问个C＋＋类单实例的问题

#include <iostream>
using namespace std;
class Temp {
        public:
                static Temp * getInstance();
                static void putInstance();
        private:
                static Temp *instance;
};
Temp * Temp::getInstance()
{
        if (!instance)
                instance = new Temp();
        return instance;
}
void Temp::putInstance()
{
        if (instance) {
                delete instance;
                instance = NULL;
        }  
}
int main()
{
        Temp *p;
        p = Temp::getInstance();
        return 0;
}

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上面的程序报错：
/tmp/ccpL8OB4.o: In function `Temp::putInstance()':
hello.cpp:(.text+0x79): undefined reference to `Temp::instance'
hello.cpp:(.text+0x82): undefined reference to `Temp::instance'
hello.cpp:(.text+0x90): undefined reference to `Temp::instance'
/tmp/ccpL8OB4.o: In function `Temp::getInstance()':
hello.cpp:(.text+0xa1): undefined reference to `Temp::instance'
hello.cpp:(.text+0xb6): undefined reference to `Temp::instance'
/tmp/ccpL8OB4.o:hello.cpp:(.text+0xbb): more undefined references to `Temp::instance' follow
collect2: ld returned 1 exit status

请问是什么原因啊，我不知道哪里错了，请帮助