How can you limit the number of class instances in…

There are several ways to limit the number of class instances:

Singleton: Ensures only one instance of the class is created.

class Singleton {
public:
    static Singleton& getInstance() {
        static Singleton instance; // Guaranteed creation of a single instance upon first call
        return instance;
    }

    // Disable copying and assignment
    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;

private:
    Singleton() {} // Private constructor

    // Additional class members
};

Instance counter: Maintain a static counter within the class.

class LimitedInstances {
public:
    LimitedInstances() {
        if (instanceCount_ >= MAX_INSTANCES) {
            throw std::runtime_error("Maximum number of instances reached.");
        }
        instanceCount_++;
    }

    ~LimitedInstances() {
        instanceCount_--;
    }

    // Disable copying and assignment
    LimitedInstances(const LimitedInstances&) = delete;
    LimitedInstances& operator=(const LimitedInstances&) = delete;

private:
    static const int MAX_INSTANCES = 3; // Maximum number of instances
    static int instanceCount_;
};

int LimitedInstances::instanceCount_ = 0; // Static variable initialization

Factory (Factory Method): Manage instance creation through a static method.

class LimitedInstancesFactory {
public:
    static LimitedInstancesFactory* createInstance() {
        if (instanceCount_ >= MAX_INSTANCES) {
            return nullptr; // Or throw an exception
        }
        instanceCount_++;
        return new LimitedInstancesFactory(); // Create an instance
    }

    ~LimitedInstancesFactory() {
        instanceCount_--;
    }

    // Disable copying and assignment
    LimitedInstancesFactory(const LimitedInstancesFactory&) = delete;
    LimitedInstancesFactory& operator=(const LimitedInstancesFactory&) = delete;

private:
    LimitedInstancesFactory() {} // Private constructor

    static const int MAX_INSTANCES = 2; // Maximum number of instances
    static int instanceCount_;
};

int LimitedInstancesFactory::instanceCount_ = 0; // Static variable initialization

Each approach has its features and is applicable depending on the required scenario. Singleton for a single instance, counter and factory for a specified number.

How can you limit the number of class instances in C++?