Private constructor in Singleton pattern

Question

I am just trying to learn design patterns. So I analyse source code but cannot figure out why private constructor is used in singleton pattern. Can anyone helps me to understand the reason?

Answer 1

Suppose you have a singleton class defined like this (the code is on C# but it does not matter):

public sealed class Singleton
{
    private static Singleton instance = null;
    private static readonly object padlock = new object();

    public Singleton() //with a public constructor
    {
    }

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

So you can have two instances of you class:

var instance1 = Singleton.Instance;
var instance2 = new Singleton();

But the pattern itself is done to avoid multiple copies.

http://csharpindepth.com/articles/general/singleton.aspx

Answer 2

Constructors with private visibility may only be invoked from within the class itself - self-only construction is one of the attributes of a singleton class.