一个简单的单例模式编译不过去,该怎么处理

一个简单的单例模式编译不过去
我经常看到别人这么写个简单的单例模式,然后我就编译了一下,编译不过啊!
WINDOWS XP + VS2010

struct A
{
int i;
};

class Singleton
{
public:
static A* GetSingletion()
{
if (m_ptr == NULL)
{
m_ptr = new A;
}
return m_ptr;
}
private:
static A *m_ptr;
};

int _tmain(int argc, _TCHAR* argv[])
{
A *pa = Singleton::GetSingletion();
[color=#FF0000][/color]
return 0;
}

错误:
1>singleton.obj : error LNK2001: 无法解析的外部符号 "private: static struct A * Singleton::m_ptr" (?m_ptr@Singleton@@0PAUA@@A)
1>C:\Documents and Settings\BOY_SO_KU\My Documents\Visual Studio 2010\Projects\singleton\Debug\singleton.exe : fatal error LNK1120: 1 个无法解析的外部命令

难道是我哪里出错了

[解决办法]

C/C++ code

class Singleton{public:        static A* GetSingletion()    {        if (m_ptr == NULL)        {            m_ptr = new A;        }        return m_ptr;    }private:    static A *m_ptr;};A* Singleton::m_ptr = NULL;int _tmain(int argc, _TCHAR* argv[]){    A *pa = Singleton::GetSingletion();    return 0;}
[解决办法]
C/C++ code
class Singleton{private:    Singleton()    {        a = 0;    }public:    static Singleton& GetInstance()    {        static Singleton ins;        return ins;    }    int a;}int _tmain(int argc, _TCHAR* argv[]){    Singleton::GetInstance().a++;    return 0;}