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Posted 4 Mar 2002


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Singleton Pattern & its implementation with C++

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4 Mar 20022 min read
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++.


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.

Image 1

C++ Implementation

Creational Pattern: SINGLETON
Author: Rajesh V.S
Language: C++

#include <iostream>

using namespace std;

class Singleton
    static bool instanceFlag;
    static Singleton *single;
        //private constructor
    static Singleton* getInstance();
    void method();
        instanceFlag = false;

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

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

int main()
    Singleton *sc1,*sc2;
    sc1 = Singleton::getInstance();
    sc2 = Singleton::getInstance();

    return 0;


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Comments and Discussions

QuestionIs this implementation correct? Pin
Member 128842562-Dec-16 20:30
MemberMember 128842562-Dec-16 20:30 
QuestionSingleton class Pin
ranjithkumar8118-Oct-13 20:31
Memberranjithkumar8118-Oct-13 20:31 
GeneralMy vote of 1 Pin
NSProgrammer4-Sep-13 18:27
MemberNSProgrammer4-Sep-13 18:27 
QuestionWhat if I init this way Pin
Balkrishna Talele19-Feb-13 22:19
MemberBalkrishna Talele19-Feb-13 22:19 
AnswerRe: What if I init this way Pin
Abdallah Al-Dalleh31-Mar-14 23:04
MemberAbdallah Al-Dalleh31-Mar-14 23:04 
AnswerRe: What if I init this way Pin
Abhijeet Pathak3-Mar-16 0:11
MemberAbhijeet Pathak3-Mar-16 0:11 
GeneralRegarding Singleton class object creation Pin
Member 884300724-Jul-12 2:18
MemberMember 884300724-Jul-12 2:18 
QuestionCould it be more easy? Pin
DmitrijNik18-Feb-12 3:06
MemberDmitrijNik18-Feb-12 3:06 
QuestionRe: memory leak Pin
Michael B Pliam14-Dec-11 8:52
MemberMichael B Pliam14-Dec-11 8:52 
GeneralMy vote of 3 Pin
Konobey19-Jul-11 12:36
MemberKonobey19-Jul-11 12:36 
GeneralMy vote of 5 Pin
aurerilo14-Jul-11 4:33
Memberaurerilo14-Jul-11 4:33 
GeneralMy vote of 1 Pin
Giri Ganji16-Sep-10 20:13
MemberGiri Ganji16-Sep-10 20:13 
GeneralCould be better... Pin
Bogdan Mustiata9-Jun-06 7:50
MemberBogdan Mustiata9-Jun-06 7:50 
Questionwhat if ... Pin
Tom Utal2-Sep-04 3:15
MemberTom Utal2-Sep-04 3:15 
AnswerRe: what if ... Pin
vish11112-Aug-08 22:51
Membervish11112-Aug-08 22:51 
Questionsimpler code? Pin
Alex Chirokov14-Jul-04 2:16
MemberAlex Chirokov14-Jul-04 2:16 
AnswerRe: simpler code? Pin
RNEELY29-Sep-04 4:10
MemberRNEELY29-Sep-04 4:10 
GeneralRe: simpler code? Pin
Alex Chirokov29-Sep-04 8:10
MemberAlex Chirokov29-Sep-04 8:10 
GeneralRe: simpler code? Pin
vish11112-Aug-08 22:53
Membervish11112-Aug-08 22:53 
GeneralRe: simpler code? Pin
vasu_sri1-Apr-12 23:50
Membervasu_sri1-Apr-12 23:50 
QuestionIs it really a singleton? Pin
ThumbUp25-Apr-04 23:22
MemberThumbUp25-Apr-04 23:22 
AnswerRe: Is it really a singleton? Pin
Bogdan Mustiata9-Jun-06 22:04
MemberBogdan Mustiata9-Jun-06 22:04 
AnswerRe: Is it really a singleton? Pin
vish11112-Aug-08 22:55
Membervish11112-Aug-08 22:55 
AnswerRe: Is it really a singleton? Pin
Member 1211239726-Apr-16 22:03
MemberMember 1211239726-Apr-16 22:03 
GeneralAlternative Thread Safe Singleton Pin
Paul Evans8-Dec-02 1:34
MemberPaul Evans8-Dec-02 1:34 

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