Getting ready

We will use the same class created in the last recipe of factory method. We will make few changes to the class so that we can prevent the creation of multiple instances of the class. We will make a copy of the class (program) shown in the previous recipe and modify it. The name of the copy is singleton_class.

How to do it...

For creating a singleton class, follow these steps:

How it works...

Similar to the previous recipe, we try to instantiate two objects emp1 and emp2, having number 0000012 and 00000014 respectively. However, in our singleton class, we have added an attribute number_of_instances, which keeps track of the number of class instances that already exist. Upon creation of the first object, the factory method increments this static attribute to 1. On the second object creation attempt, the IF statement does not allow the CREATE OBJECT statement to be called a second time. The result is that the second object is not created. No further attempts of object creation will be allowed. Rather, a message saying that only one object instantiation is allowed is outputted for the second object creation attempt.

See also