[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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