15 Preprocessing directives [cpp]

15.5 Header unit importation [cpp.import]

pp-import:
export import header-name pp-tokens ; new-line
export import header-name-tokens pp-tokens ; new-line
export import pp-tokens ; new-line
A pp-import shall not appear in a context where import or (if it is the first token of the pp-import) export is an identifier defined as an object-like macro.
The preprocessing tokens after the import preprocessing token in the import control-line are processed just as in normal text (i.e., each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens).
[Note 1:
An import directive matching the first two forms of a pp-import instructs the preprocessor to import macros from the header unit ([module.import]) denoted by the header-name, as described below.
— end note]
The point of macro import for the first two forms of pp-import is immediately after the new-line terminating the pp-import.
The last form of pp-import is only considered if the first two forms did not match, and does not have a point of macro import.
If a pp-import is produced by source file inclusion (including by the rewrite produced when a #include directive names an importable header) while processing the group of a module-file, the program is ill-formed.
In all three forms of pp-import, the import and export (if it exists) preprocessing tokens are replaced by the import-keyword and export-keyword preprocessing tokens respectively.
[Note 2:
This makes the line no longer a directive so it is not removed at the end of phase 4.
— end note]
Additionally, in the second form of pp-import, a header-name token is formed as if the header-name-tokens were the pp-tokens of a #include directive.
The header-name-tokens are replaced by the header-name token.
[Note 3:
This ensures that imports are treated consistently by the preprocessor and later phases of translation.
— end note]
Each #define directive encountered when preprocessing each translation unit in a program results in a distinct macro definition.
[Note 4:
A predefined macro name ([cpp.predefined]) is not introduced by a #define directive.
Implementations providing mechanisms to predefine additional macros are encouraged to not treat them as being introduced by a #define directive.
— end note]
Each macro definition has at most one point of definition in each translation unit and at most one point of undefinition, as follows:
  • The point of definition of a macro definition within a translation unit T is
    • if the #define directive of the macro definition occurs within T, the point at which that directive occurs, or otherwise,
    • if the macro name is not lexically identical to a keyword ([lex.key]) or to the identifiers module or import, the first point of macro import in T of a header unit containing a point of definition for the macro definition, if any.
    In the latter case, the macro is said to be imported from the header unit.
  • The point of undefinition of a macro definition within a translation unit is the first point at which a #undef directive naming the macro occurs after its point of definition, or the first point of macro import of a header unit containing a point of undefinition for the macro definition, whichever (if any) occurs first.
A macro directive is active at a source location if it has a point of definition in that translation unit preceding the location, and does not have a point of undefinition in that translation unit preceding the location.
If a macro would be replaced or redefined, and multiple macro definitions are active for that macro name, the active macro definitions shall all be valid redefinitions of the same macro ([cpp.replace]).
[Note 5:
The relative order of pp-imports has no bearing on whether a particular macro definition is active.
— end note]
[Example 1:

Importable header "a.h":#define X 123 // #1 #define Y 45 // #2 #define Z a // #3 #undef X // point of undefinition of #1 in "a.h"

Importable header "b.h":import "a.h"; // point of definition of #1, #2, and #3, point of undefinition of #1 in "b.h" #define X 456 // OK, #1 is not active #define Y 6 // error: #2 is active

Importable header "c.h":#define Y 45 // #4 #define Z c // #5

Importable header "d.h":import "c.h"; // point of definition of #4 and #5 in "d.h"

Importable header "e.h":import "a.h"; // point of definition of #1, #2, and #3, point of undefinition of #1 in "e.h" import "d.h"; // point of definition of #4 and #5 in "e.h" int a = Y; // OK, active macro definitions #2 and #4 are valid redefinitions int c = Z; // error: active macro definitions #3 and #5 are not valid redefinitions of Z — end example]