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

(2.1)
the added declaration depends on at least one program-defined type, and
(2.2)
the specialization meets the standard library requirements for the original template.159

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

(4.1)
an explicit specialization of any member function of a standard library class template, or
(4.2)
an explicit specialization of any member function template of a standard library class or class template, or
(4.3)
an explicit or partial specialization of any member class template of a standard library class or class template, or
(4.4)
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

(5.1)
depends on the name of at least one program-defined type, and
(5.2)
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]).

159)159)

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.