种的静态数据成员何时初始化?

类的静态数据成员何时初始化??
class A{
public:
A(){cout << "A() "<<endl;};
};

class B{
public:
B(){cout << "B() "<<endl;};
private:
staticA a;
};

void main () 
{ 
B b;
} 

输出: B()

为什么没有调用A的构造函数啊?? 


[解决办法]
同意2楼的yydrewdrew ：

C/C++ code

#include <iostream>using namespace std;class   A{public:A(){cout<<"A()" <<endl;};};class   B{public:B(){cout<<"B()"<<endl;};private:static A  a;//here is just declaration};A B::a;//here is definition!!!so A's constructor is invoked hereint main(){    B   b;}