面向对象语言都有类，也都有继承，继承可以很大程度上减少代码量，而让代码逻辑更清晰。继承可以使得子类（派生类）具有父类（基类）的属性和方法或者重新定义、追加属性和方法等。
比如车是基类，轿车和公交车从车类派生而来，这就是一个继承的例子。
应该有的人会问了，这车继承例子跟编程好像比较远？如果有这想法就说明还没有面向对象编程思想。
下面写下车继承例子代码，让大家明白面向对象。
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// 梁笔记 // https://zouzhongliang.com class BaseCar{ private: string brand; int wheelNum; float carL; float carW; public: BaseCar() :brand(""), wheelNum(4), carL(3),carW(1.8) { } BaseCar(string _brand, int _wheelNum, float _carL, float _carW) :brand(_brand), wheelNum(_wheelNum), carL(_carL),carW(_carW) { } }; class Car : public BaseCar{ private: bool gps; public: Car() :BaseCar(), gps(true) { } Car(string _brand, int _wheelNum, float _carL, float _carW, bool _gps) :BaseCar(_brand,_wheelNum,_carL,_carW), gps(_gps) { } }; class Bus : public BaseCar{ private: bool electric; public: Bus() :BaseCar(), electric(true) { } Bus(string _brand, int _wheelNum, float _carL, float _carW, bool _electric) :BaseCar(_brand,_wheelNum,_carL,_carW), electric(_electric) { } }; |
以上代码写出了继承的关系，不用太在意设计的成员变量是否合理，现在有了大概了解类继承，再来写下与继承有关系的内容。
类继承的语法
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class B : public A { }; class B : private A { }; class B : protected A { }; |
可以看到有三种权限的继承，public、private、protected，现在就这三种继承方式做说明
public公有继承：基类的非私有成员的访问属性在派生类中保持不变，派生类可以防问基类公有的和保护的部份
private私有继承：基类的非私有成员的访问属性在派生类变成私有成员，私有继承一般使用较少
protected保护继承：基类的非私有成员的访问属性在派生类变成保护成员
不管如何继承基类，基类中的私有成员都是无法访问。
继承后派生类在内存如何存放基类成员：
用上面车类定义代码来写一段测试
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// 梁笔记 // https://zouzhongliang.com #include <iostream> #include <ctime> using namespace std; class BaseCar{ private: string brand; int wheelNum; float carL; float carW; public: BaseCar() :brand(""), wheelNum(4), carL(3),carW(1.8) { } BaseCar(string _brand, int _wheelNum, float _carL, float _carW) :brand(_brand), wheelNum(_wheelNum), carL(_carL),carW(_carW) { } void Baseprint(){ cout<<&brand<<endl; cout<<&wheelNum<<endl; cout<<&carL<<endl; cout<<&carW<<endl; } }; class Car : public BaseCar{ private: bool gps; public: Car() :BaseCar(), gps(true) { } Car(string _brand, int _wheelNum, float _carL, float _carW, bool _gps) :BaseCar(_brand,_wheelNum,_carL,_carW), gps(_gps) { } void print(){ Baseprint(); cout<<&gps<<endl; } }; class Bus : public BaseCar{ private: bool electric; public: Bus() :BaseCar(), electric(true) { } Bus(string _brand, int _wheelNum, float _carL, float _carW, bool _electric) :BaseCar(_brand,_wheelNum,_carL,_carW), electric(_electric) { } void print(){ Baseprint(); cout<<&electric<<endl; } }; int main() { cout<<endl; Bus bus; bus.print(); cout<<endl; Car car; car.print(); return 0; } |
继承后派生类在内存中测试结果：
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0x66ff00 0x66ff04 0x66ff08 0x66ff0c 0x66ff10 0x66fee0 0x66fee4 0x66fee8 0x66feec 0x66fef0 |
从测试结果可以看出，类继承后，派生类的成员变量在内存中是连续存储的。前面基类成员变量，后面是派生类成员为变量。
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