Protocols: define a capability

A school needs to print labels for different kinds of objects. A protocol describes requirements a conforming type must provide, such as a property or method. It lets code depend on a capability without requiring every type to inherit from one common class.

Conformance means implementing the requirements under their access and type rules. Both structs and classes can conform. A parameter written any Protocol uses an existential value that can hold a conforming instance; generics offer another way to preserve more static type information.

Save this complete program as main.swift and run it with the Swift toolchain. No separate input file is needed.

Swift · complete program

protocol Labelled {
  var label: String { get }
}
struct Student: Labelled {
  let label: String
}
func display(_ item: any Labelled) { print(item.label) }
display(Student(label: "Diya"))

Expected output

Diya

Follow the execution

The protocol requires a readable String label. Student’s stored property satisfies that requirement. Display accesses only the promised capability and does not need knowledge of student-specific fields.

Try this program in the Swift compiler. Paste the complete source above.

Your task: Could a Room struct with a String label also be passed to display?

Swift · complete program

protocol Labelled {
  var label: String { get }
}
struct Student: Labelled {
  let label: String
}
func display(_ item: any Labelled) { print(item.label) }
display(Student(label: "Diya"))

Worked answer

Yes, if Room explicitly conforms to Labelled and satisfies the requirement. The function depends on the capability, not on the Student type or class inheritance.

A protocol is not an automatic implementation of all its requirements. Associated types, Self requirements and mutation introduce additional rules; use generic constraints when the relationship between types matters rather than treating every existential as interchangeable.

Yes, if Room explicitly conforms to Labelled and satisfies the requirement. The function depends on the capability, not on the Student type or class inheritance. A protocol is not an automatic implementation of all its requirements.

Further reading: official Swift language guide.