Is a C++ Singleton Class that Simple?

[gzeng]

I saw a piece of code from a website. It seems to be a simple example for a singleton class. Basically, the author creates an object in the definition of a class, which has the same name as the class. Inside the definition of the class, everything is public. One cannot instantiate another object. Is it that simple to create a singleton class? Is the all-public members in the class good enough in terms of encapsulation?

In the following is the code run successfully:

#include "stdafx.h"
#include<stdio.h>
#include <iostream>
using namespace std;

class singleton
{
public:
void print()
{
printf("hello\n");
}
int a;
}singleton;

class client : public singleton
{};

int main()
{
// singleton s; // Can define an object before singleton
// singleton singleton;
// singleton b; #cannot define a new object after singleton
singleton.a=5;
printf("a=%d\n",singleton.a);
singleton.print();

client xyz;
xyz.print();
xyz.a = 10;
cout << "xyz.a = " << xyz.a << endl;

return 0;
}

[gpraghuram]

I saw a piece of code from a website. It seems to be a simple example for a singleton class. Basically, the author creates an object in the definition of a class, which has the same name as the class. Inside the definition of the class, everything is public. One cannot instantiate another object. Is it that simple to create a singleton class? Is the all-public members in the class good enough in terms of encapsulation?

In the following is the code run successfully:

#include "stdafx.h"
#include<stdio.h>
#include <iostream>
using namespace std;

class singleton
{
public:
void print()
{
printf("hello\n");
}
int a;
}singleton;

class client : public singleton
{};

int main()
{
// singleton s; // Can define an object before singleton
// singleton singleton;
// singleton b; #cannot define a new object after singleton
singleton.a=5;
printf("a=%d\n",singleton.a);
singleton.print();

client xyz;
xyz.print();
xyz.a = 10;
cout << "xyz.a = " << xyz.a << endl;

return 0;
}

Hi,
This is not the right way to create a singleton class and in the example the class name is singleton and not the implementaion is.
Search in thie forum fo singleton class in c++.
Raghuram

[plemoine]

The singleton is a pattern where you want to make sure that no other instance can be created (you can look at "Effective C++" and "More Effective C++" from Scott Meyers).

Ex.1

class MySingleton {
  public:
    MySingleton() {
        if (s_pInstance != NULL) {
            //    run-time error
        }
    }
 
    ~MySingleton() {
        if (s_pInstance != NULL) {
            delete s_pInstance;
            s_pInstance = NULL;
        }
    }
 
  private:
    // prevents that an object of the class get copied
    //    ex: MySingleton s2(s1);
    //  ex: s2 = s1;
    MySingleton(const MySingleton&);
    MySingleton& operator=(const MySingleton&);
 
    static MySingleton*        s_pInstance;
};
 
// in C++ file
MySingleton*        MySingleton::s_pInstance = NULL;
 
 

In this example, it does a run-time check on the existence of a MySingleton instance.

class MySingleton2 {
  public:
    static MySingleton2& GetInstance();
 
  private:
    MySingleton2() {
        // ...
    }
 
    ~MySingleton2() {
        // ...
    }
};
 
// in C++ file
MySingleton2& MySingleton2::GetInstance()
{
    static MySingleton2 theInstance;
    return theInstance;
}
 
 

In this example, the CTOR and DTOR are private, and the unique instance will be created on demand only, whenever GetInstance() gets called, so there is no error upon rt check.

I saw a piece of code from a website. It seems to be a simple example for a singleton class. Basically, the author creates an object in the definition of a class, which has the same name as the class. Inside the definition of the class, everything is public. One cannot instantiate another object. Is it that simple to create a singleton class? Is the all-public members in the class good enough in terms of encapsulation?

In the following is the code run successfully:

#include "stdafx.h"
#include<stdio.h>
#include <iostream>
using namespace std;

class singleton
{
public:
void print()
{
printf("hello\n");
}
int a;
}singleton;

class client : public singleton
{};

int main()
{
// singleton s; // Can define an object before singleton
// singleton singleton;
// singleton b; #cannot define a new object after singleton
singleton.a=5;
printf("a=%d\n",singleton.a);
singleton.print();

client xyz;
xyz.print();
xyz.a = 10;
cout << "xyz.a = " << xyz.a << endl;

return 0;
}

[sicarie]

There is another example in the C/C++ Articles section.

http://www.thescripts.com/forum/thread656124.html