CSC/ECE 517 Fall 2010/ch5 5c IC

Dynamic Dispatch

Introduction

Dynamic dispatch is an object-oriented programming concept that refers to the mapping of a method to an object's dynamic runtime type. It is common in many object-oriented languages. Languages such as Java and C++ use single dispatch, while only a few, such as CLOS, use multiple dispatch. Both types of dispatch will be discussed later.

Advantages

• Flexibility
• Extensibility
• Reduces the effort required to change implementation [4]

Disadvantages

• Lookup overhead
• Counter to safety and increased compile-time knowledge
• Obstacle to optimization
• Hinders compiler in determining exact type of objects

Example in C++

Below are two example classes in C++. Class A contains one private attribute and two public virtual methods, method1 and method2. Class B inherits from A, contains one private attribute and two public methods, method1 (which overrides the method defined in class A) and method3.

class A
{
  int aMember1;
public:
  virtual int method1();
  virtual void method2();
};

class B : public A
{
  int bMember1;
public:
  virtual int method1();
  virtual double method3();
}

The next section of code shows a basic implementation of these methods within these two classes.

class A
{
  
  virtual int method1()
  {
    cout << "method1 in class A" << endl;
  }

  virtual int method2()
  {
    cout << "method2 in class A" << endl;
  }
};

class B : public A
{
  virtual int method1()
  {
    cout << "method1 in class B" << endl;
  }

  virtual int method3()
  {
    cout << "method3 in class B" << endl;
  }
}

Since C++ supports the use of polymorphism, an object can be created with a static type of A but a dynamic type of B.

A ab = new B();

Single vs Multiple Dispatch

Dynamic dispatch comes in 2 types: single and multiple dispatch.

References

[1] “Dynamic Dispatch in Object-Oriented Languages”, 2004.

[2] M. Muller, “Message Dispatch in Dynamically-Typed Object-Oriented Languages”, Master’s Thesis, University of New Mexico, 1995.

[3] http://en.wikipedia.org/wiki/Dynamic_dispatch

[4] D. Schmidt, "Dynamic Binding C++", http://www.cs.wustl.edu/~schmidt/PDF/C++-dynamic-binding4.pdf, 2006.