what is the need of private constructor in singleton design pattern?

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when i go through the below code, i couldnt find the reason why it using private constructor in the sample?

public sealed class Singleton
    {
        private static Singleton instance = null;
        private Singleton()
        {
        }

        public static Singleton Instance
        {
            get
            {
                if (instance == null)
                {
                    instance = new Singleton();
                }

                return instance;
            }
        }
    } 

...

  //Why shouldnt I use something like below.
  public class Singleton
  {
       private static Singleton instance = null;            

       static Singleton()
       {
       }

       public static Singleton Instance
       {
            get
            {
                if (instance == null)
                {
                     instance = new Singleton();
                }

                return instance;
            }
        }
    } 

instead of public class if i created a static class, i can use the class directly rather than creating instance. what is the need of creating a private constructor here, when the static keyword persists to the same job?

any other advantage for following this pattern?

Since Singleton can have one instance only, you have to prevent second instance creation. If you skip constructor declaration, e.g.

  public class clsSingleTon {
  }

one can call a default constructor:

  var one = new clsSingleTon();
  var two = new clsSingleTon(); // <- that's we try to ban

if you declare constructor being public, one can call it, so the only option is private one:

  public class clsSingleTon {
     public static int intcounter;

     // No-one will call it
     private clsSingleTon() {
     } 
     ...
  }

however, it seems that you don't want any instances at all, so drop the constructor and declare the class as static:

  // Static class - no instances are allowed 
  public static class clsSingleTon {
    //TODO: turn it into property
    public static int intcounter;

    public static void Hit() {
    }

    //TODO: turn it into property, do not mimic Java
    public static int getTotalHits() {
      return intCouner;
    }
  }

  ...

  clsSingleTon.Hit();
  MessageBox.Show(clsSingleTon.getTotalHits().ToString());

Private Constructors, are often useful where we have to control the resources, such as database connections or sockets. Singleton classes are employed extensively in concepts like Networking and Database Connectivity. Design Pattern of Singleton classes: The constructor of singleton class would be private so there must be another way to get the instance of that class. This problem is resolved using a class member instance and a factory method to return the class member.

A singleton class and a static class are different things, and it seems you're mixing that up. A static class has only static methods and static members and therefore cannot have a constructor. Static methods are called on the type, not the instance.

A singleton class, in contrast, has normal methods and is called using an instance. The private constructor is there to prevent creating multiple instances of the class and usually used by a private property that returns this only instance.

public class Singleton
{ 
    static Singleton s_myInstance = null;
    private Singleton()
    {
    }

    // Very simplistic implementation (not thread safe, not disposable, etc)
    public Singleton Instance 
    {
        get 
        { 
             if (s_myInstance == null) 
                   s_myInstance = new Singleton();
             return s_myInstance;
        }
     }
     // More ordinary members here. 
}

The advantage of singletons is that they can implement interfaces. Also, they should be preferred over static classes if they are stateful (have many members), as having many static fields in a static class is quite ugly design.

Digesting Singleton Design Pattern in Java - Keval Patel, , we prevent the class from being instantiated as well as subclassing of that class. Singleton pattern implies that you have one active instance of a class all the time. To achieve singleton pattern you can use private constructor. Private constructor prevents inheritance i.e. class cannot be inherited.

Singelton provides you the facility to implement the interfaces

Singelton pattren has wide usage specialy where you have to limit your application to create only one instance just like in many games you have only one instance

Can a constructor in Java be private?, of this class, the other class has to use the static methods. In Singleton design pattern, we restricts the instantiation of a class and ensures that only one instance of the class exists in the java virtual machine. To make a class singleton in Java, below things need to be taken care: Define all constructors to be protected or private. Define one private static attribute in the "single instance" class.

Original pattern was devised before C# and .net, see http://www.blackwasp.co.uk/gofpatterns.aspx

Canonical implementation was in c++, which doesn't have static classes, therefore you had to use private constructor trick in order to control class instantiation

How can I access a private constructor of a class?, Do we need such 'private constructors ' ? Internal Constructor chaining; Singleton class design pattern Below is an example in java illustrating the same:. A single constructor, which is private and parameterless. This prevents other classes from instantiating it (which would be a violation of the pattern). subclassed twice, and if each of those subclasses can create an instance, the pattern is violated. The factory pattern can be used if you need a single instance of a base type,

There is a private constructor so it can be called only within the class. Then you will have to define a static method which will 'lock' your class and instantiate it.

Private Constructors and Singleton Classes in Java, Otherwise everyone can create an instance of your class, so it's not a singleton any more. For a singleton by definition there can exist only one  Having the constructor private is key to the design pattern and how singletons operate. We will discover in future steps how a singleton is instantiated and accessed. The class should now look like this after moving the constructor to private (Look at associated photo) Ask Question.

Why is it mandatory to have private Constructor inside a Singleton , The designing of a singleton class is better explained by the following example. Example. Live Demo public class SingletonClassDriver { public  Singleton pattern is one of the simplest design patterns in Java. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object. This pattern involves a single class which is responsible to create an object while making sure that only single object gets created.

Private Constructors and Singleton Classes in Java Programming, We have to take help of nested class (Inner Class) or static method to Private constructor:- 1.it is use in singleton design pattern in which  There are various ways to implement the Singleton Pattern in C#. The following are the common characteristics of a Singleton Pattern. A single constructor, that is private and parameterless. The class is sealed. A static variable that holds a reference to the single created instance, if any.

Why private constructor is used ?, Singleton pattern implies that you have one active instance of a class all the time. To achieve singleton pattern you can use private constructor. My doubt in Singleton design pattern is if it's enough to just make the constructor private. I think there is no need to make the copy constructor and assignment operator private. Since we can not create the object, (object will be created by a static function and will be allocated in static pointer) then the copy constructor or assignment operator will not be invoked.

Comments
  • michaelsafyan.com/tech/design/patterns/singleton
  • This isn't a singleton, there are no instance methods. The private constructor simply prevents client code creating an instance (which isn't needed for anything). The static modifier for classes was only introduced in C# 2 - perhaps the code predates this or the author was unaware of it.
  • I have updated the code.
  • do not use static constructors i.e. static Singleton() unless you have to.
  • sorry..i am not clear what you are trying to convey.. My question was what is wrong in creating a static class instead of private constructor. exactly what you did in last snippet.
  • @Ammu: in your current implementation (taken from Java?) you don't have a static class (Java don't allow static classes) and thus the only option to prevent instance creation is to hide the constructor. In C# static class is preferable implementation (no boiler plate code); another issue is combining intcounter and getTotalHits() into property
  • @ Dmitry Bychenko what is the need of combining intcounter and getTotalHits? why cant i use the same way that you mentioned in your answer?
  • @Ammu: sure you can, but property makes your code more readable (in C# we expect method to be an active doer, say it calls database and return TotalHits while property accessor is hardly more than field reader) more safier - in your current design one can put, say clsSingleTon.intcounter = -12345; do you want to allow this? Less perplexing: what's the difference between clsSingleTon.intcounter; and clsSingleTon.getTotalHits();?
  • Can you please check the updated code in the question. I have taken this from a website.
  • Please, mark out that your implementation is the simplest one and thus not thread safe, or provide a standard thread safe version.
  • Ok. The question was so basic that I thought to prefer the really basic implementation.
  • why cant i use a static constructor in the code you gave as an example? that too will invoke only once.
  • That's also possible, however not usually done, as it could result in the instance being created even if it is never used.