c++ 单例模式 private 成员初始化

#include <iostream>
using namespace std;
 
class Singleton
{
private:
 
    static Singleton* st;
    //static Singleton* st = NULL; //错误
 
    Singleton(){}
public:
 
    static Singleton* getInstance()
    {
        if (st == NULL)
        {
            st = new Singleton();
        }
 
        return st;
    }
 
    void show()
    {
        cout << st << endl;
    }
 
};
 
Singleton* Singleton::st = NULL; //正确，只能在类外初始化,如若不在此初始化会报连接错误
 
int main()
{
    //Singleton* Singleton::st = NULL; //错误
 
    Singleton* st = Singleton::getInstance();
    Singleton* st1 = Singleton::getInstance();
 
    if (st == st1)
    {
        cout << "两个对象是相同的实例。" << endl;
    }
 
    return 0;
}