设计模式8工厂方法模式

　　回顾简单工厂模式，在工厂类里判断到底需要创建哪一个类来计算结果，如果要加一个类的话，我们需要在工厂类里来增加一个if判断；
　　修改了原有的类，违背了开放封闭原则。
　　简单工厂模式代码 #include using namespace std;  class Calc { public:     virtual double calc(double num1, double num2) {         return 0;     }; };  class AddCalc : public Calc { public:     double calc(double num1, double num2) override {         return num1 + num2;     } };  class SubCalc : public Calc { public:     double calc(double num1, double num2) override {         return num1 - num2;     } };  class MulCalc : public Calc { public:     double calc(double num1, double num2) override {         return num1 * num2;     } };  class DivCalc : public Calc { public:     double calc(double num1, double num2) override {         if (num2 == 0) {             cout << ＂除数不能为0！＂ << endl;             return 0;         }         return num1 / num2;     } };  class CalcFactory { public:     static Calc* CreateCalc(const char op) {         if (op == ＂+＂) {             return new AddCalc;          }         else if (op == ＂-＂) {             return new SubCalc;          }         else if (op == ＂*＂) {             return new MulCalc;          }         else if (op == ＂/＂) {             return new DivCalc;         }         else {             throw;         }     } };   int main(int argc, char const* argv[]) {     char op;     Calc *calc;     double num1, num2, result;     cout << ＂请输入数字A: ＂ << endl;     cin >> num1;      cout << ＂请输入运算符(+, -, *, /):＂ << endl;     cin >> op;      cout << ＂请输入数字B: ＂ << endl;     cin >> num2;      calc = CalcFactory::CreateCalc(op);     result = calc->calc(num1, num2);      cout << ＂两个数的运算结果为: ＂ << result << endl;     return 0; }
　　下面我们将学习工厂方法模式，增加新的计算类，不需要修改原来的老代码，就能增加一个新类。
　　工厂方法模式代码 #include using namespace std;  class Calc { public:     virtual double calc(double num1, double num2) {         return 0;     }; };  class AddCalc : public Calc { public:     double calc(double num1, double num2) override {         return num1 + num2;     } };  class SubCalc : public Calc { public:     double calc(double num1, double num2) override {         return num1 - num2;     } };  class MulCalc : public Calc { public:     double calc(double num1, double num2) override {         return num1 * num2;     } };  class DivCalc : public Calc { public:     double calc(double num1, double num2) override {         if (num2 == 0) {             cout << ＂除数不能为0！＂ << endl;             return 0;         }         return num1 / num2;     } };  class CalcFactory { public:     virtual Calc* CreateCalc() {         return new Calc;     } };  class AddCalcFactory : public CalcFactory { public:     virtual Calc* CreateCalc() {         return new AddCalc;     } };  class SubCalcFactory : public CalcFactory { public:     virtual Calc* CreateCalc() {         return new SubCalc;     } };  class MulCalcFactory : public CalcFactory { public:     virtual Calc* CreateCalc() {         return new MulCalc;     } };  class DivCalcFactory : public CalcFactory { public:     virtual Calc* CreateCalc() {         return new DivCalc;     } };   int main(int argc, char const* argv[]) {     char op;     Calc *calc;     double num1, num2, result;     cout << ＂请输入数字A: ＂ << endl;     cin >> num1;      cout << ＂请输入运算符(+, -, *, /):＂ << endl;     cin >> op;      cout << ＂请输入数字B: ＂ << endl;     cin >> num2;      CalcFactory *factory;     if (op == ＂+＂) {         factory = new AddCalcFactory;      }     else if (op == ＂-＂) {         factory = new SubCalcFactory;      }     else if (op == ＂*＂) {         factory = new MulCalcFactory;      }     else if (op == ＂/＂) {         factory = new DivCalcFactory;     }     else {         throw;     }      calc = factory->CreateCalc();     result = calc->calc(num1, num2);     cout << ＂两个数的运算结果为: ＂ << result << endl;     return 0; }
　　最重要的一点就是把判断运算符的逻辑从设计里挪到了调用的地方，从而在新增运算符时可以不用动原来写好的类，遵循了开放关闭原则。
　　工厂方法模式
　　定义一个用于创建对象的接口，让子类决定实例化哪一个类，工厂方法使一个类的实例化延迟到子类。
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