《21天学通C++》实现继承（1）

1.派生语法

语法如下：

class Base{};class Name:access-specifier Base{//access-specifier可以是public、private、protect};

一个简单的继承结构程序：

#include <iostream> 
using namespace std; // 定义一个基类Fish
class Fish {
public:bool isFreshWaterFish; // 一个布尔型成员变量，表示鱼是淡水鱼还是咸水鱼// 一个虚函数Swim，根据鱼的种类输出不同的游泳场所void Swim() {if (isFreshWaterFish) {cout << "swim in lake" << endl; // 如果是淡水鱼，输出在湖中游泳} else {cout << "swim in sea" << endl; // 如果是咸水鱼，输出在海中游泳}}
};// 定义一个派生类Tuna，继承自Fish
class Tuna : public Fish {
public:Tuna() {isFreshWaterFish = false; // 将isFreshWaterFish设置为false，表示Tuna是咸水鱼}
};// 定义另一个派生类Carp，同样继承自Fish
class Carp : public Fish {
public:Carp() {isFreshWaterFish = true; // 将isFreshWaterFish设置为true，表示Carp是淡水鱼}
};int main() {Carp lunch; // 创建一个Carp类型的对象lunchTuna dinner; // 创建一个Tuna类型的对象dinnercout << "lunch: " << endl; // 输出午餐的描述lunch.Swim(); // 调用lunch的Swim函数cout << "dinner: " << endl; // 输出晚餐的描述dinner.Swim(); // 调用dinner的Swim函数system("pause"); return 0;
}

2.访问限定符protected

将属性声明为protected时，允许友元类和派生类访问该属性，但禁止在继承结构之外访问

示例代码如下

class Fish
{
protected://声明为protectedbool isFreshWaterFish;
public：void Swim(){if (isFreshWaterFish){cout<<"swim in lake"<<endl;}else{cout<<"swim in sea"<<endl;}        }
};

3.基类初始化

如果基类包含重载的构造函数，可以使用初始化列表传递参数
示例代码如下：

#include <iostream> 
using namespace std; // 定义基类Fish
class Fish {
protected:bool isFreshWaterFish; // 一个受保护的布尔型成员变量，表示鱼是淡水鱼还是咸水鱼Fish(bool isFreshWater) : isFreshWaterFish(isFreshWater) {}// 构造函数，根据传入的参数初始化isFreshWaterFish
public:// 一个虚函数Swim，根据鱼的种类输出不同的游泳场所void Swim() {if (isFreshWaterFish) {cout << "swim in lake" << endl; // 如果是淡水鱼，输出在湖中游泳} else {cout << "swim in sea" << endl; // 如果是咸水鱼，输出在海中游泳}}
};// 定义派生类Tuna，继承自Fish
class Tuna : public Fish {
public:// Tuna的构造函数，使用Fish的构造函数并传入false，表示Tuna是咸水鱼Tuna() : Fish(false) {}
};// 定义另一个派生类Carp，同样继承自Fish
class Carp : public Fish {
public:// Carp的构造函数，使用Fish的构造函数并传入true，表示Carp是淡水鱼Carp() : Fish(true) {}
};int main() {Carp lunch; // 创建一个Carp类型的对象lunchTuna dinner; // 创建一个Tuna类型的对象dinnercout << "lunch: " << endl; // 输出午餐的描述lunch.Swim(); // 调用lunch的Swim函数cout << "dinner: " << endl; // 输出晚餐的描述dinner.Swim(); // 调用dinner的Swim函数system("pause");return 0; 
}

