C++ switch/case的替換方案
阿新 • • 發佈:2020-11-04
switch語句改為多型結構更好些。
1. 常規switch
#include <iostream> 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() { //常規switch showMessage(enumOne); showMessage(enumTwo); showMessage(enumThree); return 0; }
2.多型+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() { //多型+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; }
上述完全是一個面向過程到面向物件的轉變:將每個case分支都作為一個子物件,然後用C++語言的多型性去動態繫結。這樣做確實是帶來了效能上的損失,但是在當今的CPU計算能力而言,這是可以忽略的,而它帶來的好處卻很有用:
(1)分支的增減只要繼續派生即可;
(2)子類代表了一個case,比必須用type去硬編碼的case語句更加具有可讀性;
(3)程式碼的可讀性增強,使得分支的維護性增加;
(4)面向物件的思想更加符合人看世界的方式;
(5)避免了漏寫break語句造成的隱蔽錯誤。
3.函式指標+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() { //函式指標+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; }
4.狀態模式取代switch
#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;
}
5. 多型+模板取代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");
}
};
template <class S>
void show( S shape)
{
shape.showMessage();
}
int main()
{
MessageOne shape1;
MessageTwo shape2;
MessageThree shape3;
show(shape1);
show(shape2);
show(shape3);
return 0;
}
參考: JoannaJuanCV