16 Library introduction [library]

16.5 Library-wide requirements [requirements]

16.5.4 Constraints on programs [constraints]

16.5.4.2 Namespace use [namespace.constraints]

16.5.4.2.3 Namespaces for future standardization [namespace.future]

1
#
Top-level namespaces whose namespace-name consists of std followed by one or more 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
 ]