20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.4 Constraints on programs [constraints]

20.5.4.2 Namespace use [namespace.constraints]

20.5.4.2.1 Namespace std [namespace.std]

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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 otherwise specified. A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.176

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

  • (2.1)

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

  • (2.2)

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

  • (2.3)

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

  • (2.4)

    a deduction guide for any standard library class template.

A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type and the instantiation meets the standard library requirements for the original template.

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A translation unit shall not declare namespace std to be an inline namespace ([namespace.def]).

176)

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 International Standard.

20.5.4.2.2 Namespace posix [namespace.posix]

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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 9945 and other POSIX standards.

20.5.4.2.3 Namespaces for future standardization [namespace.future]

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Top level namespaces with a name starting with std and followed by a non-empty sequence of digits are reserved for future standardization. The behavior of a C++ program is undefined if it adds declarations or definitions to such a namespace. [ Example: The top level namespace std2 is reserved for use by future revisions of this International Standard.  — end example ]