How to Create a Singleton Class in C++

What is a Singleton Class?

A singleton class in C++ is a design pattern that assures a class has only a single instance and provides a universal point of access to that instance. It limits the number of objects that a class may generate since, over the lifetime of the program, only one object can be used to access that class. When you want to ensure that just one instance of a class is produced, this technique is very helpful for controlling instance creation.

Steps to Create a Singleton Class in C++

Steps to create a singleton class in C++ are as follows:

Step 1: Private Constructor:

The singleton class should have a private constructor defined.

This restricts instances from being directly created by outside programs.

Step 2: Private Static Member:

Create a class member that is private and static to hold the single instance.

This member will hold the class's single instance.

Step 3: Public Static Method (getInstance()):

Create the getInstance() public static function to grant access to the object.

This method is in charge of generating the instance and returning it.

Step 4: Initialization of Static Members:

Set the static instance member's initial value to nullptr or something comparable.

Step 5: Instance Creation:

Check to see whether the instance was initially created (nullptr) in the getInstance() function.

If not, use the private constructor to create an entirely new instance.

Step 6: Thread Safety (Optional):

If necessary, use thread-safety measures.

You can use methods like std::call_once or double-checked locking.

Step 7: Return Instance:

The instance from the getInstance() function should be returned.

Step 8: Implement the Singleton:

Use the getInstance() method to gain access to the singleton instance and use its features in your code.

Example:

Output:

Explanation:

• The Singleton pattern is implemented in this C++ code. A single instance of a class is provided by the Singleton pattern, which additionally offers a single point of access to that instance universally. The code functions as follows:
• The Singleton class is created, and it has a private static member named uniqueInstance that stores the class's single instance. The private constructor prevents external instantiation.
• The Singleton class defines the getInstance() function as a static member. Using this technique, you can get a single instance from any location in the world. If the instance already exists, the function tests it (uniqueInstance == nullptr). If not, lazy initialization is achieved by creating a new instance using the private constructor. It, however, returns the instance.
• The Singleton class's showMessage() function only outputs a message explaining that the singleton instance was successfully created.
• The uniqueInstance, which is a static member, is initialized as nullptr, and it is outside the class.
• The Singleton pattern can be seen in the main() method. Following a call to Singleton::getInstance(), two pointers, instance 1 and instance 2, are declared and given their respective values. This ensures that the instances pointed to by both pointers are the same, fulfilling the singleton condition.
• The singleton's message is printed by using instance1->showMessage() and instance2->showMessage(). The message is written just once since both pointers are pointing to the same instance.

Features and Advantages of the Singleton Class

• Single Instance: The single instance certainty provided by the singleton pattern is its main advantage. When you want to prevent instance duplication and manage the generation of a specific object, this is quite helpful.
• Global Access: The single instance is accessible from anywhere in the world, thanks to singleton. This makes it possible for other components of the program to access the instance without explicitly passing it around.
• Resource management: Singletons are useful for managing resources that the application as a whole must share. A singleton can be used, for example, to maintain a database connection, file handles, or network sockets, guaranteeing effective resource use.
• Centralized configuration: Using a singleton makes sure that all components of the application have consistent access to the same configuration data when a class is in charge of controlling configuration options or application parameters.
• Lazy Initialization: The singleton design enables lazy initialization, which means that the instance is generated only when it is requested for the first time. This is beneficial for decreasing memory utilization and enhancing speed, particularly for things that need lots of resources.
• Avoid using Global Variables: Singletons offer a regulated substitute for global variables, which can be error-prone and cause unanticipated interactions between various application components.
• Consistent State: A singleton guarantees that the instance's state is constant throughout the application since it enforces a single instance for the program's execution.
• Thread Safety (When Implemented): A singleton can allow concurrent access to the instance when it is meant to be thread-safe, preventing problems like data races and guaranteeing appropriate synchronization in a multi-threaded context.
• Memory management: Singletons, which offer a distinct point for object creation and destruction, can aid in managing memory allocation and deallocation.
• Easy to Implement: A singleton pattern may be easily implemented and does so by following a clear set of instructions. Its acceptance and appeal are made easier by its simplicity.

Disadvantages of Singleton Class in C++

• Global State: A Singleton creates a point of access for its instance on a global scale. This may result in a global state for the program, which makes it more difficult to monitor dependencies and comprehend how various pieces of code interact. Unexpected behavior may result from modifications to singleton's state affecting other areas of the program.
• Tight Coupling: When using the singleton, classes frequently depend entirely on the Singleton instance. Because of the potential for tight coupling between classes, the software may become less modular and more challenging to rework. The codebase may be significantly affected by modifications to singleton's behavior or interface.
• Testing Problems: It might be difficult to test programs that use singleton. Singleton instances make it more difficult to isolate and test specific components because several application components share them. It might be challenging to simulate or replace the Singleton instance for testing reasons, which has an impact on the unit testing procedure.
• Concurrency Problems: Singleton instances in multi-threaded situations can cause concurrency problems if there aren't enough synchronization measures in place. Data races, deadlocks, and inconsistent behavior could occur when the Singleton instance is accessed concurrently if it was not intended for thread safety.
• Hidden Dependencies: Within the class, the Singleton design hides dependencies. This may make it more challenging to identify the components that depend on the Singleton instance. Finding all the code affected when the singleton has to be replaced with a different implementation is difficult.
• Delayed Instance creation: Lazy initialization might be useful for resource-efficient memory management, but it may cause unexpected interruptions in instance formation when the getInstance() function is initially used. In some cases, this may affect how well an application performs.
• Single Responsibility Principle Violation: The Singleton class frequently performs many tasks, including managing the global state and creating new instances. The class may become less coherent and more difficult to maintain if this violates the single responsibility principle.
• Difficult to Subclass: It might be difficult to extend a Singleton class. Subclassing requires adding complicated inheritance structures or changing the getInstance() function, making the code less flexible and more difficult to maintain.
• Alternative Patterns: Depending entirely on the Singleton pattern may cause you to miss out on some of the advantages of alternative design patterns that offer comparable functionality while addressing some of the previous disadvantages.

Conclusion

In conclusion, the C++ Singleton design provides a controlled mechanism for producing a single instance of a class and ensuring its accessibility globally. Although it has many benefits, like effective resource management, centralized configuration, and worldwide access, there are also some significant disadvantages, including global state, tight coupling, and testing difficulties.

The Singleton pattern should be a design decision that complements the overall architecture and design philosophy of the software system. Understanding its strengths and limits allows developers to make better-educated decisions and produce more robust, maintainable, and adaptable codebases.