Swift - Operator Overloading

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Operator overloading is used to redefine the behavior of existing operators for custom types.

Operator Overloading in Swift

Operator overloading is a powerful technique in Swift programming. Operator overloading allows us to change the working of the existing operators like +, -, /, *, %, etc. with the customized code.

It allows you to define custom implementations for operators.

Syntax for Overloading an Operator

To overload an operator, we have to define the behaviour of that operator using the "static func" keyword.

Following is the syntax for operator overloading −

static func operatorSymbol(lhs: Type, rhs: Type) -> ReturnType {
    // Implementation
}

Consider this example, where we are overloading the addition operator (+) for a Vector type:

struct Vector {
    var x: Double
    var y: Double
}

static func + (lhs: Vector, rhs: Vector) -> Vector {
    return Vector(x: lhs.x + rhs.x, y: lhs.y + rhs.y)
}

Types of Operator Overloading

Swift supports overloading operators in three forms:

• Infix Operators: The operators which are placed between two operands.
• Prefix Operators: The operators which are placed before the operands.
• Postfix Operators: The operators which are placed after the operands.

Example of Infix Operator Overloading

Swift program to overload the + operator to add two distances represented in kilometers.

import Foundation

struct Distance {
    var km: Int

    static func + (lhs: Distance, rhs: Distance) -> Distance {
        return Distance(km: lhs.km + rhs.km)
    }
}

let morningRun = Distance(km: 5)
let eveningRun = Distance(km: 7)

let totalDistance = morningRun + eveningRun
print("Total distance run: \(totalDistance.km) km")

Output

Total distance run: 12 km

Example of Prefix Operator Overloading

Swift program to overload the prefix -- operator to decrease a vehicle’s fuel level.

import Foundation

prefix operator --

struct FuelTank {
    var liters: Int

    static prefix func -- (tank: inout FuelTank) {
        tank.liters -= 5
    }
}

var myCar = FuelTank(liters: 50)
--myCar
print("Fuel left in tank: \(myCar.liters) liters")

Output

Fuel left in tank: 45 liters

Example of Postfix Operator Overloading

Swift program to overload a postfix ++ operator to increase a passenger count in an elevator system.

import Foundation

postfix operator ++

struct Elevator {
    var passengers: Int

    static postfix func ++ (elevator: inout Elevator) {
        elevator.passengers += 1
    }
}

var officeLift = Elevator(passengers: 4)
officeLift++
print("Current passengers in elevator: \(officeLift.passengers)")

Output

Current passengers in elevator: 5

Operators Overloading More Examples

Practice the following examples to understand the concepts of operators overloading in Swift:

Example 1

Swift program to overload + operator to calculate the sum of two complex numbers.

import Foundation
struct ComplexNumber {
   var real: Double
   var imag: Double
        
   // Overloading + operator to add two complex numbers
   static func+(left: ComplexNumber, right: ComplexNumber) -> ComplexNumber {
      return ComplexNumber(real: left.real + right.real, imag: left.imag + right.imag)
   }
}
let complexNumber1 = ComplexNumber(real: 2.1, imag: 2.0)
let complexNumber2 = ComplexNumber(real: 6.1, imag: 9.0)

// Calling + operator to add two complex numbers
let sumOfComplexNumbers = complexNumber1 + complexNumber2
print(sumOfComplexNumbers)

Output

ComplexNumber(real: 8.2, imag: 11.0)

Example 2

Swift program to overload custom prefix operator.

import Foundation

struct Car {
   var price: Double

   // Overloading the custom prefix operator "++" to increase the price of car
   static prefix func ++ (carPrice: inout Car) {
      carPrice.price += 500000.0
   }
}
var currentPrice = Car(price: 2500000.0)

// Calling the custom ++ operator to increase the car price ++currentPrice 
print("Updated car price is", currentPrice.price) 

Output

Updated car price is 2500000.0

Defining a Custom Operator

Swift allows you to create a custom operator where you can define new operators with custom symbols. In this example, we create a custom infix operator ** to perform exponentiation between two integers.

Example

Swift program to define and use a custom ** operator to calculate compound growth like interest or investment value.

import Foundation

// Define custom infix operator for exponentiation
infix operator ** : MultiplicationPrecedence

func ** (lhs: Int, rhs: Int) -> Int {
    return Int(pow(Double(lhs), Double(rhs)))
}

// Calculating future value
let baseAmount = 2
let years = 5

let result = baseAmount ** years
print("\(baseAmount) raised to the power \(years) is \(result)")

Output

2 raised to the power 5 is 32

Limitation of Operator Overloading

The following are the limitations of operator overloading −

• In Swift, you can overload limited operators like arithmetic and customized operators.
• While overloading operators, you are not allowed to change the precedence or associativity of the operators.
• Swift does not support short-circuiting behaviour for overloading logical operators.
• Do not overuse operator overloading because it makes your code difficult to read and understand.

Difference Between Operator Function and Normal Functions

The following are the major differences between the operator functions and normal functions −