Use class without instance (like e.g. String)

After a class is declared you can create a "working instance" that you will use elsewhere. You can do this directly in .hpp or .cpp file for example:

class myClass {
    public:
        void doSomething () {};  
};

myClass myWorkinInstance;

You can latter use the "working instance" in other modules:

void setup () {    
    myWorkinInstance.doSomething ();
}