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学习C++编译环境：Linux

第四十五课 不同的继承方式

1.被忽略的细节

冒号（：）表示继承关系，parent表示被继承的类，public的意义是什么？

2.有趣的问题

是否可以将继承语句中的public换成protected或者private？如果可以，与public继承有什么区别？

45-1 有趣的尝试—可以运行

#include <iostream>
#include <string>
using namespace std;
class Parent
{
};
class Child_A : public Parent
{
};
class Child_B : protected Parent
{
};
class Child_C : private Parent
{
};
int main()
{   
    return 0;
}

3.不同的继承方式

C++中支持三种不同的继承方式
public继承
父类成员在子类中保持原有访问级别
private继承
父类成员在子类中变为私有成员
protected继承
父类中的公有成员变为保护成员，其他成员保持不变

45-2 继承与访问级别深度实践(**)

#include <iostream>
#include <string>
using namespace std;
class Parent
{
protected:
    int m_a;
protected:
    int m_b;
public:
    int m_c;
    void set(int a, int b, int c)
    {
	m_a = a;
	m_b = b;
	m_c = c;
    }
};
class Child_A : public Parent
{
public:
    void print()
    {
	cout << "m_a" << m_a << endl;
	cout << "m_b" << m_b << endl;
	cout << "m_c" << m_c << endl;
    }
};
class Child_B : protected Parent
{
public:
    void print()
    {
	cout << "m_a" << m_a << endl;
	cout << "m_b" << m_b << endl;
	cout << "m_c" << m_c << endl;
    }
};
class Child_C : private Parent
{
public:
    void print()
    {
	cout << "m_a" << m_a << endl;
	cout << "m_b" << m_b << endl;
	cout << "m_c" << m_c << endl;
    }
};
int main()
{   
    Child_A a;
    Child_B b;
    Child_C c;
    a.m_c = 100;
/*b.m_c = 100;//Child_B protection inherits from Parent, so all Parent members become protected members, so the outside world cannot access
//Child_B保护继承自Parent，所以所有的Parent成员全部变成了protected成员，因此外界无法访问
    c.m_c = 100;//Child_C protection inherits from Parent, so all Parent members become private members, so the outside world cannot access   */
  //Child_C保护继承自Parent，所以所有的Parent成员全部变成了private成员，因此外界无法访问
    a.set(1,1,1);
    //b.set(2,2,2);
    //c.set(3,3,3);
    a.print();
    b.print();
    c.print(); 
    return 0;
}

4.遗憾的事实

一般而言，C++工程项目中只使用public继承
C++的派生语言只支持一种继承方式（public继承）
protected和private继承带来的复杂性远大于实用性

45-3.d 45-3.cs 45-3.java C++派生语言初探

45-3.d linux运行指令：gdc 45-3.d

module D_Demo;
import std.stdio;
import std.string;
class Obj
{
protected:
    string mName;
    string mInfo;
public:
    this()
    {
        mName = "Object";
        mInfo = "";
    }
    string name()
    {
        return mName;
    }
    string info()
    {
        return mInfo;
    }
}
class Point : Obj
{
private:
    int mX;
    int mY;
public:
    this(int x, int y)
    {
        mX = x;
        mY = y;
        mName = "Point";
        mInfo = format("P(%d, %d)", mX, mY);
    }
    int x()
    {
        return mX;
    }
    int y()
    {
        return mY;
    }
}
void main(string[] args)
{
    writefln("D Demo");           // D Demo
    
    Point p = new Point(1, 2);
    
    writefln(p.name());           // Point
    writefln(p.info());           // P(1, 2)
}
运行结果
D Demo
Point
P(1, 2)

45-3.cs linux运行： gmcs 45-3.cs

class Obj
{
    protected string mName;
    protected string mInfo;
    
    public Obj()
    {
        mName = "Object";
        mInfo = "";
    }
    
    public string name()
    {
        return mName;
    }
    
    public string info()
    {
        return mInfo;
    }
}
class Point : Obj
{
    private int mX;
    private int mY;
    public Point(int x, int y)
    { 
        mX = x;
        mY = y;
        mName = "Point";
        mInfo = "P(" + mX + ", " + mY + ")";
    }
    
    public int x()
    {
        return mX;
    }
    
    public int y()
    {
        return mY;
    }
}
class Program
{
    public static void Main(string[] args)
    {
        System.Console.WriteLine("C# Demo");    // C# Demo
        
        Point p = new Point(1, 2);
        
        System.Console.WriteLine(p.name());     // Point
        System.Console.WriteLine(p.info());     // P(1, 2)
        
    }
}
运行结果
C# Demo
Point
P(1, 2)

45-3.java linux运行：javac 45-3.java

class Obj
{
    protected String mName;
    protected String mInfo;
    
    public Obj()
    {
        mName = "Object";
        mInfo = "";
    }
    
    public String name()
    {
        return mName;
    }
    
    public String info()
    {
        return mInfo;
    }
}
class Point extends Obj
{
    private int mX;
    private int mY;
    public Point(int x, int y)
    { 
        mX = x;
        mY = y;
        mName = "Point";
        mInfo = "P(" + mX + ", " + mY + ")";
    }
    
    public int x()
    {
        return mX;
    }
    
    public int y()
    {
        return mY;
    }
}
class Program {
    public static void main(String[] args){
        System.out.println("Java Demo");    // Java Demo
        
        Point p = new Point(1, 2);
        
        System.out.println(p.name());       // Point
        System.out.println(p.info());       // P(1, 2)
    }
}
运行结果
Java Demo
Point
P(1, 2)

小结
C++中支持3种不同的继承方式
继承方式直接影响父类成员在子类中的访问属性
一般而言，工程中只使用public的继承方式
C++的派生语言中只支持public继承方式