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In the current C++ draft (august 2019), the grammar for a pp-import http://eel.is/c++draft/cpp.import#nt:pp-import) allows for additional pp-tokens after the header-name or the header-name-tokens.
The current version of that section is the result of P1703: "Recognizing Header Unit Imports Requires Full Preprocessing". Before the changes caused by this proposal, the grammar still allowed for preprocessing tokens tokens after the header-name or the header-name-tokens, but in the form of a pp-import-suffix. (section [cpp.module] of P1103).
What is the reasoning behind allowing additional, unused preprocessing tokens in this context?
Thank you.
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#import and #include are preprocessor directives for bringing in the contents of a header to a file. #include is replaced by the contents of the header directly, while #import is only replaced by the contents of the header the first time that header is imported.
You can use modules side by side with header files. A C++ source file can import modules and also #include header files. In some cases, you can import a header file as a module rather than include it textually by using #include in the preprocessor.
You make the declarations in a header file, then use the #include directive in every . cpp file or other header file that requires that declaration. The #include directive inserts a copy of the header file directly into the . cpp file prior to compilation.
hpp extension, just click New Item ~> Header File (. h) and at the bottom name it with . hpp extension. Alternatively you might create .
They are there to allow for attributes.
The phase 7 (parsing and semantic analysis) grammar didn't change. Attributes are still allowed on imports. Previously the phase 4 (preprocessor) grammar tried to restrict more what was allowed as an import in order to enforce the rule that:
A pp-import is only recognized when the sequence of tokens produced by phase 4 of translation up to the import token forms an import-seq, and the import token is not within the header-name-tokens or pp- import-suffix of another pp-import.
The new rules restrict it even further by not allowing more then one import per line, so this rule and its associated grammar are no longer needed.
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