Prototype

Intent

Create new objects by cloning existing ones so creation doesn’t depend on concrete classes or complex constructors.

Use When

• You need copies of objects with runtime-specific concrete types.
• Construction is expensive or complicated, but copies are relatively cheap.
• You want to avoid “deep constructor trees” and keep object creation flexible.

Prefer Something Else When

• Copy semantics are ambiguous or dangerous (lots of shared mutable state).
• You mostly create from primitives/config and can express creation directly (Builder/Factory is clearer).

Minimal Structure

• Prototype interface with clone(): Prototype (or language idioms like copy constructors)
• Concrete prototypes implement clone semantics (deep vs shallow) explicitly
• Optional PrototypeRegistry mapping keys to prototype instances

Implementation Steps

1. Decide and document copy semantics for each field:
2. immutable primitives/value types (copy)
3. shared references (share) vs “owned” references (deep copy)
4. Implement clone() with explicit rules; handle cycles if your graph can be cyclic.
5. If you use a registry, keep keys typed (enum/ADT) and register at startup.

Pitfalls

• Shallow copy bugs: clones unintentionally share mutable children.
• Identity confusion: clones that must have new IDs/timestamps; define what resets on clone.
• Versioning: if objects evolve, update clone logic and tests.

Testing Checklist

• Modifying a clone does not mutate the original (for fields meant to be copied).
• “Reset-on-clone” fields behave correctly (IDs, timestamps, caches).
• Registry returns the expected concrete type for each key.