Practical example of Polymorphism

Question

Can anyone please give me a real life, practical example of Polymorphism? My professor tells me the same old story I have heard always about the + operator. a+b = c and 2+2 = 4 , so this is polymorphism. I really can't associate myself with such a definition, since I have read and re-read this in many books.

What I need is a real world example with code, something that I can truly associate with.

For example, here is a small example, just in case you want to extend it.

>>> class Person(object):
    def __init__(self, name):
        self.name = name

>>> class Student(Person):
    def __init__(self, name, age):
        super(Student, self).__init__(name)
        self.age = age

Answer 1

Check the Wikipedia example: it is very helpful at a high level:

class Animal:
    def __init__(self, name):    # Constructor of the class
        self.name = name
    def talk(self):              # Abstract method, defined by convention only
        raise NotImplementedError("Subclass must implement abstract method")

class Cat(Animal):
    def talk(self):
        return 'Meow!'

class Dog(Animal):
    def talk(self):
        return 'Woof! Woof!'

animals = [Cat('Missy'),
           Cat('Mr. Mistoffelees'),
           Dog('Lassie')]

for animal in animals:
    print animal.name + ': ' + animal.talk()

# prints the following:
#
# Missy: Meow!
# Mr. Mistoffelees: Meow!
# Lassie: Woof! Woof!

Notice the following: all animals "talk", but they talk differently. The "talk" behaviour is thus polymorphic in the sense that it is realized differently depending on the animal . So, the abstract "animal" concept does not actually "talk", but specific animals (like dogs and cats) have a concrete implementation of the action "talk".

Similarly, the "add" operation is defined in many mathematical entities, but in particular cases you "add" according to specific rules: 1+1 = 2, but (1+2i)+(2-9i)=(3-7i).

Polymorphic behaviour allows you to specify common methods in an "abstract" level, and implement them in particular instances.

For your example:

class Person(object):
    def pay_bill(self):
        raise NotImplementedError

class Millionare(Person):
    def pay_bill(self):
        print "Here you go! Keep the change!"

class GradStudent(Person):
    def pay_bill(self):
        print "Can I owe you ten bucks or do the dishes?"

You see, millionares and grad students are both persons. But when it comes to paying a bill, their specific "pay the bill" action is different.

Answer 2

A common real example in Python is file-like objects . Besides actual files, several other types, including StringIO and BytesIO , are file-like. A method that acts as files can also act on them because they support the required methods (eg read , write ).

Answer 3

A C++ example of polymorphism from the above answer would be:

class Animal {
public:
  Animal(const std::string& name) : name_(name) {}
  virtual ~Animal() {}

  virtual std::string talk() = 0;
  std::string name_;
};

class Dog : public Animal {
public:
  virtual std::string talk() { return "woof!"; }
};  

class Cat : public Animal {
public:
  virtual std::string talk() { return "meow!"; }
};  

void main() {

  Cat c("Miffy");
  Dog d("Spot");

  // This shows typical inheritance and basic polymorphism, as the objects are typed by definition and cannot change types at runtime. 
  printf("%s says %s\n", c.name_.c_str(), c.talk().c_str());
  printf("%s says %s\n", d.name_.c_str(), d.talk().c_str());

  Animal* c2 = new Cat("Miffy"); // polymorph this animal pointer into a cat!
  Animal* d2 = new Dog("Spot");  // or a dog!

  // This shows full polymorphism as the types are only known at runtime,
  //   and the execution of the "talk" function has to be determined by
  //   the runtime type, not by the type definition, and can actually change 
  //   depending on runtime factors (user choice, for example).
  printf("%s says %s\n", c2->name_.c_str(), c2->talk().c_str());
  printf("%s says %s\n", d2->name_.c_str(), d2->talk().c_str());

  // This will not compile as Animal cannot be instanced with an undefined function
  Animal c;
  Animal* c = new Animal("amby");

  // This is fine, however
  Animal* a;  // hasn't been polymorphed yet, so okay.

}