Dynamic Binding (Section 10.4)

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Dynamic Binding(Section 10.4)

• CSCI 431 Programming Languages
• Fall 2003

A compilation of material developed by Felix
Hernandez-Campos and Mircea Nicolescu
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Fundamental Concepts in OOP

• Encapsulation
• Data Abstraction
• Information hiding
• The notion of class and object
• Inheritance
• Code reusability
• Is-a vs. has-a relationships
• Polymorphism
• Dynamic method binding

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Inheritance

• Encapsulation improves code reusability
• Abstract Data Types
• Modules
• Classes
• However, it is generally the case that the code a
  programmer wants to reuse is close but not
  exactly what the programmer needs
• Inheritance provides a mechanism to extend or
  refine units of encapsulation
• By adding or overriding methods
• By adding attributes

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InheritanceNotation
Base Class (or Parent Class or Superclass)
Java.awt.Dialog
Is-a relationship
Derived Class (or Child Class or Subclass)
Java.awt.FileDialog
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Dynamic Method Binding

• Consequence of inheritance
• derived class D has all members of its base class
  B

• can use an object of class D everywhere an object
  of class B is expected
• a form of polymorphism

// Both student and professor objects have all
properties of // a person object // Both can be
used in a person context
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Dynamic Method Binding
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Dynamic Method Binding
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Dynamic Method Binding

• Two alternatives for choosing the method to call

• according to the types of variables (references)
  x and y static method binding (will call the
  method of person in both cases)
• according to the types of objects s and p to
  which x and y refer dynamic method binding
  (will call the methods of student / professor)

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Dynamic Method Binding

• Simula, C, Ada 95
• static method binding by default

• how do we specify dynamic binding in C?

• label individual methods as virtual

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Virtual and Non-Virtual Methods
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Virtual and Non-Virtual Methods
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Virtual and Non-Virtual Methods
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Abstract Classes
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Member Lookup
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Member Lookup

• The compiler will generate

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Member Lookup
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Member Lookup
ok references through q will use prefixes of B's
data space and vtable
static semantic error F lacks the additional
data and vtable entries of a bar
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Type Checking
still an error
ok, but risky programmer must ensure that q
refers to a bar object
C specific involves dynamic type check
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Type Checking
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Type Checking Implementation
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Member Lookup

• Disadvantages of virtual methods

• run-time cost
• preclude in-line expansion of methods at compile
  time
• performance decrease for small and frequently
  called methods

• Potential problem with static lookup

• fragile base class problem (Java)
• large standard library, evolving over time
  (adding new features)
• user may have old library and run code designed
  for new library
• code may use the new features

• static lookup invalid memory access
• dynamic lookup better, produce error message
  ("member not found")

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Classes as "Closures"
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Classes as "Closures"
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Classes as "Closures"