[转]C++ 取代switch的三种方法

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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;
}