I was doing some research on C++ and Unity and for the most part it seems impossible.
Does Unity offer any type of low API ? I see a big push on Unity part to reach into cinimatic and film studios with updates like Timeline. The largest hurdle in adapting Unity is all our software like custom cloth/hair solvers are written in C++ and intergrated with a low level api like Maya.
Sure something similar must exist for unity
you can buy the full open source version (if that is still possible at all), and if you have the money for that. Logic scripting are done in c# (that follows almost the same OO principle as c++ just without the memory handling) or java script. C++ is not an advantage to use when it comes to logic implementation. Every thing performance wise is done in the engine in native code.
If you already have code written in C/C++ that you want to use in Unity, you can build it as a DLL and then write C# wrappers for it using the [DllImport] attribute. You cannot interact with the Unity API directly from C/C++, though.