16 Library introduction [library]

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.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.

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.

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]).

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.

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.