关于多态代码和运行结果

#include <iostream>
#pragma hdrstop
using std::cout;
using std::endl;
//---------------------------------------------------------------------------
#pragma argsused
//---------------------------------------------------------------------------
class Base {
public:
	Base() { };
	virtual void DisplayMessage() {
		cout << "Displaying Message from an object of Base class" << endl;
	};
};
//---------------------------------------------------------------------------
class DerivedFirst : public Base {
public:
	DerivedFirst() { };
	void DisplayMessage() {
		cout << "Displaying Message from an object of DerivedFirst class" << endl;
	}
};
//---------------------------------------------------------------------------
class DerivedSecond : public Base {
public:
	DerivedSecond() { };
	void DisplayMessage() {
		cout << "Displaying Message from an object of DerivedSecond class" << endl;
	}
};
//---------------------------------------------------------------------------
class DerivedThird : public Base {
public:
	DerivedThird() { };
};
//---------------------------------------------------------------------------
int main()
{
	// create a base class object
	Base* bc = new Base();
	bc->DisplayMessage();
	// delete the base class object and assign it to DerivedFirst object
	delete bc;
	bc = new DerivedFirst();
	bc->DisplayMessage();
	// delete the base class object and assign it to DerivedSecond object
	delete bc;
	bc = new DerivedSecond();
	bc->DisplayMessage();
	// delete the base class object and assign it to DerivedThird object
	delete bc;
	bc = new DerivedThird();
	bc->DisplayMessage();
	delete bc;
	return EXIT_SUCCESS;
}
//---------------------------------------------------------------------------

虚函数主要作用：用基类声明的指针（Base* bc），指向派生类，如果派生类中有与基类中相同的方面就调用子类的方法，如果子类没有该方面，就调用基类自身的方法。
基类中如果不加virtual 关键字，用用基类声明的指针，调用函数时只能调用基类自身的函数。