Question

It seems to me that a static class variable is identical to an extern variable, because you only declare it in the static int x / extern int x statement, and actually define it elsewhere (usually in a .cpp file)

static class variable

// .h file
class Foo
{
    static int x ;
} ;

// .cpp file
int MyClass::x = 0 ;

Extern variables:

// .h file
extern int y;

// .cpp file
int y = 1;

In both cases the variable is declared once somewhere, and defined in a file that will not be included more than once in the compilation (else linker error)

Answer 1

Yes, both have static storage duration and external linkage; they have essentially the same run-time properties, only differing in (compile-time) visiblity.