Define and get/set a global variable or a variable of type class in Omnet++

I want to define and access C++ variable in the whole of the Omnet++ project (any function including the Inet functions) from start simulation to end of simulation. How to implement it? Where is the initialize or constructor function of a Omnet++ project? Also finilize or destructure? What is the first point of the Omnet++ project? Also the last point of the Omnet++ project?