Singleton Design Pattern in C++

The Singleton design pattern ensures a class has only one instance and provides a global point of access to it. Here’s a concise summary of a C++ Singleton implementation and its considerations.

C++98 Singleton Implementation

In C++98, implementing a Singleton involved several steps to ensure lazy evaluation, guaranteed destruction, and thread safety. However, this approach wasn’t inherently thread-safe:

class S {
public:
    static S& getInstance() {
        static S instance; // Guaranteed to be destroyed, instantiated on first use
        return instance;
    }
private:
    S() {}                    // Constructor needed

    // C++98 method to prevent copying
    S(S const&);              // Don't Implement
    void operator=(S const&); // Don't implement
};

C++11 Singleton Implementation

With C++11, Singleton implementation became cleaner and inherently thread-safe using the static variable within the function scope, which is initialized in a thread-safe manner:

class S {
public:
    static S& getInstance() {
        static S instance; // Guaranteed to be destroyed, instantiated on first use
        return instance;
    }
private:
    S() {} // Constructor needed

    // C++11 method to prevent copying
    S(S const&) = delete;
    void operator=(S const&) = delete;
};

In C++11, you can delete the copy constructor and copy assignment operator to prevent copying, ensuring a single instance.

Key Points

• Lazy Evaluation: The Singleton instance is created only when it is first accessed.
• Guaranteed Destruction: The instance is automatically destroyed when the program exits, ensuring resource cleanup.
• Thread Safety: C++11 ensures that the static local variable is initialized in a thread-safe manner.

Additional Resources

1. When to Use a Singleton: Singleton: How should it be used
2. Initialization Order Issues:
   • Static variables initialization order
   • Finding C++ static initialization order problems
3. Lifetime of Static Variables: What is the lifetime of a static variable in a C++ function?
4. Threading Implications: Singleton instance declared as static variable of GetInstance method, is it thread-safe?
5. Double-Checked Locking Issue: C++ and The Perils of Double-Checked Locking

Using the Singleton pattern sparingly and understanding its implications on multithreading and resource management are crucial for robust C++ programming.