1

I’m excited for the imminent release of Swift 5.9 and I was thinking about the new possibilities unlocked by C++ interop. Swift on Android has worked for a while, but you got little more than the stdlib and bridging was a pain.

With C++ interop, I assume we could at least write Swift code and use it on Android by bridging through C++, but I’m wondering if we could just skip the C++ intermediate layer and just call Swift directly from Kotlin through the NDK as if it were C++?

Also I’m guessing we can use the existing system C / C++ libraries normally from Swift? Though, if they are non-modular I guess we might have to use some kind of bridging header?

2

Not true, I have been distributing a full SDK for years, which can be used to compile many Swift packages for Android with minimal changes.

There is a wrapper to make this easier, been around even longer.

As in, without using whatever Kotlin<->C++ interop there is now, but using some new Kotlin<->Swift interop? Somebody would have to work on that, but I don't know of anybody who is.

For C, this has long been available through Swift's C interop. I don't believe there's much system C++ APIs, if any.

Keep in mind that google doesn't want you using C/C++ on Android, so what exists in the NDK is the bare minimum to port over legacy code. For example, you cannot access the Android GUI APIs directly from C++, only from the JVM, even though that GUI is implemented in C++.

The Android libc++ uses a modified version of the upstream LLVM modulemap, so that works, but the Bionic libc doesn't have one. I opened an issue for it years ago, but I doubt anybody will work on it anytime soon. Swift's C++ Interop is in a semi-broken state on Android, partially because of those modulemap issues, as we currently just use the glibc modulemap that comes with the Swift toolchain.

For other C system headers, yes, you may need a modulemap, just like that JNI wrapper has.

To sum up, Android has almost no system C++ APIs, Swift's C++ Interop is currently incomplete on Android (note all the Interop/*Cxx tests marked as unsupported on the community Android CI), and Swift already works fairly well on Android, so support for C++ interop would add almost nothing on that platform.

2 Likes
3

You can run a full C++ application in the NDK, it's just that interactions with the Android event loop have to be creatively managed.

4

Sure, that is mainly meant for porting existing games that have their own UI widgets rendering to OpenGLES. But you have to ship the C++ stdlib with your app, you can't even use the one in the system. If they were really trying to enable C++ devs, they could easily make available C or C++ headers/libraries for the native Android GUI, but they choose not to.

I can't blame them for trying to sunset use of C/C++ in mobile apps, I just wish they would provide a modern natively-compiled alternative like Apple is. I tried Kotlin/Native years ago, it was not a good dev experience. There is some interest in adding Rust, but the NDK devs say it "is not being worked on, nor is it planned."

5

Ah, when I used it I did so using a non-standard standard library so I didn't encounter this.

I think you can blame them. Seems like bad behavior on their part. You wouldn't encounter so many arbitrary decisions using let's say a Pinephone or even an iPhone.