[module.import] (original) (raw)

10 Modules [module]

10.3 Import declaration [module.import]

A module-import-declarationshall inhabit the global namespace scope.

A module-import-declaration imports a set of translation units determined as described below.

[Note 1:

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

— _end note_]

A module-import-declaration that specifies a module-name Mimports all module interface units of M.

Such a declaration imports the so-named module partition of M.

A module-import-declaration that specifies a H imports a synthesized , which is a translation unit formed by applying phases 1 to 7 of translation ([lex.phases]) to the source file or header nominated by H, which shall not contain a module-declaration.

[Note 2:

A header unit is a separate translation unit with an independent set of defined macros.

All declarations within a header unit are implicitly exported ([module.interface]), and are attached to the global module ([module.unit]).

— _end note_]

An is a member of animplementation-defined set of headers that includes all importable C++ library headers ([headers]).

H shall identify an importable header.

Given two such module-import-declarations:

if their s identify different headers or source files ([cpp.include]), they import distinct header units;
otherwise, if they appear in the same translation unit, they import the same header unit;
otherwise, it is unspecified whether they import the same header unit.
[Note 3:
It is therefore possible that multiple copies exist of entities declared with internal linkage in an importable header.
— _end note_]

[Note 4:

A module-import-declaration nominating a 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].

Any other module-import-declarationdoes not make macros visible.

— _end note_]

A declaration of a name with internal linkage is permitted within a header unit despite all declarations being implicitly exported ([module.interface]).

[Note 5:

A definition that appears in multiple translation units cannot in general refer to such names ([basic.def.odr]).

— _end note_]

A header unit shall not contain a definition of a non-inline function or variable whose name has external linkage.

When a module-import-declaration imports a translation unit T, it also imports all translation units imported by exported module-import-declaration_s_in T; such translation units are said to be exported by T.

Additionally, when a module-import-declarationin 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_s_in the module unit purview of U.85

These rules can in turn lead to the importation of yet more translation units.

[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]).

Macros can be made available by directly importing header units as described in [cpp.import].

— _end note_]

A module implementation unit shall not be exported.

[Example 1:

Translation unit #1:module M:Part;

Translation unit #2:export module M;export import :Part; — _end example_]

A module implementation unit of a module Mthat is not a module partition shall not contain a module-import-declarationnominating M.

[Example 2: module M;import M; — _end example_]

A translation unit has an interface dependency on a translation unit Uif it contains a declaration (possibly a module-declaration) that imports U or if it has an interface dependency on a translation unit that has an interface dependency on U.