未定义对班级成员的引用

Question

class Singleobject
{
    private:
        static Singleobject* instance;
        Singleobject() {
            instance = 0;
        }
    public:
        static Singleobject* get_instance()
        {
            if (!instance)
                instance = new Singleobject;
            return instance;
        }

        void showMessage()
        {
            cout << "Inside Singleobject::showMessage()\n";
        }
};

//Singleobject *Singleobject::instance = 0;

int main()
{
    Singleobject *obj = Singleobject::get_instance();
    (*obj).showMessage();
}

在注释Singleobject *Singleobject::instance = 0; 并编译代码时，给出上面的代码，它将返回错误，如下所示：

/tmp/ccWE4wn4.o: In function `Singleobject::Singleobject()':singletondemo.cpp:(.text._ZN12SingleobjectC2Ev[_ZN12SingleobjectC5Ev]+0xb): undefined reference to `Singleobject::instance'
/tmp/ccWE4wn4.o: In function `Singleobject::get_instance()':
singletondemo.cpp:(.text._ZN12Singleobject12get_instanceEv[_ZN12Singleobject12get_instanceEv]+0xc): undefined reference to `Singleobject::instance'
singletondemo.cpp:(.text._ZN12Singleobject12get_instanceEv[_ZN12Singleobject12get_instanceEv]+0x2d): undefined reference to `Singleobject::instance'
singletondemo.cpp(.text._ZN12Singleobject12get_instanceEv[_ZN12Singleobject12get_instanceEv]+0x34): undefined reference to `Singleobject::instance'
collect2: error: ld returned 1 exit status

但是当我删除该注释时，代码将成功编译。我相信我已经在私有构造函数本身中有效地完成了Singleobject *Singleobject::instance = 0;的操作，因此该注释代码是否保留都无关紧要。谁能解释为什么我会得到这种行为？