[namespace.future]

# 16 Library introduction [[library]](./#library)

## 16.4 Library-wide requirements [[requirements]](requirements#namespace.future)

### 16.4.5 Constraints on programs [[constraints]](constraints#namespace.future)

#### 16.4.5.2 Namespace use [[namespace.constraints]](namespace.constraints#namespace.future)

#### 16.4.5.2.3 Namespaces for future standardization [namespace.future]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3117)

Top-level namespaces whose [*namespace-name*](namespace.def.general#nt:namespace-name "9.9.2.1 General [namespace.def.general]") consists of std followed by one or more [*digit*](lex.name#nt:digit "5.11 Identifiers [lex.name]")*s* ([[lex.name]](lex.name "5.11 Identifiers"))
are reserved for future standardization[.](#1.sentence-1)

The behavior of a C++ program is undefined if
it adds declarations or definitions to such a namespace[.](#1.sentence-2)

[*Example [1](#example-1)*:

The top-level namespace std2 is reserved
for use by future revisions of this International Standard[.](#1.sentence-3)

— *end example*]