What does HackerNews think of include-what-you-use?

> * Remove all unnecessary headers. Template expansion is slow, and preprocessing is even slower. Some of the standard includes (like `` and ``) are notorious for slowing individual translation units to a crawl. `#pragma once` for your own headers also helps with cpp-time performance.

> * Forward-declare as much as you can. Forward type declarations mean that the compiler doesn't need to process all of `Foo` when it sees `Foo&` or `Foo`.

I've found the include-what-you-use (IWYU) tool [1][2] can help immensely with automating this process, especially on large code-bases.

It uses LLVM/Clang to analyze a .cpp file / translation unit and produces the minimal subset of exactly which includes are necessary and what types can be forward declared.

[1] https://include-what-you-use.org/

[2] https://github.com/include-what-you-use/include-what-you-use

If the issue is with including transitive dependencies that are in your own codebase, then you should annotate the public interface header to the implementation details with IWYU Pragmas [1] that export the implementation (for example [2]).

If this is in third-party libraries, you can use IWYU Mappings [3] to map the "private" headers (usually the transitive include) to the public interface. An example that I use for the PEGTL library [4].

[1]: https://github.com/include-what-you-use/include-what-you-use...

[2]: https://github.com/anand-bala/signal-temporal-logic/blob/800...

[3]: https://github.com/include-what-you-use/include-what-you-use...

[4]: https://github.com/anand-bala/signal-temporal-logic/blob/800...

I've found that using IWYU Pragmas [1] for codebases you own and IWYU Mappings [2] for third-party libraries __almost__ entirely eliminates weird IWYU suggestions (there are a few annoyingly stupid suggestions from the tool I just ignore).

I've also recently been making libraries I write compatible with users that run IWYU by annotating all public headers with IWYU pragma comments that export symbols/transitive includes correctly, etc.

[1]: https://github.com/include-what-you-use/include-what-you-use...

[2]: https://github.com/include-what-you-use/include-what-you-use...

iwyu is Google's project originally. It has worked for them for more than a decade on their ginormous monorepo.

Sometimes it gets some things wrong, so you have these escape hatches to control it: https://github.com/include-what-you-use/include-what-you-use...

You can use the IWYU "export" pragma to inform IWYU that this is what you're doing. https://github.com/include-what-you-use/include-what-you-use...

seems so. https://github.com/include-what-you-use/include-what-you-use...