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单一职责原则

news 2025/8/5 11:56:17

单一职责原则：

就一个类而言，应该只有一个引起它变化的原因，如果一个类承担的职责过多就等于把这些职责耦合在一起，至少会造成以下两方面的问题：

我们要去修改该类中的一个职责可能会影响到该类的其它职责。这种耦合会导致脆弱的设计，当变化发生时，设计会遭受到意想不到的破坏。
当客户端仅需要该对象的某一个职责时，不得不将其他不需要的职责全都包含进来，从而造成冗余代码或代码的浪费。

我们在设计一个类时要学会发现职责，并把那些职责相互分离，其实要去判断是否应该分离出一个类来并不难，前面说过，一个类应该只有一个引起它变化的原因，如果你能想到其它的原因也能去改变这个类，那么这个类就具有多于1个的职责，就应该考虑类的职责分离。

在之前的这篇博客中，https://blog.csdn.net/weixin_44049823/article/details/128907849，我们实现的计算器实际上也用到了单一职责原则，这里我们选出其中最经典的3.0版本和5.0版本来学习单一职责原则。

3.0版本计算器代码如下：

#include<iostream>
using namespace std;
#include<string>//业务逻辑
//异常类用于处理异常情况
class opeException
{
public:void getMessage(){cout << "您的输入有误！" << endl;}
};//运算类用于处理运算
class Operation
{
public:Operation(string& _num1, string& _num2, string& _ope) :num1(_num1), num2(_num2), ope(_ope){}//获取运算结果int getResult(){if (!(isStringNum(num1) && isStringNum(num2) && (ope == "+" || ope == "-" || ope == "*" || ope == "/")))throw opeException();if (ope == "+"){re = stoi(num1) + stoi(num2);}else if (ope == "-"){re = stoi(num1) - stoi(num2);}else if (ope == "*"){re = stoi(num1) * stoi(num2);}else if (ope == "/"){if (stoi(num2) != 0){re = stoi(num1) / stoi(num2);}elsethrow opeException();}return re;}
private:int re;string num1;string num2;string ope;//判断一个字符串是不是数字bool isStringNum(string& s){bool flag = true;for (auto e : s)if (!(isdigit(e))){flag = false;break;}return flag;}
};//界面逻辑
int main()
{try{string _num1 = " ";string _num2 = " ";string _ope = " ";cout << "请输入左操作数：" << endl;cin >> _num1;cout << "请输入右操作数：" << endl;cin >> _num2;cout << "请输入操作符" << endl;cin >> _ope;Operation operation(_num1, _num2, _ope);cout << operation.getResult() << endl;}catch (opeException &ex){ex.getMessage();}return 0;
}

仅仅一个运算类Operation就实现了加减乘除4种功能，很明显在这个类中我至少有4个原因去修改这个类，我修改加法算法的时候可能会影响到其它的运算算法，这个类的耦合太高且严重违反了单一职责原则。

修改后的5.0版本如下：

#include<iostream>
using namespace std;
#include<string>//业务逻辑//异常类用于处理异常情况
class opeException
{
public:void getMessage(){cout << "您的输入有误！" << endl;}
};//运算类
class Operation
{//判断一个字符串是不是数字bool isStringNum(string& s){bool flag = true;for (auto e : s)if (!(isdigit(e))){flag = false;break;}return flag;}protected:bool isError(string& _strNum1, string& _strNum2, string& _ope){if (!(Operation::isStringNum(_strNum1) && Operation::isStringNum(_strNum2) && (_ope == "+" || _ope == "-" || _ope == "*" || _ope == "/"))){return false;}}
public:virtual int getResult() = 0;
};//加法运算类
class addOperation :public Operation
{
private:string strNum1;string strNum2;string ope;int re;
public:addOperation(string& _strNum1, string& _strNum2, string& _ope) :strNum1(_strNum1), strNum2(_strNum2), ope(_ope), re(0) {}virtual int getResult() override{if (!isError(strNum1, strNum2, ope))throw opeException();elsere = stoi(strNum1) + stoi(strNum2);return re;}
};//减法运算类
class subOperation :public Operation
{
private:string strNum1;string strNum2;string ope;int re;
public:subOperation(string& _strNum1, string& _strNum2, string& _ope) :strNum1(_strNum1), strNum2(_strNum2), ope(_ope), re(0) {}virtual int getResult() override{if (!isError(strNum1, strNum2, ope))throw opeException();elsere = stoi(strNum1) - stoi(strNum2);return re;}
};//乘法运算类
class mulOperation :public Operation
{
private:string strNum1;string strNum2;string ope;int re;
public:mulOperation(string& _strNum1, string& _strNum2, string& _ope) :strNum1(_strNum1), strNum2(_strNum2), ope(_ope), re(0) {}virtual int getResult() override{if (!isError(strNum1, strNum2, ope))throw opeException();elsere = stoi(strNum1) * stoi(strNum2);return re;}
};//除法运算类
class divOperation :public Operation
{
private:string strNum1;string strNum2;string ope;int re;
public:divOperation(string& _strNum1, string& _strNum2, string& _ope) :strNum1(_strNum1), strNum2(_strNum2), ope(_ope), re(0) {}virtual int getResult() override{if (!isError(strNum1, strNum2, ope))throw opeException();else if (stoi(strNum2) != 0)re = stoi(strNum1) / stoi(strNum2);elsethrow opeException();return re;}
};//运算工厂类
class OpeFactory
{
public:Operation& choose(string &_strNum1,string &_strNum2,string &_ope){if (_ope == "+"){operation = new addOperation(_strNum1, _strNum2, _ope);}else if (_ope == "-")operation = new subOperation(_strNum1, _strNum2, _ope);else if (_ope == "*")operation = new mulOperation(_strNum1, _strNum2, _ope);else if (_ope == "/"){operation = new divOperation(_strNum1, _strNum2, _ope);}elseoperation = nullptr;return *operation;}private:Operation* operation;
};//界面逻辑
int main()
{try{string _strNum1 = " ";string _strNum2 = " ";string _ope = " ";cout << "请输入左操作数：" << endl;cin >> _strNum1;cout << "请输入右操作数：" << endl;cin >> _strNum2;cout << "请输入操作符：" << endl;cin >> _ope;OpeFactory factory;Operation* re = &factory.choose(_strNum1, _strNum2, _ope);if (re != nullptr)cout << (*re).getResult() << endl;elsecout << "您的输入有误！" << endl;}catch (opeException ex){cout << "您的输入有误" << endl;}return 0;
}