예제랑 같이 설명하겠습니다

class Animal
{
public:
    void eat() { std::cout << "I'm eating generic food."; }
};

class Cat : public Animal
{
public:
    void eat() { std::cout << "I'm eating a rat."; }
};

int main(){
    Animal *animal = new Animal;
    Cat *cat = new Cat;
    animal->eat(); // outputs: "I'm eating generic food."
    cat->eat();    // outputs: "I'm eating a rat."
}

I'm eating generic food.
I'm eating a rat.

인 코드에서 void func(Animal *xyz) { xyz->eat(); }를 추가시켜 다음같은 코드를 만들었다고 하면

class Animal
{
public:
    void eat() { std::cout << "I'm eating generic food."; }
};

class Cat : public Animal
{
public:
    void eat() { std::cout << "I'm eating a rat."; }
};
void func(Animal *xyz) { xyz->eat(); }

int main(){
    Animal *animal = new Animal;
    Cat *cat = new Cat;
    func(animal); // outputs: "I'm eating generic food."
    func(cat);    // outputs: "I'm eating generic food."
}

I'm eating generic food.
I'm eating generic food.

func(cat)을 호출했는데 "I'm eating a rat." 이 아닌 "I'm eating generic food."이 나왔지요 여기서 더 많은 동물들을 추가하면 어떻게 될까요? 동물 하나하나를 위해 func()를 오버로딩 해야 할까요?

virtual 메소드는 이런 상황에서 씁니다.

class Animal
{
public:
    virtual void eat() { std::cout << "I'm eating generic food."; }
};
class Cat : public Animal
{
public:
    void eat() { std::cout << "I'm eating a rat."; }
};
void func(Animal *xyz) { xyz->eat(); }

int main(){
    Animal *animal = new Animal;
    Cat *cat = new Cat;
    func(animal); // outputs: "I'm eating generic food."
    func(cat);    // outputs: "I'm eating a rat."
}

I'm eating generic food.
I'm eating a rat.