Slight off-topic: is it possible to use C++ code cross-platform way for performance-critical parts? i.e. to have single C++ code for iOS and Android, in React Native.
Yup you can! That's actually how Expo's `GLView` (which exposes OpenGL ES as a WebGL'y API in React Native's JS context) works. The binding is done in common C++ code for both iOS and Android. Calling to that from Obj-C is straightforward, for Java you use JNI. This is binding is done manually and built once (not auto-exposed), then you can use the GL stuff in JS!
https://github.com/exponent/exponent/blob/master/cpp/EXGL.cp... Here's the code
It is definitely possible to create a cross-platform C++ library to share performant across platforms, but unfortunately you need to do the plumbing for each platform by hand.