C++ :: How To Define Static Member Data In Abstract Class

For example, in a header file A.h, I define an abstract class,

Code:

// A.h
class A {
public:
virtual void foo() = 0;
private:
static int _x;
};

How'd I initialize static member data _x?Normally, we initialize a static member data in a cpp file. However, there is not cpp file for A.h. If I intialize _x in header file, there will be linker errors like mulitple defined symbols. What is appropriate way to do that?