
  • an explicit specialization of any member function of a standard library class template, or
  • an explicit specialization of any member function template of a standard library class or class template, or
  • an explicit or partial specialization of any member class template of a standard library class or class template, or
  • a deduction guide for any standard library class template.
A program may explicitly instantiate a class template defined in the standard library only if the declaration
  • depends on the name of at least one program-defined type, and
  • the instantiation meets the standard library requirements for the original template.
Let F denote a standard library function ([global.functions]), a standard library static member function, or an instantiation of a standard library function template.
Unless F is designated an addressable function, the behavior of a C++ program is unspecified (possibly ill-formed) if it explicitly or implicitly attempts to form a pointer to F.
[Note 2: 
Possible means of forming such pointers include application of the unary & operator ([expr.unary.op]), addressof ([specialized.addressof]), or a function-to-pointer standard conversion ([conv.func]).
— 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]) or an instantiation of a standard library member function template.
A translation unit shall not declare namespace std to be an inline namespace ([namespace.def]).
155)155)
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.

16.4.5.2.2 Namespace posix [namespace.posix]

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace posix or to a namespace within namespace posix unless otherwise specified.
The namespace posix is reserved for use by ISO/IEC/IEEE 9945 and other POSIX standards.

16.4.5.2.3 Namespaces for future standardization [namespace.future]

Top-level namespaces whose namespace-name consists of std followed by one or more digits ([lex.name]) are reserved for future standardization.
The behavior of a C++ program is undefined if it adds declarations or definitions to such a namespace.
[Example 1: 
The top-level namespace std2 is reserved for use by future revisions of this International Standard.
— end example]