Does iOS let you run it in another process? That's a common technique to isolate your app from crashy 3rd party components. This can work if you don't pass it untrusted data. If there's untrusted data coming in and you give it to a crashy c++ component, you're just asking to be pwned.

For containing legacy C++ codebases https://fil-c.org/ looks promising as well, and could fit the bill better if the data was user supplied. It's been discussed on HN many times, most recently here https://news.ycombinator.com/item?id=45133938 .. but currently doesn't support iOS.