[module.import]

# 10 Modules [[module]](./#module)

## 10.3 Import declaration [module.import]

[module-import-declaration:](#nt:module-import-declaration "10.3 Import declaration [module.import]")  
import-keyword [*module-name*](module.unit#nt:module-name "10.1 Module units and purviews [module.unit]") [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt ;  
import-keyword [*module-partition*](module.unit#nt:module-partition "10.1 Module units and purviews [module.unit]") [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt ;  
import-keyword [*header-name*](lex.header#nt:header-name "5.6 Header names [lex.header]") [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt ;

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L412)

A [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") shall inhabit the global namespace scope[.](#1.sentence-1)

In a module unit, all [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]")*s* and [*export-declaration*](module.interface#nt:export-declaration "10.2 Export declaration [module.interface]")*s* exporting[*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]")*s* shall appear before all other [*declaration*](dcl.pre#nt:declaration "9.1 Preamble [dcl.pre]")*s* in
the [*declaration-seq*](dcl.pre#nt:declaration-seq "9.1 Preamble [dcl.pre]") of the[*translation-unit*](basic.link#nt:translation-unit "6.7 Program and linkage [basic.link]") and of the [*private-module-fragment*](module.private.frag#nt:private-module-fragment "10.5 Private module fragment [module.private.frag]") (if any)[.](#1.sentence-2)

The optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") appertains to the [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]")[.](#1.sentence-3)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L425)

A [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") [*imports*](#def:import "10.3 Import declaration [module.import]") a set of
translation units determined as described below[.](#2.sentence-1)

[*Note [1](#note-1)*:

Namespace-scope declarations exported by the imported translation units
can be found by name lookup ([[basic.lookup]](basic.lookup "6.5 Name lookup"))
in the importing translation unit
and declarations within the imported translation units
become reachable ([[module.reach]](module.reach "10.7 Reachability"))
in the importing translation unit
after the import declaration[.](#2.sentence-2)

— *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L438)

A [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") that specifies
a [*module-name*](module.unit#nt:module-name "10.1 Module units and purviews [module.unit]") M imports all module interface units of M[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L443)

A [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") that specifies
a [*module-partition*](module.unit#nt:module-partition "10.1 Module units and purviews [module.unit]") shall only appear after
the [*module-declaration*](module.unit#nt:module-declaration "10.1 Module units and purviews [module.unit]") in a module unit of
some module M[.](#4.sentence-1)

Such a declaration imports the so-named
module partition of M[.](#4.sentence-2)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L451)

A [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") that specifies
a [*header-name*](lex.header#nt:header-name "5.6 Header names [lex.header]") H imports
a synthesized [*header unit*](#def:header_unit "10.3 Import declaration [module.import]"),
which is a translation unit formed by applying
phases 1 to 7 of translation ([[lex.phases]](lex.phases "5.2 Phases of translation"))
to the source file or header nominated by H,
which shall not contain a [*module-declaration*](module.unit#nt:module-declaration "10.1 Module units and purviews [module.unit]")[.](#5.sentence-1)

[*Note [2](#note-2)*:

A header unit is a separate translation unit with
an independent set of defined macros[.](#5.sentence-2)

All declarations within a header unit are implicitly
exported ([[module.interface]](module.interface "10.2 Export declaration")),
and are attached to the global module ([[module.unit]](module.unit "10.1 Module units and purviews"))[.](#5.sentence-3)

— *end note*]

An [*importable header*](#def:header,importable "10.3 Import declaration [module.import]") is a member of animplementation-defined
set of headers that
includes all importable C++ library headers ([[headers]](headers "16.4.2.3 Headers"))[.](#5.sentence-4)

H shall identify an importable header[.](#5.sentence-5)

Given two such [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]")*s*:

- [(5.1)](#5.1)

if their [*header-name*](lex.header#nt:header-name "5.6 Header names [lex.header]")*s* identify
different headers or source files ([[cpp.include]](cpp.include "15.3 Source file inclusion")),
they import distinct header units;

