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]

1
#
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.
2
#
Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.177
3
#
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.
4
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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.
5
#
A program may explicitly instantiate a class template defined in the standard library only if the declaration (a) depends on the name of at least one program-defined type and (b) the instantiation meets the standard library requirements for the original template.
6
#
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 1:
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.
7
#
Other than in namespace std or in a namespace within namespace std, a program may provide an overload for any library function template designated as a customization point, provided that (a) the overload's declaration depends on at least one user-defined type and (b) the overload meets the standard library requirements for the customization point.178
[Note 2:
This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments.
— end note]
8
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A translation unit shall not declare namespace std to be an inline namespace ([namespace.def]).
177)
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.
 ⮥
178)
Any library customization point must be prepared to work adequately with any user-defined overload that meets the minimum requirements of this document.
Therefore an implementation can elect, under the as-if rule ([intro.execution]), to provide any customization point in the form of an instantiated function object ([function.objects]) even though the customization point's specification is in the form of a function template.
The template parameters of each such function object and the function parameters and return type of the object's operator() must match those of the corresponding customization point's specification.
 ⮥

16.4.5.2.2 Namespace posix [namespace.posix]

1
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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.

16.4.5.2.3 Namespaces for future standardization [namespace.future]

1
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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]