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Polymorphism & Virtual Methods
Week 6
Gaddis:15.6-15.
CS 5301
Fall 2013
Jill Seaman
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Polymorphism
! The Greek word poly means many, and the
Greek word morphism means form.
! So, polymorphism means 'many forms'.
! In object-oriented programming (OOP),
polymorphism refers to
- identically named (and redefined) methods
- that have different behavior depending on the
(specific derived) type of object that they are called
on.
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Example of polymorphism?
class Animal { private: ... public: void speak() { cout << “none ”; } }; class Cat : public Animal { private: ... public: void speak() { cout << “meow “; } }; class Dog : public Animal { private: ... public: void speak() { cout << “bark “; } }; string name1 = “Steve Jobs”; cout << “Name” << name1 << endl; 4
Example of polymorphism?, part 2
void f (Animal a) { a.speak(); } int main() { Cat c; Dog d; f(c); f(d); } string name1 = “Steve Jobs”; cout << “Name” << name1 << endl;
! IF the output is “meow bark”, this (function f)
is an example of polymorphism.
- The behavior of a in f would depend on its
specific (derived type).
! IF the output is “none none”, it’s not
polymorphism.
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Polymorphism in C++
! Polymorphism in C++ is supported through:
- virtual methods AND
- pointers to objects OR reference variables/
parameters.
! without these, C++ determines which method to
invoke at compile time (using the variable type).
! when virtual methods and pointer/references are
used together, C++ determines which method to
invoke at run time (using the specific type of the
instance currently referenced by the variable).
cout << “Name” << name1 << endl; 6
Virtual methods
! Virtual member function: function in a base class
that expects to be redefined in derived class
! Function defined with key word virtual:
! Supports dynamic binding: functions bound at
run time to function that they call
! Without virtual member functions, C++ uses
static (compile time) binding
cout << “Name” << name1 << endl; virtual^ void^ Y()^ {...} 7
Example virtual methods
string name1 = “Steve Jobs”; cout << “Name” << name1 << endl; class Animal { public: virtual void speak(); int age(); }; class Cat : public Animal { public: virtual void speak(); //redefining a virtual int age(); //redefining a normal function }; int main() { Cat morris; Animal *pA = &morris; pA -> age(); // Animal::age() is invoked (base) (not virtual) pA -> speak(); // Cat::speak() is invoked (derived) ... } 8
Virtual methods
! In compile-time binding, the data type of the
pointer resolves which method is invoked.
! In run-time binding, the type of the object
pointed to resolves which method is invoked.
string name1 = “Steve Jobs”; cout << “Name” << name1 << endl; void f (Animal &a) { a.speak(); } int main() { Cat c; Dog d; f(c); f(d); }
! Assuming speak is virtual,
and a is passed by
reference, the output is:
meow bark
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Example: Abstract Class
! An abstract class may not be used as an
argument type, as a function return type,or as
the type of an explicit conversion.
! Pointers and references to an abstract class
may be declared.
cout << “Name” << name1 << endl; class CShape { public: CShape ( ) { } virtual void vDraw ( ) const = 0; // pure virtual method }; CShape CShape1; // Error: object of abstract class CShape* pCShape; // Ok CShape CShapeFun(); // Error: return type void vg(CShape); // Error: argument type CShape& rCShapeFun(CShape&); // Ok 14
Example: Abstract Class
! Pure virtual functions are inherited as pure
virtual functions.
! Or else:
cout << “Name” << name1 << endl; class CAbstractCircle : public CShape { private: int m_iRadius; public: void vRotate (int) {} // CAbstractCircle ::vDraw() is a pure virtual function }; class CCircle : public CShape { private: int m_iRadius; public: void vRotate (int) {} void vDraw(); //define here or in impl file }; 15
Heterogeneous collections
string name1 = “Steve Jobs”; cout << “Name” << name1 << endl; class Animal { private: string name; public: Animal(string n) {name = n;} virtual void speak() = 0; }; class Cat : public Animal { public: Cat(string n) : Animal(n) { }; void speak() {cout << "meow "; } }; class Dog : public Animal { public: Dog(string n) : Animal(n) { }; void speak() {cout << "bark "; } }; class Pig : public Animal { public: Pig(string n) : Animal(n) { }; void speak() {cout << "oink "; } }; 16
Heterogeneous collections
! Driver:
string name1 = “Steve Jobs”; cout << “Name” << name1 << endl; int main() { Animal* animals[ ] = { new Cat("Charlie"), new Cat("Scamp"), new Dog("Penny"), new Cat("Libby"), new Cat("Patches"), new Dog("Milo"), new Pig("Wilbur") }; for (int i=0; i< 7; i++) { animals[i]->speak(); } } meow meow bark meow meow bark oink
