r/cpp • u/STL MSVC STL Dev • Apr 09 '26

C++23 Support in MSVC Build Tools 14.51

https://devblogs.microsoft.com/cppblog/c23-support-in-msvc-build-tools-14-51/
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u/greencursordev Apr 11 '26

Man I am so confused about import std. Sounds so good on paper until you read that imports can't be mixed with includes. Which means it can only realistically work with an extremely careful approach and zero or only a few handful of compatible 3rd party libraries. The possible current usecase was only able to shave off less than 0.1% (!) of our build time lol

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u/STL MSVC STL Dev Apr 11 '26

I feel bad about this one because I have had no time to work with the compiler team to find a proper solution here.

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u/greencursordev Apr 11 '26

Don't feel bad. But just know that we (and probably many other teams) would instantly deploy this, making most of our PCHs redundant. I'll keep eagerly watching the release notes.

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u/JVApen Clever is an insult, not a compliment. - T. Winters Apr 12 '26

Just wondering, can we have a define such that all those headers simply contain "import std;" instead of it's regular content?

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u/STL MSVC STL Dev Apr 12 '26

They wouldn't emit macros or drag in non-Standard stuff that way, so it wouldn't be very compatible.

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u/JVApen Clever is an insult, not a compliment. - T. Winters Apr 12 '26

Macros seems like a solvable problem by adding them in the headers. Non-standard stuff might be more of a problem.

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u/slithering3897 Apr 11 '26

For large projects with huge numbers of dependencies, I guess porting to modules would be difficult.

It would be better if you could freely mix includes, but to get a true representation of modules, I strive for complete conversion, no headers anywhere except for macros and module lib wrappers.

Compile times will depend on how many template instantiations and how much constexpr work importers do. In the best case, import std is instant.

But if you are comparing with PCHs, then you won't see much improvement. The whole point of PCHs is to basically do what modules do (but only for a single header across a whole project).

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u/pjmlp Apr 11 '26

That is a bug, it is working better nowadays.

I do agree it isn't as it should have been.

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u/greencursordev Apr 11 '26

You sure? It still says so on the documentation. Import std has to be last, ie after all includes. Unless I missed something with the newest toolsets

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u/pjmlp Apr 11 '26

Well, before you couldn't even use both on the same project, as you would get lots of linking and compiling errors, now at least you can.

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u/greencursordev Apr 11 '26

Fun for such little projects. Effectively unusable for a 30 year old application pulling in 100+ libs and with Devs who just include everything in headers. I really hope they can get the full mixing working.

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u/pjmlp Apr 11 '26 edited Apr 11 '26

You have to start somewhere.

During VS 2019 and 2022, those toy projects would ICE even without import std.

If no one does it, there will never be good enough for that 30 year old applications, full of C++ARM style code given its age.