If you require a shared asset that is utilized across multiple segments of the application, such as a database connection, a singleton will ensure that one instance is consistently shared with the resource.
In this article, we will elucidate the singleton design pattern, the procedure to implement singleton design patterns in C++, along with its testing and effective practices to equip you with a strong understanding of this C++ concept.
The Singleton Design Pattern guarantees that only one instance is generated throughout the application and provides a global access point to that instance. This singleton approach is employed when precisely one object is required to execute the operations across the application.
Applications of Singleton Design Pattern:
Singleton ensures a singular instance that remains globally accessible across an application.
Singleton assists in managing shared resources such as database connections, file systems, and network connections.
The Singleton approach is also utilized for overseeing the cache, ensuring only one cache is available throughout the application.
The logging function maintains a single instance across the application to regulate the log file.
Types of Singleton Design Patterns
There are four prevalent types of singleton design patterns, which are:
1. Eager Initialization of Singleton Design Pattern
The instance of a class is instantiated when the class is loaded, i.e., at the commencement of the program. This method creates the object even if it is never employed.
Example:
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In this code, the Singleton class holds a static instance (Singleton instance) that is created eagerly at program startup. The getInstance() method provides a reference to the static instance.
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verifying a solitary occurrence of the class.
2. Lazy Initialization of Singleton Design Pattern
The instance of the class is generated solely when it is required for the initial time. It is instantiated only when the getInstance() method is invoked for the first time.
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Within this code, the Singleton class features a static pointer (Singleton* instance), which will be initialized exclusively to nullptr. If the instance is nullptr (not yet created), the getInstance() function generates a new instance of the class using new. This ensures that only one instance of the class is created in a lazy manner.
3. Thread-Safe
A variant of the Lazy Initialization Singleton where instance creation is safeguarded, thereby preventing various threads from concurrently generating the instance by utilizing mutex locks.
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Nonetheless, this approach adopts a thread-safe Singleton model with a lazy initialization of the instance implemented through a std::mutex, ensuring that numerous threads cannot create multiple singletons concurrently. Afterward, we instantiate it using std::lock_guard, which locks a mutex during instance formation.
4. Static Local Variable
From C++ 11 onwards, this method guarantees thread-safety during the creation of the local static variable, while the lazy initialization strategy ensures that only one instance is created via getInstance().
Example:
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In the preceding code, the singleton class employs a static local variable instance within the getInstance() method, confirming that only one instance is instantiated, which is thread-safe from C++ 11 onwards.
Steps to Execute Singleton Pattern in C++
There are 4 steps to execute the singleton pattern in C++
Step 1. Private Constructor: Guaranteeing Single Instance Creation
The constructor should be designated as private to inhibit other parts of the program from generating an object of the singleton class. Instead of creating multiple instances, this method ensures that only the singleton class itself has the authority to create and manage its sole instance.
Note: If the private constructor were absent here, other segments of the program could easily instantiate objects using a new or default constructor, which contradicts the singleton principle.
Example:
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In this illustration, the constructor is private, meaning no external code can instantiate the class.
Step 2. Static instance: Retaining the Singleton Instance
The static instance is essential as it serves as a class variable that can hold only a single instance. It ensures that only one instance exists throughout the application’s lifecycle. The static instance enables the reuse of the same instance across different calls, thereby preserving the Singleton pattern.
Example:
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The static instance holds significance as, if the class possessed a standard instance, invoking the getInstance() method would result in the creation of a new instance each time.
Step 3. Public Static Method: Accessing the Singleton Instances
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The public static function utilizes the getInstance() to offer the global access point to the Singleton entity. This function ensures that a single instance is instantiated and distributed throughout the application.
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The getInstance() function verifies the existence of the instance. If it is nonexistent, it generates a new instance. Conversely, if it is present, it returns the current instance.
Upholding singleton integrity is the final aspect in the execution of the singleton design pattern of the singleton instance. This mechanism prevents the duplication of singleton instances, especially in multi-threaded contexts where the singleton can be unintentionally replicated. Should the singleton class be utilized in a multi-threaded setting, it is essential to ensure that the getInstance() method is thread-safe.
Illustration:
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// Function to duplicate code to clipboard
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Output:
The getInstance() method guarantees that only a single instance of the singleton class is established, while the showMessage() method outputs a message. The if statement (instance1 == instance2) verifies whether both pointers lead to the same instance.
