ProtocolVsClassWhenToUse [rule] v0.1.0

When defining a new type, ask what is being modelled. (1) A capability shared across unrelated concrete types → protocol. (2) A value without identity (a coordinate, a measurement, a record) → struct or enum. (3) A thing with identity — a network connection, a file handle, a long-lived UI controller, a node in a shared mutable graph — → class (or actor if it carries state across concurrent contexts). The default in Swift is value-with-protocol; classes are the exception that buys reference semantics.

Checks

• New types start as struct unless an explicit reason for reference semantics applies.
• Shared behaviour across unrelated concrete types is expressed via protocol, with default implementations in a protocol extension where useful — not via a class hierarchy.
• Inheritance for code reuse is treated as a smell; protocol composition is preferred (Identifiable & Hashable & Sendable).
• final is applied to classes that are not designed for subclassing — which is most classes.
• When state must be shared safely across concurrency boundaries, actor is preferred to a class plus locks.

Label

Reach for protocols + value types first; reach for classes only when reference identity is the point

ProtocolVsClassWhenToUse [rule] v0.1.0

When defining a new type, ask what is being modelled. (1) A capability shared across unrelated concrete types → protocol. (2) A value without identity (a coordinate, a measurement, a record) → struct or enum. (3) A thing with identity — a network connection, a file handle, a long-lived UI controller, a node in a shared mutable graph — → class (or actor if it carries state across concurrent contexts). The default in Swift is value-with-protocol; classes are the exception that buys reference semantics.

Checks

• New types start as struct unless an explicit reason for reference semantics applies.
• Shared behaviour across unrelated concrete types is expressed via protocol, with default implementations in a protocol extension where useful — not via a class hierarchy.
• Inheritance for code reuse is treated as a smell; protocol composition is preferred (Identifiable & Hashable & Sendable).
• final is applied to classes that are not designed for subclassing — which is most classes.
• When state must be shared safely across concurrency boundaries, actor is preferred to a class plus locks.

Label

Reach for protocols + value types first; reach for classes only when reference identity is the point

Label

Reach for protocols + value types first; reach for classes only when reference identity is the point