The const after the function can only be used on member functions. These functions have an implicit hidden parameter, which is a pointer to the object:
int GetNum(/* Class *this */);
If the function is followed by const, the this pointer is const:
int GetNum(/* const Class *this */);
* You can't change member variables in a function declared as const.
* If an object is declared as const, you can only call functions that are declared const.
* From a const member function, you can only call other member functions that are also const.