Double Dispatch Pattern

Problem

Sometimes the computational path in a program needs to differ based on the runtime arguments it was called with.

Solution

The Double Dispatch pattern prescribes a mechanism to call different concrete functions depending on the types of objects involved in the call.

Related Patterns

Visitor

Discussion

Double Dispatch is prominently used in the Visitor pattern, where different operations on an object produce different results based on the type of the calling object.

Examples

The Visitor pattern usees an implementation of Double Dispatch.

Code

Foo and bar are C++ classes. If bar::foobar() is called, the overloaded method will be selected based on the type of the argument selected by the method signature.

class foo{
public:
  virtual void foobar(foo& f);
}
class bar{
  void foobar(foo& f){
    cout << "Foo" << endl;
  }
  void foobar(int& i){
    cout << "I am an int" << endl;
  }
public:
  void getType(foo& f){
    f.foobar(*this);
  }
}