singleton problem

[Magcialking]

I'm writting a code using singleton idea much like this one:

//I have it simplified as to be brief
class a{
public:
static a&get(){return obj;}

void fun(){}

private:
static a obj;
}

int main(){
a::get().fun();
}

the problem occurs during the linking process I guess, because the
message returned as follow

"main.cpp:: undefined reference to `filelist::_obj' "

without line number

I don't know what's going wrong here

Could anybody help me?