3-21 开闭原则

开闭原则是一种开发原则，表述为
对拓展进行开放，对修改进行关闭。
也就是对拓展功能进行开放，但是修改代码时不被允许，源代码具有拓展性，那么就可以直接进行拓展，不建议修改源码。
例如下面的计算机案例

#include <iostream>
#include <string>
using namespace std;
class Calculator {
public:
    int getResult(string optr) {
        if (optr == "+") {
            return m_Num1 + m_Num2;
        }
        else if (optr == "-") {
            return m_Num1 - m_Num2;
        }
        else if (optr == "*") {
            return m_Num1 * m_Num2;
        }
        else return 0;
    }
    int m_Num1;
    int m_Num2;
};
void example() {
    Calculator c1;
    c1.m_Num1 = 2;
    c1.m_Num2 = 3;
    cout << c1.m_Num1 << "+" << c1.m_Num2 << "=" << c1.getResult("+") << endl;
}
int main() {
    example();
    return 0;
}

上面的案例中，每次增加功能时，都必须要更改类。而且对源码的更改涉及到数据结构的变化，不符合开闭原则。
那么就需要用多态来实现将计算机作为总体抽象出来，然后再具体添加功能类。

#include <iostream>
#include <string>
using namespace std;
class AbstractCalculator {
public:
    virtual int getResult() {
        return 0;
    }
    int m_Num1;
    int m_Num2;
};
class Add :public AbstractCalculator {
public:
    int getResult() {
        return m_Num1 + m_Num2;
    }
};
class miu :public AbstractCalculator {
public:
    int getResult() {
        return m_Num1 - m_Num2;
    }
};
class mul :public AbstractCalculator {
public:
    int getResult() {
        return m_Num1 * m_Num2;
    }
};
void example() {
    AbstractCalculator *Calc = new Add;
    Calc->m_Num1 = 10;
    Calc->m_Num2 = 20;
    cout << Calc->m_Num1 << "+" << Calc->m_Num2 << "=" << Calc->getResult()<< endl;
    delete Calc;
    Calc = new miu;
    Calc->m_Num1 = 40;
    Calc->m_Num2 = 10;
    cout << Calc->m_Num1 << "-" << Calc->m_Num2 << "=" << Calc->getResult() << endl;
    delete Calc;
    Calc = new mul;
    Calc->m_Num1 = 4;
    Calc->m_Num2 = 5;
    cout << Calc->m_Num1 << "*" << Calc->m_Num2 << "=" << Calc->getResult() << endl;
    delete Calc;
}
int main() {
    example();
    return 0;
}

虽然再整体上代码量更高，但是在实际开发场景中，拓展性可读性都将大大增强。
总结：C++开发提倡利用多态设计程序架构，因为多态优点很多