How to Implement the Singleton Design Pattern in C ?

Singleton in C

In C , there are several ways to implement the Singleton design pattern, a technique that ensures a class has only a single instance. Following are two common approaches to creating a Singleton class:

The Classic Singleton with a Pointer Return

In this method, the Singleton class returns a pointer to its instance:

<code class="cpp">class A {
    private:
        static A* m_pA;
        A();
        virtual ~A();

    public:
        static A* GetInstance();
        static void FreeInstance();
        
        void WORK1();
        void WORK2();
        void WORK3();
};</code>

However, this approach has a potential issue if the FreeInstance() method is called while the object still has references. To address this, avoid manually freeing Singleton objects.

Singleton with a Reference Return

Returning the Singleton as a reference is another approach that offers better safety:

<code class="cpp">class A {
    private:
        static A* m_pA;
        explicit A();
        void A(const A& a);
        void A(A &a);
        const A& operator=(const A& a);
        virtual ~A();

    public:
        static A* GetInstance();
        static void FreeInstance();
        
        void WORK1();
        void WORK2();
        void WORK3();
};</code>

This implementation ensures that you can't accidentally destroy the Singleton object.

Additional Considerations

• Make the constructor private to prevent direct object creation.
• Override the default copy constructor and assignment operator to prevent copying.
• Use a static method to access the Singleton instance, guaranteeing its creation and destruction only once.

For further reading and best practices on Singleton design, refer to the suggested resources:

• [Singleton: How Should It Be Used](https://refactoring.guru/design-patterns/singleton)
• [C Singleton Design Pattern](https://www.learncpp.com/cpp-tutorial/singleton-design-pattern/)

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