struct Basics / Memberwise Initializer

struct StructName {
    // Stored properties
    var property1: Type
    let property2: Type

    // Custom initializer (auto-generated if omitted)
    init(property1: Type, property2: Type) {
        self.property1 = property1
        self.property2 = property2
    }

    // Method (use mutating to modify properties)
    mutating func methodName() {
        property1 = newValue
    }
}

// Define a struct
struct Point {
    var x: Double
    var y: Double

    // Method that calculates the distance to another point
    func distance(to other: Point) -> Double {
        let dx = x - other.x
        let dy = y - other.y
        return (dx * dx + dy * dy).squareRoot()
    }

    // Mutating method that modifies properties
    mutating func move(dx: Double, dy: Double) {
        x += dx
        y += dy
    }

    // Computed description property
    var description: String {
        return "(\(x), \(y))"
    }
}

// Using the memberwise initializer (no custom init defined)
var p1 = Point(x: 0.0, y: 0.0)
let p2 = Point(x: 3.0, y: 4.0)

print("p1: \(p1.description)")
print("p2: \(p2.description)")
print("Distance: \(p1.distance(to: p2))")

// Value type: copy semantics
var p3 = p1      // A copy is made
p3.move(dx: 1.0, dy: 1.0)
print("p1 (unchanged): \(p1.description)")
print("p3 (after move): \(p3.description)")

// A struct declared with let cannot have its properties modified
let fixedPoint = Point(x: 5.0, y: 5.0)
// fixedPoint.move(dx: 1, dy: 1)  // Error: cannot mutate a let constant

A Swift struct is a value type, so a copy is created whenever you assign it to a variable or pass it to a function. This makes it easier to avoid bugs caused by unintended shared state.

If you do not define a custom initializer, Swift automatically generates a memberwise initializer that accepts all stored properties as arguments. An instance declared with let makes all of its properties immutable — you cannot call mutating methods on it either.

For reference types, see class basics / init() / deinit().