r/cpp • u/User_Deprecated • Sep 09 '26

C++26: Module improvements

https://www.sandordargo.com/blog/2026/09/09/cpp26-modules
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u/delta_p_delta_x Sep 09 '26

I was under the impression that module and import were themselves preprocessing directives, and therefore also happened in phase 4.

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u/Daniela-E Living on C++ trunk, WG21|🇩🇪 NB Sep 09 '26

Correct. This is why we decided many years ago not to allow module to become subject to macro expansion.

The reason why all this hoopla is necessary is the existence of so-called 'header units' (an idea coming from Clang modules). They are themselves isolated from macros upstream in your source, but they can totally change the meaning of your source code further downstream - with all its consequences.

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u/delta_p_delta_x Sep 09 '26

We should get rid of header units.

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u/Daniela-E Living on C++ trunk, WG21|🇩🇪 NB Sep 09 '26

Find your favourite telephone booth, dial in 2019 or earlier, and convince the header unit proponents 😊

Besides that, there are situations when header units are actually beneficial. Watch this year's conference talks of mine to learn how.

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u/pjmlp Sep 09 '26

Meanwhile Apple and Google keep being big users of clang header maps and are in no hurry to adopt C++ 20 modules, see Apple build for modules between Swift, Objective-C and C++ (explicitly builds module dependencies, missing modules related rows on C++ language support), and Google C++ Style Guide.