[cxx] Request for C++ Style Exception: std::function for Swift/C++ Interop

[Asami Doi]

Hi C++ Style Owners,

We are reaching out to request a targeted exception to the style guide's ban on std::function for a specific use case involving Swift/C++ interoperability.

We are currently investigating Swift/C++ interoperability support in Chromium that requires passing callbacks across the C++-to-Swift boundary. Swift's C++ interop layer provides native bridging support for std::function, making it the most viable path for this integration.

We are not proposing to use std::function for general callback handling or to replace Chromium's callback system.
Instead, we plan to use std::function strictly as a wrapper to transport base::RepeatingCallback and base::OnceCallback objects across the C++-to-Swift boundary. By restricting its use to a transport mechanism, we ensure that Chromium's standard callbacks still handle the underlying lifecycle and memory management.
Here is a WIP CL: https://chromium-review.googlesource.com/c/chromium/src/+/7572162

Could you let us know if this approach and exception are acceptable?

Thanks,
Asami

[Avi Drissman]

I would vote for this exception given the narrow scope. (I will admit to bias, being on the Mac team, so I'm hoping to hear more diverse voices as well.)

Avi

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[Joe Mason]

I think https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/c++-features.md#banned-features-and-third_party-code covers this use case:

Chromium code that calls functions exported from a third-party library may use banned library types that are required by the interface, as long as:

• The disallowed type is used only at the interface, and converted to and from an equivalent allowed type as soon as practical on the Chromium side.

 

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[Asami Doi]

Thank you for your feedback.

The guideline mentions banned features in a third-party library. In our case, this Swift/C++ interop code will reside within Chromium's own codebase (not in //third_party).
Could you confirm if this existing exception still applies to our use case?

Assuming it does, since std::function is actively flagged by Chromium's automated presubmit checks, can we bypass the presubmit entirely for those specific warnings when uploading/landing CLs? Or do you have any other guidance for landing CLs with std::function?

Thank you.

[Joe Mason]

In my interpretation of that style guide passage, Swift's "native bridging support" is the third-party library, and the "Swift/C++ interop code... within Chromium's own codebase" would be the Chromium code. 

I'm not sure how to get an exception from the presubmit, though.

[Avi Drissman]

I would argue that this is effectively equivalent, as being at a boundary to code we don't control, whether that is third-party code or interop code.

Avi

[Asami Doi]

Updating a list of exceptions in PRESUBMIT.py works well. I think this is better than bypassing the presubmit. 
https://chromium-review.googlesource.com/c/chromium/src/+/7572162

Thank you everyone for your opinions and feedback.

On Mon, Feb 23, 2026 at 8:09 PM Justin Novosad <ju...@google.com> wrote:

To codify the exception, just go to src/PRESUBMIT.py and find the "BanRule" for "std::function".  There's already a list of exceptions to that rule. You just add your file to the list.  Reference this thread in a review comment for the PRESUBMIT owner's approval.

[Justin Novosad]

To codify the exception, just go to src/PRESUBMIT.py and find the "BanRule" for "std::function".  There's already a list of exceptions to that rule. You just add your file to the list.  Reference this thread in a review comment for the PRESUBMIT owner's approval.

On Mon, Feb 23, 2026 at 11:01 AM Avi Drissman <a...@google.com> wrote: