C++、C#父类子类互转机制

简单来说子类可以直接转换为父类，但是父类如果想转换为子类是有条件的，首先被转换的父类对象起初必须声明为相同的子类对象，也就是说首先是某种子类对象转换来的父类对象可以在转换回去。C#的转换机制比较简单，对于父类、子类没有什么特殊要求，但是C++要求父类必须是含有虚函数的类（实现了多态机制），才能有条件实现父类转为子类。下面用代码说明
C#代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace inheritCS
{
    public class person
    {
        public string name;
        public int age;
    }

    class student : person
    {
        public string school;
        public int grade;
    }

    class teacher : person
    {
        public string subject;
    }
    class Program
    {
        static void Main(string[] args)
        {
            student stu = new student();
            stu.name = "pupil";
            stu.school = "school1";
            stu.grade = 2;
            stu.age = 10;

            teacher t = new teacher();
            t.name = "teacher";
            t.age = 30;
            t.subject = "maths";

            person p1 = stu as person;
            if(p1!=null)
            {
                Console.WriteLine(string.Format("{0} 由 student 转 person 成功", stu.name));
            }
            else
            {
                Console.WriteLine(string.Format("{0} 由 student 转 person 失败", stu.name));
            }
            person p2 = t as person;
            if (p2 != null)
            {
                Console.WriteLine(string.Format("{0} 由 teacher 转 person 成功", t.name));
            }
            else
            {
                Console.WriteLine(string.Format("{0} 由 teacher 转 person 失败", t.name));
            }
            
            student stuReceiver = p1 as student;
            if (stuReceiver != null)
            {
                Console.WriteLine(string.Format("{0} 由 person 转 student 成功", p1.name));
            }
            else
            {
                Console.WriteLine(string.Format("{0} 由 person 转 student 失败", p1.name));
            }

            stuReceiver = p2 as student;
            if (stuReceiver != null)
            {
                Console.WriteLine(string.Format("{0} 由 person 转 student 成功", p2.name));
            }
            else
            {
                Console.WriteLine(string.Format("{0} 由 person 转 student 失败", p2.name));
            }
            if (Console.ReadKey(true).Key == ConsoleKey.Enter)
            {
                return;
            }
        }
    }
}

执行结果如下图。这里person是父类，student、teacher是子类，student、teacher转person完全没问题，student转化为的父类对象可以成功转换回student，但是teacher转化为的父类对象无法转换为student

C++代码

// inheritCPP.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <stdlib.h>
#include <string.h>

class person 
{
public:
	char name[10];
	int age;
	virtual void Show() { printf("This is Base calss"); }
};

class student :public person
{
public:
	char school[10];
	int grade;
};

class teacher :public person
{
public:
	char subject[10];
};


int main()
{
	student stu;
	strcpy_s(stu.name, "pupil");
	strcpy_s(stu.school, "school1");
	stu.grade = 2;
	stu.age = 10;

	teacher t;
	strcpy_s(t.name, "teacher");
	t.age = 30;
	strcpy_s(t.subject, "maths");

	person* p1 = NULL;
	person* p2 = NULL;
	student* stuReceiver;

	if ((p1 = &stu) != NULL)
	{
		printf("%s 由 student 转 person 成功\n", stu.name);
	}
	else
	{
		printf("%s 由 student 转 person 失败\n", stu.name);
	}
	if ((p2 = &t) != NULL)
	{
		printf("%s 由 teacher 转 person 成功\n", t.name);
	}
	else
	{
		printf("%s 由 teacher 转 person 失败\n", t.name);
	}

	if (stuReceiver = dynamic_cast<student*>(p1))
	{
		printf("%s 由 person 转 student 成功\n",p1->name);
	}
	else
	{
		printf("%s 由 person 转 student 失败\n", p1->name);
	}

	if (stuReceiver = dynamic_cast<student*>(p2))
	{
		printf("%s 由 person 转 student 成功\n", p2->name);
	}
	else
	{
		printf("%s 由 person 转 student 失败\n", p2->name);
	}
	getchar();
    return 0;
}

整体思路和C#一样，但是实现时父类必须含有虚函数已实现多态机制，这样才能实现有条件的父类转换为子类，执行结果如下图