C++ 的设计模式之 工厂方法加单例

在下面的示例中，我将演示如何创建一个工厂类，该工厂类能够生成四个不同类型的单例对象，每个单例对象都通过单独的工厂方法进行创建。

#include 
#include 

// Singleton base class
class Singleton {
protected:
    Singleton() {}

public:
    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;

    static Singleton& getInstance() {
        static Singleton instance;
        return instance;
    }

    void doSomething() {
        std::cout << "Singleton is doing something." << std::endl;
    }
};

class Singleton1 : public Singleton {
private:
    Singleton1() {}

public:
    static Singleton1& getInstance() {
        static Singleton1 instance;
        return instance;
    }
};

class Singleton2 : public Singleton {
private:
    Singleton2() {}

public:
    static Singleton2& getInstance() {
        static Singleton2 instance;
        return instance;
    }
};

class Singleton3 : public Singleton {
private:
    Singleton3() {}

public:
    static Singleton3& getInstance() {
        static Singleton3 instance;
        return instance;
    }
};

class Singleton4 : public Singleton {
private:
    Singleton4() {}

public:
    static Singleton4& getInstance() {
        static Singleton4 instance;
        return instance;
    }
};

int main() {
    // Using the factory methods to get instances of different singletons
    Singleton1& singleton1 = Singleton1::getInstance();
    Singleton2& singleton2 = Singleton2::getInstance();
    Singleton3& singleton3 = Singleton3::getInstance();
    Singleton4& singleton4 = Singleton4::getInstance();

    // Verify that they are all the same instance
    if (&singleton1 == &singleton2 && &singleton2 == &singleton3 && &singleton3 == &singleton4) {
        std::cout << "All Singletons are the same instance." << std::endl;
    }

    singleton1.doSomething();
    singleton2.doSomething();
    singleton3.doSomething();
    singleton4.doSomething();

    return 0;
}
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在这个示例中，我们创建了一个基类 Singleton，它实现了单例模式的基本机制，然后创建了四个派生类 Singleton1、Singleton2、Singleton3 和 Singleton4，每个派生类都有自己的工厂方法 getInstance 来创建对应的单例对象。

通过这种方式，每个派生类继承了单例的行为，但每个单例对象都是独立的实例。当我们调用工厂方法来获取这些单例对象时，它们确保只有一个实例存在，而且每个工厂方法创建的实例是不同的，即每个单例类都有自己的单例实例。