单例模式

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单例模式：

确保一个类只有一个实例，并提供一个全局访问点。

单例模式类图：

经典单例模式实现：

class Singleton{private:static Singleton *uniqueInstance;Singleton(){}public:static Singleton *getInstance(){if (uniqueInstance == NULL) uniqueInstance = new Singleton();return uniqueInstance;}};

下面来看如何将一个普通的巧克力工厂类转为单例类：

class ChocolateBoiler{private:bool empty;bool boiled;public:ChocolateBoiler(){//这是原来的构造函数empty = true;boiled = false;}void fill(){if (isEmpty()){empty = false;boiled = false;}}void drain(){ //排出煮沸的巧克力和牛奶if (!isEmpty() && isBoiled()) empty = true;}void boil(){//加热if (!isEmpty() && !isBoiled()) boiled = true;}bool isEmpty(){return empty;}bool isBoiled(){return boiled;}};

改成单例类后如下：

class ChocolateBoiler{private:static ChocolateBoiler *uniqueInstance;  //*bool empty;bool boiled;ChocolateBoiler(){//*这是新的构造函数empty = true;boiled = false;}public:static ChocolateBoiler *getInstance(){//*if (uniqueInstance == NULL) uniqueInstance = new ChocolateBoiler();return uniqueInstance;}void fill(){if (isEmpty()){empty = false;boiled = false;}}void drain(){ //排出煮沸的巧克力和牛奶if (!isEmpty() && isBoiled()) empty = true;}void boil(){//加热if (!isEmpty() && !isBoiled()) boiled = true;}bool isEmpty(){return empty;}bool isBoiled(){return boiled;}};

上面的经典单例模式存在多线程安全问题，解决方法参考面试题2：实现单例模式（offer）

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