First, thinking

class Child : public Parent{    };

In the code, the colon represents the inheritance relationship, and the Parent represents the inherited class. What is the meaning of public?

Programming experiment:

#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;
}

2, Different ways of inheritance

Three different inheritance methods are supported in C + +

• public inheritance
  
  • The parent class members keep the original access principle in the child class
• private inheritance
  
  • Parent member becomes private in child
• protected inheritance
  
  • The public member in the parent class becomes a protected member and the other members remain unchanged

The default inheritance method in C + + is private

Programming experiment -- Inheritance and access level deep practice

#include<iostream>
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 public members become protected members, so the outside world cannot access them
//  c. M ﹣ C = 100; / / child ﹣ C inherits from the Parent, so all members become private members, so the outside world cannot access them
    a.set(1,1,1);
//  b.set(2,2,2);
//  c.set(3,3,3);
    a.print();
    b.print();
    c.print();
    return 0;
} 

Print results:

m_a1
m_b1
m_c1
m_a0
m_b0
m_c28
m_a1
m_b0
m_c4254425

Regrettable facts:

• Generally speaking, only public inheritance is used in C + + project
• C + + only supports one inheritance method (public inheritance)
• The complexity of protected and private inheritance is far greater than the utility

3, On C + + derived language

D language:

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)
}

C# language:

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)

    }
}

Java language:

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 //Use keywords to indicate inheritance
{

    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)
    }
}

Four, summary

• Three different inheritance methods are supported in C + +
• Inheritance directly affects the access properties of the parent class members in the child class
• Generally speaking, only public inheritance is used in the project
• Only public inheritance is supported in C + + derived languages