[using.headers]

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

## 16.4 Library-wide requirements [[requirements]](requirements#using.headers)

### 16.4.3 Using the library [[using]](using#headers)

#### 16.4.3.2 Headers [using.headers]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1686)

The entities in the C++ standard library are defined in headers,
whose contents are made available to a translation unit when it contains the appropriate#include preprocessing directive ([[cpp.include]](cpp.include "15.3 Source file inclusion"))
or the appropriateimport declaration ([[module.import]](module.import "10.3 Import declaration"))[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1698)

A translation unit may include library headers in any order ([[lex.separate]](lex.separate "5.1 Separate translation"))[.](#2.sentence-1)

Each may be included more than once, with no effect different from
being included exactly once, except that the effect of including either[<cassert>](assertions.general#header:%3ccassert%3e "19.3.1 General [assertions.general]") or [<assert.h>](support.c.headers.general#header:%3cassert.h%3e "17.15.1 General [support.c.headers.general]") depends each time on the lexically current definition ofNDEBUG[.](#2.sentence-2)[149](#footnote-149 "This is the same as the C standard library.")

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1712)

A translation unit shall include a header only outside of anydeclaration or definition and,
in the case of a module unit,
only in its [*global-module-fragment*](module.global.frag#nt:global-module-fragment "10.4 Global module fragment [module.global.frag]"), and
shall include the header or import the corresponding header unit lexically
before the first reference in that translation unit to any of the entities
declared in that header[.](#3.sentence-1)

No diagnostic is required[.](#3.sentence-2)

[149)](#footnote-149)[149)](#footnoteref-149)

This is the same as the C standard library[.](#footnote-149.sentence-1)