c/c++取代switch的用法

Posted 2021-12-01 ims-

目录

• 1.常规switch
• 2.多态+std::map取代switch
• 3.函数指针+std::map取代switch
• 4.状态模式取代switch

1.常规switch

enum EnumType 
{ 
    enumOne, 
    enumTwo, 
    enumThree 
}; 
   
void showMessage(int type) 
{ 
    switch(type) 
    { 
    case enumOne: 
        printf("This is message one\n"); 
        break; 
    case enumTwo: 
        printf("This is message two\n"); 
        break; 
    case enumThree: 
        printf("This is message three\n"); 
        break; 
    default: 
        printf("This is wrong message\n"); 
        break; 
    } 
} 
   
int main() 
{ 
//常规switch 
    showMessage(enumOne); 
    showMessage(enumTwo); 
    showMessage(enumThree); 
   
    return 0; 
} 
 

2.多态+std::map取代switch

#include <map> 
   
enum EnumType 
{ 
    enumOne, 
    enumTwo, 
    enumThree 
}; 
   
class Base 
{ 
public: 
    Base(){} 
    virtual ~Base(){} 
    virtual void showMessage(){} 
}; 
   
class MessageOne:public Base 
{ 
public: 
    MessageOne(){} 
    ~MessageOne(){} 
    void showMessage() 
    { 
        printf("This is message one\n"); 
    } 
}; 
   
class MessageTwo:public Base 
{ 
public: 
    MessageTwo(){} 
    ~MessageTwo(){} 
    void showMessage() 
    { 
        printf("This is message two\n"); 
    } 
}; 
   
class MessageThree:public Base 
{ 
public: 
    MessageThree(){} 
    ~MessageThree(){} 
    void showMessage() 
    { 
        printf("This is message three\n"); 
    } 
}; 
   
int main() 
{ 
//多态+std::map取代switch 
    std::map<int,Base*> baseMap; 
    baseMap.insert(std::make_pair(enumOne,new MessageOne)); 
    baseMap.insert(std::make_pair(enumTwo,new MessageTwo)); 
    baseMap.insert(std::make_pair(enumThree,new MessageThree)); 
    baseMap[enumOne]->showMessage(); 
    baseMap[enumTwo]->showMessage(); 
    baseMap[enumThree]->showMessage(); 
   
    return 0; 
} 

　　上述完全是一个面向过程到面向对象的转变：将每个case分支都作为一个子对象，然后用C++语言的多态性去动态绑定。这样做确实是带来了性能上的损失，但是在当今的CPU计算能力而言，这是可以忽略的，而它带来的好处却很有用：
(1)分支的增减只要继续派生即可；
(2)子类代表了一个case，比必须用type去硬编码的case语句更加具有可读性；
(3)代码的可读性增强，使得分支的维护性增加；
(4)面向对象的思想更加符合人看世界的方式；

(5)避免了漏写break语句造成的隐蔽错误。

3.函数指针+std::map取代switch

#include <map> 
   
enum EnumType 
{ 
    enumOne, 
    enumTwo, 
    enumThree 
}; 
   
void showMessageOne() 
{ 
    printf("This is message one\n"); 
} 
   
void showMessageTwo() 
{ 
    printf("This is message two\n"); 
} 
   
void showMessageThree() 
{ 
    printf("This is message three\n"); 
} 
   
int main() 
{ 
//函数指针+std::map取代switch 
    typedef void (*func)(); 
   
    std::map<int,func> funcMap; 
    funcMap.insert(std::make_pair(enumOne,showMessageOne)); 
    funcMap.insert(std::make_pair(enumTwo,showMessageTwo)); 
    funcMap.insert(std::make_pair(enumThree,showMessageThree)); 
    funcMap[enumOne](); 
    funcMap[enumTwo](); 
    funcMap[enumThree](); 
   
    return 0; 
} 

值得注意的是函数指针要用typedef定义，否则报错。

4.状态模式取代switch

关于设计模式中的状态模式可参考：C++设计模式——状态模式

直接上代码。

#include <stdio.h> 
class Context; 
class State 
{ 
public: 
    State(){} 
    virtual ~State (){} 
    virtual void showMessage(Context *pContext)=0; 
}; 
   
class MessageOne:public State 
{ 
public: 
    MessageOne(){} 
    ~MessageOne(){} 
    void showMessage(Context *pContext) 
    { 
        printf("This is message one\n"); 
    } 
}; 
   
class MessageTwo:public State 
{ 
public: 
    MessageTwo(){} 
    ~MessageTwo(){} 
    void showMessage(Context *pContext) 
    { 
        printf("This is message two\n"); 
    } 
}; 
   
class MessageThree:public State 
{ 
public: 
    MessageThree(){} 
    ~MessageThree(){} 
    void showMessage(Context *pContext) 
    { 
        printf("This is message three\n"); 
    } 
}; 
   
class Context 
{ 
public: 
     Context(State *pState) : m_pState(pState){} 
   
     void Request() 
     { 
          if (m_pState) 
          { 
               m_pState->showMessage(this); 
          } 
     } 
   
     void ChangeState(State *pState) 
     { 
          m_pState = pState; 
     } 
   
private: 
     State *m_pState; 
}; 
   
int main() 
{ 
     State *pStateA = new MessageOne(); 
     State *pStateB = new MessageTwo(); 
     State *pStateC = new MessageThree(); 
     Context *pContext = new Context(pStateA); 
     pContext->Request(); 
   
     pContext->ChangeState(pStateB); 
     pContext->Request(); 
   
     pContext->ChangeState(pStateC); 
     pContext->Request(); 
   
     delete pContext; 
     delete pStateC; 
     delete pStateB; 
     delete pStateA; 
   
     return 0; 
}