What's the best testing or test scripting tool which works both in iOS and Android?

I'm looking for a a common testing platform which can be shared across iOS and Android. I don't want to go with heavy lifting tools such as Appium. Agenda is to test few part of codes for functional behavior. I'm looking for a solution which is light weight and can be shared across both the platforms.

My primary thoughts are writing something in C++ which can used in both iOS and Android. Not sure, if I'm on the right path.

Any help/suggestion around this would be helpful.