Static Data Member Initialization

Question

Why must static data member initialization be outside the class?

class X
{
public:
      int normalValue = 5; //NSDMI
      static int i;
};

int X::i = 0;

Why is the static data member (here "i") only a declaration, not a definition?

Answer 1

You need to supply a separate definition for a static data member (if its odr-used, as defined in C++11) simply because that definition shall reside somewhere - in one and only one translation unit. Static class data members are basically global objects (global variables) declared in class scope. The compiler wants you to choose a specific translation unit that will hold the actual "body" of each global object. It is you who has to decide which translation unit to place the actual object to.