The first day of learning C++ singleton pattern

After reading the examples of three singleton patterns on Thinking in C++, I have a preliminary understanding of it.

Code first:

Example 1:

#include<iostream>
using namespace std;
class Singleton{
   static Singleton s;//Create a private static instance (object) step1
   int i;
   Singleton(int x):i(x){}
   Singleton& operator=(Singleton&);// disallowed will not be called
   Singleton(const Singleton&);//disallowed will not be called
public:
   static Singleton& instance(){return s;}//public static member function, call the only channel of s step2
   int getValue(){return i ;}
   void setValue(int x){i=x;}
};

Singleton Singleton::s(47);//Initialization, an error will be reported if not initialized
//cout<<&s<<endl;
int main()
{
   Singleton& s =Singleton::instance();
   cout<<s.getValue()<<endl;
   Singleton& s2=Singleton::instance();
   cout<<&s<<endl<<&s2<<endl;//The address is the same, it can be seen that there is only one instance of Singleton
   s2.setValue(9);
   cout<<s.getValue()<<endl;
   return 0;

}

Example 2:

#include<iostream>
#include<math.h>
using namespace std;
class Singleton
{
   int i;
   Singleton(int x):i(x){}
   void operator=(Singleton&);
   Singleton(const Singleton&);
public:
   static Singleton& instance(){
       static Singleton s(111);
       return s;
   }//Create a static instance in a static member function. Because it is static, it is already created at compile time, and multiple functions will not be created because of multiple calls. object
   int getValue(){return i;}
   void setValue(int x){i=x;}
};
int main()
{
   Singleton& s=Singleton::instance();
   cout<<s.getValue()<<endl ;
   Singleton& s2=Singleton::instance();
   s2.setValue(9);
   cout<<s.getValue()<<endl;

}

Example 3:

#include<iostream>
using namespace std;
template<class T>
class Singleton{
   Singleton(const Singleton&);
   Singleton& operator=(const Singleton&);
   protected:
       Singleton(){}
       virtual~ Singleton(){}//虚析构函数，防止内存泄漏
   public:
       static T& instance(){
           static T theInstance;
           return theInstance;
       }
};
class MyClass:public Singleton<MyClass>{
   int x;
protected:
   friend class Singleton<MyClass>;
   MyClass(){x=0;}
public:
   void setValue(int n){x=n;}
   int getValue()const{return x;}
};
int main()
{
   MyClass& m=MyClass::instance();
   cout<<m.getValue()<<endl;
   m.setValue(1);
   cout<<m.getValue()<<endl;
}//相当于单件模板

https://blog.csdn.net/windboyzsj/article/details/2790485

//About the C++ singleton mode to release the object

https://www.cnblogs.com/cxjchen/p/3148582.html

//Singleton mode multithreading

http://www.cnblogs.com/qicosmos/p/3145019.html

//Singleton mode template plus advanced version

The first day of learning C++ singleton pattern

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