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Singleton Pattern & its implementation with C++

, 4 Mar 2002
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Singleton is one of the commonly used patterns in object oriented developments. In this article I am discussing abt this pattern in general and how we can implement this pattern with C++.

Introduction

Suppose we have to use a single object of a class throughout the lifetime of an application. In C++, it is possible to declare a global object, which can be used anywhere inside the program. But a good object oriented design strictly prohibits the use of global variables or methods, since they are against the fundamental principles of object orientation like data encapsulation or data hiding. More over, most latest object oriented programming languages like JAVA or C# do not support global variables or functions.

Another practical solution to get a single object is by declaring a class, which contains only static methods. A static class is loaded into memory when the execution of the program starts and it remains there till the application ends. Remember that for invoking a static method of a class, it is not necessary to create an instance of the class. But remember that a class with only static methods and variables are not a good object oriented design. A class of static methods unfortunately breaks down to a list of functions or utilities.

When we want to create only one instance of a class in a truly object oriented fashion by adhering to the basic principles of object oriented programming, the Singleton patterns are used. The Singleton Pattern comes under that classification of Creational Pattern, which deals with the best ways to create objects. The Singleton Design pattern is used, where only one instance of an object is needed throughout the lifetime of an application. The Singleton class is instantiated at the time of first access and same instance is used thereafter till the application quits.

There are very good non-software examples available in real world for Singleton patterns. The office of the Principal of my college is a Singleton. The University specifies the means by which a principal is selected, limits the term of office, and defines the order of succession. As a result, there can be at most one active principal at any given time. Regardless of the personal identity of the principal, the title, "The Principal" is a global point of access that identifies the person in the office.

The Singletons are often used to control access to resources such as database connections or sockets. Suppose we have a license for only one connection for our database. A Singleton connection object makes sure that only one connection can be made at any time.

It is pretty easy to implement the Singleton Pattern in any object oriented programming languages like C++, JAVA or C#. There are lots of different ways to implement the Singleton Pattern. But by using a private constructor and a static method to create and return an instance of the class is a popular way for implementing Singleton Pattern. The UML representation of a Singleton Pattern is shown below.

C++ Implementation

/*
Creational Pattern: SINGLETON
Author: Rajesh V.S
Language: C++
Email: rajeshvs@msn.com
*/

#include <iostream>

using namespace std;

class Singleton
{
private:
    static bool instanceFlag;
    static Singleton *single;
    Singleton()
    {
        //private constructor
    }
public:
    static Singleton* getInstance();
    void method();
    ~Singleton()
    {
        instanceFlag = false;
    }
};

bool Singleton::instanceFlag = false;
Singleton* Singleton::single = NULL;
Singleton* Singleton::getInstance()
{
    if(! instanceFlag)
    {
        single = new Singleton();
        instanceFlag = true;
        return single;
    }
    else
    {
        return single;
    }
}

void Singleton::method()
{
    cout << "Method of the singleton class" << endl;
}

int main()
{
    Singleton *sc1,*sc2;
    sc1 = Singleton::getInstance();
    sc1->method();
    sc2 = Singleton::getInstance();
    sc2->method();

    return 0;
}

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vsrajeshvs

United States United States
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Comments and Discussions

 
QuestionSingleton Pattern Example PinmemberDeveloper Hemu Patel8-Aug-14 5:13 
QuestionSingleton class Pinmemberranjithkumar8118-Oct-13 21:31 
GeneralMy vote of 1 PinmemberNSProgrammer4-Sep-13 19:27 
QuestionWhat if I init this way PinmemberBalkrishna Talele19-Feb-13 23:19 
AnswerRe: What if I init this way PinmemberAbdallah Al-Dalleh1-Apr-14 0:04 
GeneralRegarding Singleton class object creation PinmemberMember 884300724-Jul-12 3:18 
QuestionCould it be more easy? PinmemberDmitrijNik18-Feb-12 4:06 
QuestionRe: memory leak PinmemberMichael B Pliam14-Dec-11 9:52 
I think it's pretty good. Certainly simple and straightforward.
 
Because the class recognizes whether or not a class pointer is NULL or not, you cannot delete the class pointer after each use because when you call the class getInstance again, a new instance will be instantiated and the class will no longer function as a Singleton. So simply delete the class pointer once, after you're done using it.
 
Singleton *sc1,*sc2, *sc3, *sc4, *sc5;
sc1 = NULL; sc2 = NULL; sc3 = NULL; sc4 = NULL; sc5 = NULL;
sc1 = Singleton::getInstance(); sc1->method(); //delete sc1; sc1 = NULL;
sc2 = Singleton::getInstance(); sc2->method(); //delete sc2; sc2 = NULL;
sc3 = Singleton::getInstance(); sc3->method(); //delete sc3; sc3 = NULL;
sc4 = Singleton::getInstance(); sc4->method(); //delete sc4; sc4 = NULL;
sc5 = Singleton::getInstance(); sc5->method(); delete sc5; sc5 = NULL;
 
No leak!! Only one instance constructed.
 
Not sure about how to make it thread safe. Any ideas how to add a critical section ? Smile | :)
GeneralMy vote of 3 PinmemberKonobey19-Jul-11 13:36 
GeneralMy vote of 5 Pinmemberaurerilo14-Jul-11 5:33 
GeneralMy vote of 1 PinmemberGiri Ganji16-Sep-10 21:13 
GeneralCould be better... PinmemberBogdan Mustiata9-Jun-06 8:50 
Questionwhat if ... PinmemberTom Utal2-Sep-04 4:15 
AnswerRe: what if ... Pinmembervish11112-Aug-08 23:51 
Questionsimpler code? PinmemberAlex Chirokov14-Jul-04 3:16 
AnswerRe: simpler code? PinmemberRNEELY29-Sep-04 5:10 
GeneralRe: simpler code? PinmemberAlex Chirokov29-Sep-04 9:10 
GeneralRe: simpler code? Pinmembervish11112-Aug-08 23:53 
GeneralRe: simpler code? Pinmembervasu_sri2-Apr-12 0:50 
QuestionIs it really a singleton? PinmemberThumbUp26-Apr-04 0:22 
AnswerRe: Is it really a singleton? PinmemberBogdan Mustiata9-Jun-06 23:04 
AnswerRe: Is it really a singleton? Pinmembervish11112-Aug-08 23:55 
GeneralAlternative Thread Safe Singleton PinmemberPaul Evans8-Dec-02 2:34 
QuestionMemory leak ? PinsussGizMoCuz9-Jul-02 12:31 
AnswerRe: Memory leak ? Pinmemberjan larsen25-Jul-02 3:51 

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