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17 Aug 2019 : Querying the cost of sharing code between iOS and Android #
Eyal Guthmann, a Dropbox software engineer, has written an interesting piece about the difficulties of sharing C++ code across mobile platforms. I'm not questioning the truth of the difficulties Dropbox experienced, but as someone who's part of the mobile C++ dev community*, and in past lives has helped maintain C/C++ libraries shared across multiple platforms (Android, iOS, Windows, Linux, Sailfish), I don't buy all of the arguments he presents.

Let's take the points he raises one-by-one.

1) The overhead of custom frameworks and libraries - replacing language defaults

I admit this can be painful and intricate, but the main platforms already have support for cross-language library binding. When it comes to threading I'd argue the sane approach is to keep the threading in one place, on the platform-specific side, given each platform has its own slightly different approach. You can still share plenty of useful code without trying to homogenise a threading implementation across platforms.

Eyal also brings up threading in the context of debugging across language boundaries. I'd apply the same rule here: keep the threading out of the C/C++ code. That doesn't mean you can't share plenty of the code that executes inside each thread, of course.

2) The overhead of custom frameworks and libraries - replacing language defaults

Eyal cites two examples (json11 and nn) of custom libraries for replacing language defaults that Dropbox has to maintain. Combined they amount to 1812 lines of code, including comments. I find it difficult to believe Dropbox struggles with the overhead of maintaining these libraries.

3) The C++ mobile community is almost non-existent

Eyal needs to look harder. Either that or he's putting more weight on that "almost" than I think the word can reasonably sustain. Maybe he should have spoken to the devs at Qt?

4) Difference between platforms

Perhaps I'm misunderstanding what Dropbox were trying to achieve, but I'd argue the key to using cross platform C/C++ libraries is through good architecting: choosing which parts to work on cross-platform and which to leave as platform-specific. In some cases such as UI, control-flow/event handling and hardware access, it just makes more sense to use the good, bespoke, vendor-supplied tools and approaches.

5) The overhead of a custom dev environment

At least this arguments has some force for me. My personal experience is that tooling is quite painful even when you stick to the most standard of environments and approaches on a single platform. Adding in multiple toolchains and environments into a single project is going to introduce some interesting and new ways to experience pain.

6) The overhead of training, hiring, and retaining developers

I work for a company that employs many C++ mobile devs and getting quality talent certainly isn't easy. Then again I've never worked anywhere that found recruiting easy. If Dropbox find it easier to recruit mobile devs with Swift or Kotlin experience, then I'm not going to argue. Reading between the lines though, it sounds like Dropbox lost a big chunk of their C++ team and failed to keep the knowledge within the company. Sometimes even the best planning can't avoid something like that happening, but it doesn't follow that the technology in question is to blame.

So, to summarise, what I'm saying is that unless you're writing your complete application using some fully cross-platform toolkit (e.g. Qt, Xamarin, etc.) in which case you accept the compromises that come with that, then you can still use C/C++ for reducing maintenance with good partitioning. Use C/C++ for core library functionality but anything less generic, including control flow and UI, should stay as platform-specific code where vendors already provide good tooling but with largely incompatible approaches anyway.

I have to say, I feel greatly privileged that I'm now being paid to develop for a single platform that's perfectly tailored for C/C++ development across the entire stack. But I acknowledge that cross-platform development is a reality for a company like Dropbox and that it's hard. It's a shame that Dropbox feel they have to give up on code-sharing for their solution.

* I'm a C/C++ developer working in the mobile space, so that makes me "part of the community", right?