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I am confused when I read this (regarding singleton design pattern):
How do we ensure that a class has only one instance and that the instance is easily accessible? A global variable makes an object accessible, but it doesn't keep you from instantiating multiple objects.
So what is the use of singleton pattern if we can create multiple instances?
SOURCE:
Design Patterns - Elements Of Reusable Object Oriented Software (1995) - Gamma, Helm, Johnson, Vl
Without the full text this is not sure, but my (somehow educated) guess:
They only warn that a global variable is not the right way to ensure that you have a singleton. The following text should then show how to do this inside the class that should be a singleton.
In the quote they don't talk about how to do it, but how not to do it.
Some approach for getting a singleton is to make the constructor private and write a own static method, that creates a new element on the first call, saves it in a static variable and always returns this object when called again.
There are very few languages that prevent you from shooting your foot if you really want to do it. The singleton pattern makes maintaining exactly one instance of the class quite trivial and creating multiple instances of it quite difficult; creating multiple instances unintentionally - impossible.
Most design patterns are to make programmer's life easier and remove their headaches. They can be circumvented and exceptions to them can be made. The singleton is to remove the headache of watching when and where you instantiate a class there should be only one instance of.
It's the kind of thinking "I'll catch cold on purpose, to spite my parents" if you try to create more than one instance of a class designated as singleton. Adult people don't do this.
In case of a singelton class. I can create multipe object by using Malloc. For example:-
Singeltone* ptr = (Singeltone*) malloc(sizeof(Singeltone));
