Is there any way to pass Ruby objects to a C++ application ? I have never done that kind of thing before and was wondering if that would be possible. Would it require to modify the Ruby core code ?
Yes, and no, respectively.
Ruby is written in C. C++ is, by design, C-compatible.
All objects in Ruby are held by a VALUE object (which is a union type), which can be passed around quite easily.
Any directions you find for extending Ruby with C apply in C++ with little modification. Alternatively, you can use something like SWIG to simplify writing your extensions.