15 Preprocessing directives [cpp]

15.4 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
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).
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.
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.
In all three forms of pp-import, the import token is replaced by the import-keyword token.
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
:
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
:
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
 ]
Importing macros from a header unit makes macro definitions from a translation unit visible in other translation units.
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 is the point at which its #define directive occurs (in the translation unit containing the #define directive), or, 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 of a translation unit containing a point of definition for the macro definition, if any (in any other translation 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 translation 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
:
The relative order of pp-imports has no bearing on whether a particular macro definition is active.
— end note
 ]
Example
:

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 "a.h";   // point of definition of #1, #2, and #3, point of undefinition of #1 in "d.h"
import "c.h";   // point of definition of #4 and #5 in "d.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
 ]