polymorphism - overriding & virtual keyword - late binding

#include <iostream> 
using namespace std;

class base {
public:
	void fun_1() { cout << "base-1\n"; }
	virtual void fun_2() { cout << "base-2\n"; }
	virtual void fun_3() { cout << "base-3\n"; }
	virtual void fun_4() { cout << "base-4\n"; }
};

class derived : public base {
public:
	void fun_1() { cout << "derived-1\n"; }
	void fun_2() { cout << "derived-2\n"; }
	void fun_4(int x) { cout << "derived-4\n"; }
};

int main()
{
	base base_obj;
	base* base_ptr;
	derived derived_obj;
	base_ptr = &derived_obj;	// base *bp = new derived;

	cout << "base_obj :\n";
	base_obj.fun_1();
	base_obj.fun_2();
	base_obj.fun_3();
	base_obj.fun_4();

	cout << "derived_obj :\n";
	derived_obj.fun_1();
	derived_obj.fun_2();
	derived_obj.fun_3();
	derived_obj.fun_4(1);

	cout << "derived_obj pointed by base_ptr :\n";
	// Early binding because fun1() is non-virtual 
	// in base 
	base_ptr->fun_1();

	// Late binding (RTP) 
	base_ptr->fun_2();				

	// Late binding (RTP) 
	base_ptr->fun_3();				// overriding되지 않음 : 기초 클래스와 동일함 함수명의 파생 클래스 함수 누락됨

	// Late binding (RTP) 
	base_ptr->fun_4();				// overriding되지 않음 : 기초/파생 클래스의 함수명과 반환형이 같으나 인자의 개수가 달라 overriding 조건 누락됨

	// Early binding but this function call is 
	// illegal(produces error) becasue pointer 
	// is of base type and function is of 
	// derived class 
	// base_ptr->fun_4(5); 
}

출처 : https://www.geeksforgeeks.org/virtual-function-cpp/