Converting any class into a Singleton class in C++

Singleton is one of the creational design patterns. A singleton class has only one instance and provides a global point of access to that instance. It's commonly used when you want to restrict the instantiation of a class to a single object, which can be useful for scenarios where you need a single point of control or coordination.

Following are some of the prominent features of Singleton class:

• Private constructor: The key idea behind the Singleton pattern is to make the class's constructor private, preventing external code from directly creating instances of the class.

• Static instance: Inside the class, you have a static object that holds the single instance of the class. This instance is usually created the first time it's requested and then reused.

• Static method: You provide a static method that acts as the global access point to the single instance. This method checks if an instance already exists and either returns the existing one or creates a new instance if needed.

• Thread safety: In multithreaded environments, it's important to ensure that the creation of the instance is thread-safe.

• Global access: The Singleton instance can be accessed globally, allowing different parts of your codebase to interact with the same instance of the class.

In this article, I am introducing an approach using which we can convert any class into a Singleton class with minimum effort. To achieve this we are going to create what is called a mixin. Without further ado let's create the SingletonMixin.

• SingletonMixin is going to be a template class

• The default constructor is protected to prevent direct instantiation

• The destructor is virtual and protected so that it can be overridden in a derived class

• The copy constructor and copy assignment constructor are also disabled to ensure a single instance is present at any point in time

• The core logic of the singleton instance creation is implemented inside the getInstance method
  

#include <iostream>

using namespace std;

template <typename T>
class SingletonMixin {
public:
    static T& getInstance() {
        static T instance;  // Guaranteed to be created and destroyed once in a thread-safe way
        return instance;
    }

    SingletonMixin(const SingletonMixin&) = delete;             // Disable copy constructor
    SingletonMixin& operator=(const SingletonMixin&) = delete;  // Disable copy assignment constructor

protected:
    SingletonMixin() {
        // Protected constructor to prevent direct instantiation
        cout << "SingletonMixin constructor called: " << this << endl;
    }
    virtual ~SingletonMixin() {
        cout << "SingletonMixin destructor called: " << this << endl;
    }
};

Now let's take for example the following code. It represents a simple class called MyClass which has a default constructor and a single public method implemented. Whereas in real-world use cases, we can have many more methods.

class MyClass {
public:
    void showMessage() {
        cout << "Hello from MyClass!" << endl;
    }
    MyClass() {
        cout << "MyClass constructor called: " << this << endl;
    };
};

We can convert MyClass into a singleton class in the following way:

• First, we inherit from SingletonMixin<MyClass>

• Make the default constructor private and SingletonMixin<MyClass> a friend class of MyClass, so that MyClass can only be instantiated from the SingletonMixin<MyClass> class.
  

class MyClass : public SingletonMixin<MyClass> {
public:
    void showMessage() {
        cout << "Hello from MyClass!" << endl;
    }

private:
    MyClass() {
        cout << "MyClass constructor called: " << this << endl;
    };
    friend class SingletonMixin<MyClass>;
};

To create/access the singleton instance of MyClass we can call the getInstance method on MyClass class. We can call it as many times as we want, we are going to receive the same instance of the MyClass class. We can verify it printing out the address of the instance.

#include <thread>

void thread_function(const char * s) {
    this_thread::sleep_for(chrono::milliseconds(1000));
    MyClass& singleton = MyClass::getInstance();
    cout << s << ": " << &singleton << endl << flush;
}

int main() {
    thread thread_1(thread_function, "thread 1");
    thread thread_2(thread_function, "thread 2");

    MyClass& singleton1 = MyClass::getInstance();
    MyClass& singleton2 = MyClass::getInstance();
    MyClass& singleton3 = MyClass::getInstance();

    cout << &singleton1 << ", " << &singleton2 << ", " << &singleton3 << endl;

    thread_1.join();
    thread_2.join();

    return 0;
}

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Since this approach uses the default constructor to create the singleton instance, one could use getter and setter methods to access (read/write) any member variables that may become part of MyClass.

Link to the code

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