Linker error when passing static const int as reference [duplicate]

This question already has an answer here:

• Defining static const integer members in class definition 6 answers

Why do I get undefined reference errors to NUMBER when compiling this with gcc 4.6.3?:

g++ -std=c++0x -pipe -Wall -pedantic file.c

file.c:

#include <stdio.h>
#include <utility>

class Thing {
public:
    Thing() {
        foo(NUMBER);
        const int* p = &NUMBER;
        printf("%d\n", *p);
    }
    void foo(const int& i) {}
    static const int NUMBER = 123;
};

int main() { Thing thing; return 0; }

Error:

chetic@home:~/Documents$ g++ -std=c++0x -pipe -Wall -pedantic test.c 
/tmp/cceBY2zr.o: In function `Thing::Thing()':
statest.c:(.text._ZN5ThingC2Ev[_ZN5ThingC5Ev]+0x11): undefined reference to `Thing::NUMBER'
statest.c:(.text._ZN5ThingC2Ev[_ZN5ThingC5Ev]+0x21): undefined reference to `Thing::NUMBER'

I have been referred to #3 here: http://ift.tt/1IuYStb But I was hoping somebody could explain it in simpler terms.

Why doesn't the compiler give a readable error that slaps me on my hands, or alternatively, why doesn't it treat NUMBER as a variable?

Also, why does this appear as a linker error?