You have to include the class scope because when you are dealing with this kind of variable, the declaration of the static variable does not define it, so you must define it outside the class. I think to myself this is just a rule to remember.
Take in mind that this is a special variable which can even be used without declaring any myclass object.
navarromoral >> I believe you are referring to a public static variable. In his case, the default scope is private. he can only assign value by the proper get/set method which is assigned in the class. The variable here, is defined, but not assigned.
david91 >> I would think that the variable is declared static for "persistence" reasons. Once the variable holds a value, it can be pulled from all values that access that static variable - hence object1 and object2 have the same value. If you defined it locally, it would not hold value.
I was talking in general about static member variables. When the scope is private only function members are allowed to access this variable(and there is no need to include classname::), but I think all I have said holds true.