Disclaimer: I am an application developer, and one of the maintainers of the Drogon C++ web framework.
C++ & apps don’t fit well together technologically these days.
Android? Go Kotlin.
iPhone? Go Swift.
Cross platform? Meh, kind of dislike middleware as they always lag behind the vendor. This was the case three decades ago as it is today.
Usually you are far better off by defining a REST API as your border between platforms, then just use whatever easy (de)serialization techniques the language will give you.
Personally I have a cross-platform C++ stack built on top of Drogon that I can use on multiple platforms, then it’s usually a native UI layer (Kotlin + Compose on Android, Swift + SwiftUI on Apple platforms) on top of the cross-platform layer. This way business layer code can be shared across every platform while only the UI needs to be adapted to its host OS.
Drogon website: https://drogon.org