- [(5.2)](#5.2)

otherwise, if they appear in the same translation unit,
they import the same header unit;

- [(5.3)](#5.3)

otherwise, it is unspecified whether they import the same header unit[.](#5.sentence-6)
  [*Note [3](#note-3)*:
  It is therefore possible that multiple copies exist of entities
declared with internal linkage in an importable header[.](#5.3.sentence-2)
 — *end note*]

[*Note [4](#note-4)*:

A [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") nominating
a [*header-name*](lex.header#nt:header-name "5.6 Header names [lex.header]") is also recognized by the
preprocessor, and results in macros defined at the
end of phase 4 of translation of the header unit
being made visible as described in [[cpp.import]](cpp.import "15.6 Header unit importation")[.](#5.sentence-7)

Any other [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") does not make macros visible[.](#5.sentence-8)

— *end note*]

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L497)

A declaration of a name with internal linkage is
permitted within a header unit despite all
declarations being implicitly exported ([[module.interface]](module.interface "10.2 Export declaration"))[.](#6.sentence-1)

[*Note [5](#note-5)*:

A definition that appears in multiple translation units
cannot in general refer to such names ([[basic.def.odr]](basic.def.odr "6.3 One-definition rule"))[.](#6.sentence-2)

— *end note*]

A header unit shall not contain
a definition of a non-inline function or variable
whose name has external linkage[.](#6.sentence-3)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L509)

When a [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") imports
a translation unit T, it also imports
all translation units imported by
exported [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]")*s* in T; such translation units are
said to be [*exported*](#def:module,exported "10.3 Import declaration [module.import]") by T[.](#7.sentence-1)

Additionally, when a [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") in a module unit of some module M imports another module unit U of M,
it also imports all translation units imported by
non-exported [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]")*s* in the module unit purview of U[.](#7.sentence-2)[85](#footnote-85 "This is consistent with the lookup rules for imported names ([basic.lookup]).")

These rules can in turn lead to the importation of yet more
translation units[.](#7.sentence-3)

[*Note [6](#note-6)*:

Such indirect importation does not make macros available,
because a translation unit is
a sequence of tokens in translation phase 7 ([[lex.phases]](lex.phases "5.2 Phases of translation"))[.](#7.sentence-4)

Macros can be made available by directly importing header units
as described in [[cpp.import]](cpp.import "15.6 Header unit importation")[.](#7.sentence-5)

— *end note*]

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L535)

A module implementation unit shall not be exported[.](#8.sentence-1)

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

Translation unit #1:module M:Part;

Translation unit #2:export module M;export import :Part; // error: exported partition :Part is an implementation unit — *end example*]

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L548)

A module implementation unit of a module M that is not a module partition
shall not contain a [*module-import-declaration*](#nt:module-import-declaration "10.3 Import declaration [module.import]") nominating M[.](#9.sentence-1)

[*Example [2](#example-2)*: module M;import M; // error: cannot import M in its own unit — *end example*]

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L560)

A translation unit has an [*interface dependency*](#def:interface_dependency "10.3 Import declaration [module.import]") on a translation unit U if it contains a declaration (possibly a [*module-declaration*](module.unit#nt:module-declaration "10.1 Module units and purviews [module.unit]"))
that imports U or if it has
an interface dependency on a translation unit that has an interface dependency on U[.](#10.sentence-1)

A translation unit shall not have an interface dependency on itself[.](#10.sentence-2)

[*Example [3](#example-3)*:

Interface unit of M1:export module M1;import M2;

Interface unit of M2:export module M2;import M3;

Interface unit of M3:export module M3;import M1; // error: cyclic interface dependency M3 →M1 →M2 →M3 — *end example*]

[85)](#footnote-85)[85)](#footnoteref-85)

This is consistent
with the lookup rules for imported names ([[basic.lookup]](basic.lookup "6.5 Name lookup"))[.](#footnote-85.sentence-1)