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 <iostream>
#include <mutex>// 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 singletonsSingleton1& singleton1 = Singleton1::getInstance();Singleton2& singleton2 = Singleton2::getInstance();Singleton3& singleton3 = Singleton3::getInstance();Singleton4& singleton4 = Singleton4::getInstance();// Verify that they are all the same instanceif (&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;
}

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。