Diagram explaining the Singleton Pattern in C++

The Singleton pattern is a creational design pattern that ensures a class has only one instance and provides a global point of access to that instance.

Key Components:

1. Singleton Class: This is the class that you want to make a singleton. It has a private constructor, private destructor, and a private static member variable to hold the single instance of the class.
2. Static Member Variable: This is a private static member variable within the Singleton class that holds the single instance of the class. It is usually initialized to nullptr or an instance of the class itself.
3. Static Method (getInstance): A public static method within the Singleton class called getInstance. This method is responsible for creating the instance if it doesn’t exist or returning the existing instance if it does. It ensures that there’s only one instance of the class.
4. Delete Copy Constructor and Assignment Operator: To prevent copying of the Singleton instance, the copy constructor and assignment operator should be deleted within the Singleton class.

Above diagram of singleton pattern works as follows:

1. Initialization: The static member variable in the Singleton class is initialized to nullptr (or an instance of the class) when the program starts.
2. First Access (Lazy Initialization): When the getInstance method is called for the first time, it checks if the static member variable is nullptr. If it is, it creates a new instance of the Singleton class using its private constructor. If it’s not nullptr, it returns the existing instance.
3. Subsequent Accesses: For all subsequent calls to getInstance, the method simply returns the existing instance created during the first access.
4. Usage: Clients access the Singleton instance by calling getInstance() and can then use the methods and data members of the Singleton as needed.

In the diagram above, Singleton is the Singleton class, instance is the static member variable holding the single instance, and getInstance() is the static method responsible for managing access to that instance.

A singleton pattern is a design pattern that ensures that only one instance of a class can exist in the entire program. This means that if you try to create another instance of the class, it will return the same instance that was created earlier.

The Singleton pattern is useful when we need to have only one instance of a class, for example, a single database connection shared by multiple objects as creating a separate database connection for every object may be costly.