Nfy VS CppProperties

For me, the answer is a generic multicast notification system. It utilizes a template based approach that allows you to define prototype messages using a class-based interface that you wish to send to another library. For example, your input library might respond to things like a button down / up, mouse clicks, etc, etc. So you'd create an interface that defines all your input notifications as functions. I won't go into detail, because it's well documented in that link.

In my own code, I make use a generic notification system which allows high-level systems to register themselves with lower-level libraries to help invert dependencies. An example of this is how my game engine's UI library can register itself with the Input library, so it can receive mouse, keyboard, and gamepad events, necessary for manipulating UI controls. But the Game itself also needs to register for some UI events itself, not to mention debug functionality like ImGui for debug windows, etc. So I use this notification system to ensure the low-level Input library can push events out to multiple subscribers without needing to know about them.

See if this is close to what you're looking for: https://github.com/JamesBoer/Nfy

https://github.com/KonanM/CppProperties/blob/master/include/cppproperties/Signal.h#L38-L87

I'm not sure if this is exactly what you want, but I have written a small library (https://github.com/KonanM/CppProperties) that mimics how my old employee implemented such a system. I would say what you are looking for is something that I would call a proxy property?