Comparable

The Comparable protocol in Swift is used to define a sort order for instances of a type. To add Comparable conformance to custom types, define the < and == operators as static methods of those types.

Syntax

struct StructName: Comparable {
    let var: Int

    static func < (lhs: StructName, rhs: StructName) -> Bool {
        return lhs.var < rhs.var
    }

    static func == (lhs: StructName, rhs: StructName) -> Bool {
        return lhs.var == rhs.var
    }
}

In the implementation of the Comparable protocol of the custom type defined below, the Comparable keyword is used in the struct declaration, then static methods for < and == are defined.

Example

struct Building: Comparable {
    let height: Int

    static func < (lhs: Building, rhs: Building) -> Bool {
        return lhs.height < rhs.height
    }

    static func == (lhs: Building, rhs: Building) -> Bool {
        return lhs.height == rhs.height
    }
}

let eiffelTower = Building(height: 300)
let burjKhalifa = Building(height: 828)
let sydneyTower = Building(height: 300)

In this example the custom type, Building, was created with a height constant, and static methods for the < and == comparison operators were defined. There are three instances of this type: eiffelTower, burjKhalifa and sydneyTower. The example conforms to the Comparable protocol, allowing each of the instances to be compared:

print(eiffelTower < burjKhalifa) // true
print(eiffelTower > sydneyTower) // false
print(sydneyTower == burjKhalifa) // false
print(sydneyTower != eiffelTower) // false