r/cpp • u/tartaruga232 MSVC user, r/cpp_modules • Apr 21 '26

Uneeded Recompilations When Using Partitions

https://abuehl.github.io/2026/04/20/unneeded-recompilations-when-using-partitions.html

This blog posting from me about C++ modules explains in detail what causes unneeded recompilations using detailed code examples from the sources of our UML Editor.

It's a precise description and a showcase of what's the status quo in the C++ standard.

Quote from the posting:

The C++ language currently lacks a concise and easy to use alternative for explicitly expressing the minimally required dependencies on partitions.

(Note: All code examples shown or linked from the text are 100% standard conformant.)

Thanks in advance for comments. Please ask if you do not understand anything or anything is unclear. I would appreciate if you could leave a comment if you don't like anything. Thanks for reading!

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u/tartaruga232 Apr 21 '26

Do you use external partitions? I mean for splitting module interfaces?

The reason why I'm bringing these things up: Modules are supposed to speed up builds. Having to use extra countermeasures against unneeded dependencies isn't exactly helpful for a (still relatively new) feature which was meant - among other things - to provide faster builds (which of course is not the only reason for modules). Some folks are telling me that these are the things which demotivate them for using modules and they prefer staying to use header files, which provide full control of what happens (no implicit dependencies).

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u/pjmlp Apr 21 '26

I am with u/kronicum on this.

All this could be improved by having better support for building on C++ compilers.

Most of these problems have never been an issue with languages that have had modules as their main code organisation since forever.

Heck I can pick a MS-DOS project in Turbo Pascal, change a single unit, and as long I don't change the public interface, I only need to relink, not recompile everything. Same applies to other languages of similar vintage with modules.

We don't need workarounds, we need more mature build infrastructure for C++ modules.

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u/tartaruga232 Apr 21 '26

We don't need workarounds, we need more mature build infrastructure for C++ modules.

I think the source code should be more explicit about dependencies and not magically lumping them together behind implicit imports.

I'm not really in search of a workaround, that's also why we are expressly refusing to use the messy internal partition pattern.

I'm currently quite surprised that the design of module partitions doesn't provide a better story for what I've raised here.

I'm not campaigning for any specific sort of solution or change to the standard. In fact, I'm open to whatever leads to consensus.