Why can't we declare static variables in the class definition?

If I write code that looks like

#include <string>
class C {
  static const std::string MY_SPECIAL_STRING = "hi";
};

and I try to compile it, even in C++11 mode, g++ will complain:

static data member of type 'const std::string' must be initialized out of line

Why do we have this rule?

---

I mean, I can still kind of 'cheat' and have static variables inside static methods, e.g.:

#include <string>
class C {
  static const std::string& MY_SPECIAL_STRING() {
    static const std::string& ss("hi");
    return ss;
  }
};

This way, I don't need to put a declaration in a translation unit, and I effectively have the same thing, albeit with uglier syntax.