多态和强制类型转换探索

不bb,直接上代码

static_cast不管向上转换和向下转换都能成功，

1. 向上转换：把子类指针赋值给基类指针，该基类指针调用的还是子类的函数，发生错误
2. 向下转换： 把基类指针赋值给子类对象，成功实现多态；但是有两种情况，如下：
   1. 基类指针本来就是指向子类对象，那么向下转换还能访问子类成员（如下面程序的变量a）
   2. 但是从指向基类对象的基类指针硬转，访问子类成员会发生错误

#include<iostream>
using namespace std;

class Base
{
public:
	Base() {};
	virtual void Show() { cout << "This is Base calss"; }
	void Fun() {
		cout << "Base" << endl;
	}
};
class Derived :public Base
{
public:
	Derived() {};
	void Show() { cout << "This is Derived class"; }
	void Fun() {
		cout << "Derived" << endl;
	}
	void F1() {
		cout << "子类独有" << endl;
	}
	int  a = 1;
};
int main()
{
	//这是第一种情况
	Base* base = new Derived;
	//   base->a;// 错误，Base类型无成员a
	//   base->F1();//错误，Base类型无成员函数F1();
	base->Show();
	cout << endl;
	Derived* der2 = new Derived;
	Base* bb = static_cast<Base*> (der2);
	bb->Show();  //这是不安全的，把派生类指针转换为基类指针会发生错误，这里调用的还是子类的函数
	delete bb;
	if (Derived *der = static_cast<Derived*>(base))
	{
		cout << endl;
		cout << "第一种情况转换成功" << endl;
		der->Show();
		cout << endl;
		der->Fun();
		cout << endl;
		cout << der->a << endl;
	}
	//这是第二种情况
	Base * base1 = new Base;
	Derived *der1 = static_cast<Derived*>(base1);
	if (Derived *der2 = static_cast<Derived*>(base1))
	{
		cout << "第二种情况转换成功" << endl;
		der1->Show(); //把基类指针转换为子类指针，转换成功，这里调用的就是基类的函数
		cout << endl;
		cout << der1->a << endl; //这里输出的a就发生了错误，和上面的区别是：这里是纯从base转过来，而上面是base类型本来就指向Derived对象
	}
	else
	{
		cout << "第二种情况转换失败" << endl;
	}

	delete(base);
	delete(base1);
	system("pause");
}