it's a value! it's a reference!
Michael van Acken
michael.van.acken at gmail.com
Sun Feb 24 09:04:02 UTC 2019
[Only subscribed to valhalla-dev this morning, so I cannot do a
proper reply to John Rose's email.]
| So we might get the pleasing symmetry of:
| class (something with fields, methods, and supers)
| reference* class (classic, identity-testable, stateful, with natural
| value* class (new fangled, identity-free, pure, with natural by-value
| object (anything instantiated from a class, incl. int some day: just
| reference* object (instance of a reference class; you can test its
identity via. ref==)
| value* object (instance of a value class; you can only test its value)
| The starred terms are adjectives which modify the following nouns.
| The starred terms are also candidates for replacement; get out your
| scrabble dictionaries!
I would like to toss in "entity" from the domain-driven design
glossary at http://dddcommunity.org/resources/ddd_terms/
[[Entity]] An object fundamentally defined not by its attributes, but
by a thread of continuity and identity.
Note: In the context of DDD, "identity" usually refers to a property
of a real world "object", modeled in software for example as an
attribute "customerID" of a class "Customer". Eric Evans has more
on this in his DDD book (89pp "Entities (a.k.a Reference Objects)").
For completeness some related entries from the same glossary page:
[[Value Object]] An object that describes some characteristic or
attribute but carries no concept of identity.
[[Immutable]] The property of never changing observable state after
[[Life Cycle]] A sequence of states an object can take on between
creation and deletion, typically with constraints to ensure integrity
when changing from one state to another. May include migration of an
[[Entity]] between systems and different [[Bounded Contexts]].
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