Virtual Function

[jimin-kiim]

• Virtual Function
• Pure Virtual Function

[jimin-kiim]

• the behavior of virtual function is defined within an inheriting class by a function with the same signature
• without virtual :
  • member function call depends on the class type of the pointer
• with virtual :
  • member function call depends on the actual object to which it points to

[jimin-kiim]

#include <iostream> 

class Animal { 
  public: 
    virtual void eat() 
    { 
      std::cout << "I eat like a generic Animal.\n"; 
    } 
}; 

class Wolf : public Animal { 
  public: 
    void eat() { std::cout << "I eat like a wolf!\n"; } 
}; 

class Fish : public Animal { 
  public: 
    void eat() { std::cout << "I eat like a fish!\n"; } 
}; 

class OtherAnimal : public Animal { 
}; 

int main() 
{ 
  Animal *anAnimal[4]; 
  // dynamic binding
  anAnimal[0] = new Animal(); 
  anAnimal[1] = new Wolf(); 
  anAnimal[2] = new Fish(); 
  anAnimal[3] = new OtherAnimal(); 

  for(int i = 0; i < 4; i++) anAnimal[i]->eat();
  return 0; 
} 

1. member function of the base class is virtual function

• virtual void eat()

2. the variable * type of the base class

• Animal *anAnimal[4];

-> Dynamic Binding works

[jimin-kiim]

#include <iostream>
using namespace std;

class B
{
public:
    virtual int get_val() { return 0; }; 
};

class D1 : public B
{
public:
    D1(int x_val) : x(x_val){};
    virtual int get_val() { return x; };

private:
    int x;
};

class D2 : public B
{
public:
    D2(int y_val) : y(y_val){};
    virtual int get_val() { return y * y; };

private:
    int y;
};

int main()
{
    const int MAX = 5;
    D1 Zero(0), One(1), Two(2);
    D2 Three(3), Four(4);

    B *B_Array[MAX];
    B_Array[0] = &Zero;
    B_Array[1] = &One;
    B_Array[2] = &Two;
    B_Array[3] = &Three;
    B_Array[4] = &Four;

    for (int i = 0; i < MAX; ++i)
    {
        cout << i << ": " << B_Array[i]->get_val() << endl;
    }

    return 0;
}

1. member function of the base class is virtual function
   • virtual int get_val() { return 0; };
2. the variable * type of the base class
   • B *B_Array[MAX];

-> Dynamic Binding works

[jimin-kiim]

Pure Virtual Function

• if a class holds at least one pure virtual function -> abstract class
• declared with "=0"
• no implementation in self
• always defined in a derived class
• cannot be used as the member function
• making derived class define and use it is the purpose