Implementing Singleton Design Pattern With Logger Class
This design pattern assures that only one instance of the logger exists during the program's duration, which can be managed universally. Rather than generating multiple instances, the logger is designed to maintain just one instance. The logger is crucial for recording events and errors.
Example:
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document.getElementById("runBtn5195").addEventListener("click", runCode5195);
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Output:
This application employs the Singleton design pattern for a Logger class, ensuring a single instance is utilized in multi-threaded contexts. It features methods to log messages of various types (Information, Error). Three threads concurrently log messages while maintaining thread safety through a mutex.
Meyer’s Singleton Implementation
Meyer’s singleton pertains to a thread-safe implementation of the singleton design pattern in C++, introduced by Scott Meyers, a renowned C++ specialist. This method employs local static variables within a function to guarantee that the Singleton instance is generated only when required and is automatically destroyed upon program termination.
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Output:
The code above illustrates Meyer's singleton design pattern in which a static local variable ensures that only a single instance of the singleton is created when required. The constructor is private and allows class instantiation solely through external access. The 'showMessage()' function would trigger operations permitted from a single instance.
Static vs Dynamic Initialization
For the creation of singleton instances, there are primarily two methods when employing the singleton design pattern:
Static Initialization
Dynamic Initialization
1. Static Initialization of Singleton
When singletons are instantiated at the beginning and persist until the application's conclusion, this is considered Static Initialization, with their lifespan tied to the application itself. This is accomplished in C++ with the utilization of static variables.
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Output:
In the code above, a single instance of the singleton class is created when the getInstance() function is invoked. We are checking if singleton1 and singleton2 refer to the same instance, thus comparing their memory locations.
2. Dynamic Initialization of Singleton
The sole distinction in the dynamic initialization of a singleton is that an instance is instantiated upon the first use of that instance, or it can also be generated at runtime. Generally, this method employs dynamic allocation instead of static initialization and often necessitates manual clean-up or deletion if required. Typically, managing the instance via a static pointer is implemented.
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Result:
In this context, Singleton:: instance is a static pointer which is initialized statically to nullptr, while the getInstance() function comes into play during instance creation to instantiate it only when necessary. Mutex also ensures thread safety and prevents any currently executing thread from spawning additional threads. It may incorporate various std::unique_ptr for automatic memory management.
Challenges Associated with Singleton Class
Only one instance of the application can be created as a singleton, which may lead to issues with thread safety, isolation problems, or concerns regarding when an instance is initialized.
A singleton models the global state by referring to a singular object instance that is utilized throughout the application. This inherent sharing complicates the isolation of tests for that instance, potentially affecting the test outcomes.
Should your singleton class engage with outside systems (such as databases or files), the tight coupling of the singleton means those interactions cannot be substituted. This presents challenges for unit testing, as the instance is instantiated at the program's outset and cannot be replaced with mock versions.
Lastly, if the Singleton is designed for use in a multi-threaded application, its thread safety during testing complicates matters, as concurrent threads operating on the Singleton instance might lead to unintended behavior.
Best Practices for Singleton Implementation in C++
Private Constructor: Making the constructor private prevents direct instantiation.
Thread Safety: It's safeguarded by features ensuring thread safety (like std::mutex or std::call_once) to avoid race conditions.
Avoid Copying: The copy constructor and assignment operator are deleted, thus preventing copying.
Benefits of the Singleton Design Pattern
Controlled Access to a Single Instance: It allows controlled access to the singular instance.
Global Access Point: It provides a single instance of the class that can be accessed throughout the application.
Lazy Initialization: The instance gets generated only when required, potentially conserving memory and resources if the instance is never utilized.
Reduced Memory Usage: Only one instance of the class is created, avoiding unnecessary memory consumption.
Consistency: It guarantees that the state of the object remains stable across the application, as all components interact with the same instance.
Drawbacks of the Singleton Design Pattern
Global State: The singleton permits global access across the class, which may result in unpredictable behavior when multiple pieces of code alter their states.
Unit Testing: The singleton leads to tight coupling, making unit testing unfeasible.
Obscured Dependencies: The existence of a singleton globally does not correctly declare dependencies within the code, complicating maintenance.
Challenges in Extending: Modifying or extending the singleton is challenging due to the global access to its instance.
Conclusion
The Singleton pattern offers a global access point while ensuring the existence of only one instance of a specific class. It maintains the structure of the Singleton by defining a public static method, a static instance, and a private constructor. Notably, Meyer’s method is regarded as the optimal approach for implementing the Singleton pattern in C++.
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