Reclassification

Scenario

In a first person shooter:

enemies tend to change their behavior when they become aware of theplayer’s presence (and then again when they empty their magazines),

under the hood, they are typically represented by objects and

references pointing at them are often spread across the whole memory.

Michal Pıse (CTU in Prague) Object Programming Lect. 7: Reclassification November 7, 2010 2 / 14

Problem

How to represent enemies?

Single class with a typecode—clumsy.

Multiple classes, events and pointer redirection—too complicated, notfeasible.

The strategy pattern.


Problem






Problem






Reclassification

Change of a class of an object during its lifetime.

Not implemented in current industrial programming languages.


Issues

Implementation (flexible and efficient).

Type safety.

Halfway executed methods invoked on the reclassified object.

Overall design: cleanliness, consistency and interaction withother features of the language.


Issues


Type safety.




Issues


Type safety.




Issues


Type safety.




Issues


Type safety.




The Strategy Pattern

Implementation: indirect pointers.

Envelope defines its own type.

Halfway executed methods continue their execution on the originalobject—they are not aware of the replacement. However, they maydamage the context.












Smalltalk’s become:

Implementation: redirect pointer referencing class object.

No type safety (as in the rest of the language).

Halfway executed methods continue their execution—it is up to theprogrammer to make sure they cause no harm.

Overall design in sync with the rest of the language.




















Gilgul


Optionally, an exception is raised when reclassifying an object withmethods on the stack. When all methods invoked on that objects areunwound, the last one is restarted.

Reclassification is a global issue—indirectly or directly reclassifyingmethods should either be side-effect free or abortable/restartable.


Gilgul





Gilgul





Fickle

Three types of classes: normal, root and state.

Root classes can only inherit from normal classes.Root classes can only be parents to state classes.State classes can only inherit from root classes or state classes.

State classes can not be used as field types.

Methods have to declare root classes of directly or indirectlyreclassified objects.

Static types of variables change conservatively to accommodatefor the effects of reclassification.


Fickle







Fickle







Fickle







Example

root class Player {bool brave;

abstract Weapon kissed(){Player}}

state class Frog extends Player {Vocal pouch;

Weapon kissed(){Player}{this⇓Prince; sword = new Weapon}}

state class Prince extends Player {Weapon sword;

Weapon kissed(){Player}{sword}}


Example (II)

bool play(Player p, Frog f){Player} {f.pouch; // correct

p.kissed();

f.pouch; // incorrect

p.brave; //correct

}


Predicate Classes

Combination of multiple dispatch with automatic reclassification.

An object is reclassified once a predicate is fulfilled.


Typing Issues of Predicate Classes

The system uses multimethods.

It guarantees no method invocation is ambiguous.

Compiler raises an error if it can not deduce no ambiguity can occur.

A programmer may provide an additional information which helps todispel suspicions of ambiguity using disjoint and cover primitives.




















See

Sophia Drossopoulou, Ferruccio Damiani, Mariangiola Dezani-Ciancaglini,and Paola Giannini. More Dynamic Object Reclassification: FickleII.ACM Transactions on Programming Languages and Systems 24, 2 (March2002). 153–191. http://doi.acm.org/10.1145/514952.514955

Craig Chambers. Predicate Classes. Proceedings of the 7th EuropeanConference on Object-Oriented Programming (ECOOP ’93), OscarNierstrasz (Ed.). Springer-Verlag, London, UK. 268–296.


Reclassification

Education

prague object programming

class object

issues michal pse ctu

reclassied object

reclassication change

gilgul implementation

issues implementation

executed methods