Should mojom-forward.h files be more widely publicised?

[Joe Mason]

Chrome's style guide recommends forward declarations over includes (https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/c++.md#forward-declarations-vs_includes), and there are some forward-only headers like "callback_forward.h" to facilitate this.

IIUC the generated mojo code for foo.mojom creates "foo.mojom-forward.h", which has only forwards, and "foo.mojom.h" which has the full definitions. So by the spirit of the style guide, header files should #include mojom-forward.h files where possible. But I can't find this explicitly documented anywhere.

Should we update https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/c++.md#forward-declarations-vs_includes to explicity recommend .mojom-forward.h files?

Come to think of it, is it worth explicitly mentioning forwarding headers like "callback_forward.h"?

Joe

[Peter Kasting]

I think you're right that forwarding headers are preferred when they suffice. I am ambivalent towards explicitly mentioning that (I don't think it will change behavior much).

I would love to see a limited form of misc-include-cleaner or IWYU suggest these automatically where applicable. 

PK

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[danakj]

On Fri, Oct 18, 2024 at 12:03 PM 'Joe Mason' via cxx <c...@chromium.org> wrote:

Chrome's style guide recommends forward declarations over includes (https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/c++.md#forward-declarations-vs_includes), and there are some forward-only headers like "callback_forward.h" to facilitate this.

IIUC the generated mojo code for foo.mojom creates "foo.mojom-forward.h", which has only forwards, and "foo.mojom.h" which has the full definitions. So by the spirit of the style guide, header files should #include mojom-forward.h files where possible. But I can't find this explicitly documented anywhere.

Should we update https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/c++.md#forward-declarations-vs_includes to explicity recommend .mojom-forward.h files?

Good idea :)

Come to think of it, is it worth explicitly mentioning forwarding headers like "callback_forward.h"?

Maybe as this one is particularly widespread, but don't want to maintain a list of such headers in the style guide either. Maybe a big "USE THE FORWARDING HEADER IN HEADERS" at the top of callback.h would do more?

[Joe Mason]

On Fri, Oct 18, 2024 at 2:44 PM Peter Kasting <pkas...@chromium.org> wrote:

I think you're right that forwarding headers are preferred when they suffice. I am ambivalent towards explicitly mentioning that (I don't think it will change behavior much).

I would love to see a limited form of misc-include-cleaner or IWYU suggest these automatically where applicable. 

Yeah, an automated warning would be better, but I still think it's worthwhile to mention in the docs so reviewers have something to point to.

On Tue, Oct 22, 2024 at 2:52 PM danakj <dan...@chromium.org> wrote:

Come to think of it, is it worth explicitly mentioning forwarding headers like "callback_forward.h"?

Maybe as this one is particularly widespread, but don't want to maintain a list of such headers in the style guide either. Maybe a big "USE THE FORWARDING HEADER IN HEADERS" at the top of callback.h would do more?

I just meant mentioning the concept of forwarding headers, not a complete list. I put up https://crrev.com/c/5953868 for review, which uses callback_forward.h as "an example" of a forwarding header, and also mentions the mojo-generated ones since they're harder to find in a code search. WDYT?

Also, callback.h already has a blurb like that. Maybe it could be bigger, though.

[Claudio DeSouza]

I was doing a few test today with IWYU, and actually these headers get automatically selected with a tool like that. Something to bear in mind.

Claudio.