Reclassification

Scenario
In a ﬁrst person shooter:
enemies tend to change their behavior when they become aware of the
player’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 P´ıˇse (CTU in Prague) Object Programming Lect. 7: Reclassiﬁcation November 7, 2010 2 / 14

Problem
How to represent enemies?
Single class with a typecode—clumsy.

Problem
Multiple classes, events and pointer redirection—too complicated, not
feasible.

Problem
Multiple classes, events and pointer redirection—too complicated, not
feasible.
The strategy pattern.

Reclassiﬁcation
Change of a class of an object during its lifetime.
Not implemented in current industrial programming languages.

Issues

Issues
Implementation (ﬂexible and eﬃcient).

Issues
Type safety.

Issues
Type safety.
Halfway executed methods invoked on the reclassiﬁed object.

Issues
Type safety.
Halfway executed methods invoked on the reclassiﬁed object.
Overall design: cleanliness, consistency and interaction with
other features of the language.

The Strategy Pattern
Implementation: indirect pointers.

Envelope deﬁnes its own type.

Envelope deﬁnes its own type.
Halfway executed methods continue their execution on the original
object—they are not aware of the replacement. However, they may
damage 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 the
programmer to make sure they cause no harm.

Halfway executed methods continue their execution—it is up to the
programmer to make sure they cause no harm.
Overall design in sync with the rest of the language.

Gilgul

Gilgul
Optionally, an exception is raised when reclassifying an object with
methods on the stack. When all methods invoked on that objects are
unwound, the last one is restarted.

Gilgul
Optionally, an exception is raised when reclassifying an object with
methods on the stack. When all methods invoked on that objects are
unwound, the last one is restarted.
Reclassiﬁcation is a global issue—indirectly or directly reclassifying
methods should either be side-eﬀect free or abortable/restartable.

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.

Fickle
State classes can not be used as ﬁeld types.

Fickle
Methods have to declare root classes of directly or indirectly
reclassiﬁed objects.

Fickle
Methods have to declare root classes of directly or indirectly
reclassified objects.
Static types of variables change conservatively to accommodate
for the effects of reclassification.

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.

Compiler raises an error if it can not deduce no ambiguity can occur.
A programmer may provide an additional information which helps to
dispel 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 (March
2002). 153–191. http://doi.acm.org/10.1145/514952.514955
Craig Chambers. Predicate Classes. Proceedings of the 7th European
Conference on Object-Oriented Programming (ECOOP ’93), Oscar
Nierstrasz (Ed.). Springer-Verlag, London, UK. 268–296.

Reclassification

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