这样可以强迫在构造一个派生类时，必须定义一个这样的函数来为其赋值，避免不定义的情况下，随机数值产生影响

4.在派生类中覆盖基类方法

只要派生类中实现了从基类中继承的函数，且返回值和标值相同，就可以覆盖

class Fish
{
protected:bool isFreshWaterFish;Fish(bool isFreshWater):isFreshWaterFish(isFreshWater){};public:void Swim(){if (isFreshWaterFish){cout<<"swim in lake"<<endl;}else{cout<<"swim in sea"<<endl;}        }
};class Tuna:public Fish{
public:Tuna():Fish(false){};void Swim(){//覆盖基类中的方法cout<<"fu gai"<<endl;}};

5.调用基类中已经被覆盖的方法

使用作用域解析运算符::

Dinner.Fish::Swim();//调用基类中的方法
Dinner.Swim();//调用覆盖后的方法

6.在派生类中的覆盖方法中，调用基类方法

同样使用作用域解析运算符::

class Fish
{
protected:bool isFreshWaterFish;Fish(bool isFreshWater):isFreshWaterFish(isFreshWater){};public:void Swim(){if (isFreshWaterFish){cout<<"swim in lake"<<endl;}else{cout<<"swim in sea"<<endl;}        }
};class Tuna:public Fish{
public:Tuna():Fish(false){};void Swim(){//覆盖基类中的方法cout<<"fu gai"<<endl;Fsih::Swim();//调用基类方法}};

7.派生类中隐藏的基类方法

覆盖是一种极端情况，即派生类中的重载方法会隐藏基类中方法的所有重载版本，导致需要调用时无法调用，示例代码如下

#include <iostream> 
using namespace std;
// 定义基类Fish
class Fish {
public:// 定义一个成员函数Swimvoid Swim() {cout << "Fish swim" << endl;}// 重载Swim函数，带有一个布尔参数isFreshWaterFishvoid Swim(bool isFreshWaterFish) {if (isFreshWaterFish) {cout << "swim in lake" << endl;} else {cout << "swim in sea" << endl;}}
};// 定义派生类Tuna，
class Tuna : public Fish {
public:// 重写基类Fish的Swim成员函数void Swim() {cout << "fu gai" << endl;}
};int main() {Tuna dinner; // 创建Tuna类的对象dinnercout << "dinner: " << endl; dinner.Swim(); // 调用dinner对象的Swim函数，由于Tuna类重写了Swim，所以调用的是Tuna类的Swimdinner.Swim(false); //ERROR，因为派生类中的重载函数Swim(bool isFreshWaterFish)已经被覆盖了system("pause"); return 0;

若想调用基类中的方法

解决办法1： 使用作用域解析符::

dinner.Fish::Swim(false);

解决方法2： 在派生类中使用using解除隐藏

#include <iostream>
using namespace std;class Fish
{public:void Swim(){cout<<"Fish swim"<<endl;}void Swim(bool isFreshWaterFish){if (isFreshWaterFish){cout<<"swim in lake"<<endl;}else{cout<<"swim in sea"<<endl;}        }
};class Tuna:public Fish{
public:using Fish::Swim;//解除隐藏void Swim(){cout<<"fu gai"<<endl;}};int main(){Tuna dinner;cout<<"dinner: "<<endl;dinner.Swim();dinner.Swim(false);//可以正常调用了system("pause");return 0;}

解决方法3： 在派生类中，把基类中的方法全部重载一次，且在重载方法中就调用基类方法

#include <iostream>
using namespace std;class Fish
{public:void Swim(){cout<<"Fish swim"<<endl;}void Swim(bool isFreshWaterFish){if (isFreshWaterFish){cout<<"swim in lake"<<endl;}else{cout<<"swim in sea"<<endl;}        }
};class Tuna:public Fish{
public://重载基类中的方法Swim()void Swim(){cout<<"fu gai"<<endl;}//重载基类中的重载方法Swim(bool isFreshWaterFish)void Swim(bool isFreshWaterFish){Fish::Swim(isFreshWaterFish);//直接调用基类中的方法}};int main(){Tuna dinner;cout<<"dinner: "<<endl;dinner.Swim();dinner.Swim(false);//可以正常运行了system("pause");return 0;}