Init

In Swift, types like classes and structs have init methods. The init() method sets up all the initial values of the type instance.

With multiple init methods, and convenience init methods, we set up values in an efficient way. And failable init let us exit the init method early if the arguments are invalid.

Init, convenience

This example shows a designated initializer, which uses the init keyword, and a convenience initializer, which has the "convenience" keyword.

Part 1 We have an init method. This receives any number of parameters and uses the arguments received to set up the fields within the class.

Part 2 A convenience init method calls into the self.init() method. It can make a class easier to create by specifying fewer arguments.

Part 3 To use an init method, we specify the type name with the arguments to init(). The init method is then executed.

Part 4 The convenience init method can also be accessed by specifying the type name, with the appropriate arguments.

class Bird {
    var color = ""
    var feathers = 0
    // Part 1: designated initializer in class.
    init(c: String, f: Int) {
        self.color = c
        self.feathers = f
    }
    // Part 2: convenience initializer, which calls the designated initializer.
    convenience init(c: String) {
        self.init(c: c, f: 100)
    }
}

// Part 3: use designated initializer.
var b = Bird(c: "blue", f: 200)
print("Bird: \(b.color) \(b.feathers)")

// Part 4: use convenience initializer.
var b2 = Bird(c: "red")
print("Bird: \(b2.color) \(b2.feathers)")Bird: blue 200
Bird: red 100

Multiple `init`

Sometimes a class can benefit from multiple separate init methods. It is sometimes possible to use a convenience init in this case, but not necessary.

class Example {
    var size1 = 0
    var size2 = 0
    // An initializer.
    init(size1: Int, size2: Int) {
        self.size1 = size1
        self.size2 = size2
    }
    // Another initializer.
    init(allSizes: Int) {
        self.size1 = allSizes
        self.size2 = allSizes
    }
}

// Use the first initializer method.
var e = Example(size1: 10, size2: 20)
print("Example \(e.size1) \(e.size2)")
// Use the second initializer.
var e2 = Example(allSizes: 30)
print("Example \(e2.size1) \(e2.size2)")Example 10 20
Example 30 30

Failable

Suppose we want to create a class or struct but only if the arguments to the init() method are valid. Otherwise, we want to return a nil value.

Part 1 We specify a failable init. We use a question mark after the "init" keyword for this purpose.

Part 2 A failable initializer is used like any other init method, but it returns an optional value.

Part 3 We can test the value returned by a failable initializer with an if statement, or an if-let statement.

struct Test {
    var id = 0
    // Part 1: use failable initializer to avoid structs with id of 0.
    init?(id: Int) {
        if id == 0 { return nil }
        self.id = id
    }
}

// Part 2: use failable initializer.
let t = Test(id: 20)!
print("Test \(t.id)")

// Part 3: get nil struct from failable initializer.
let t2 = Test(id: 0)
if t2 == nil {
    print("Nil Test optional")
}Test 20
Nil Test optional

Init methods in classes and structs help us create objects in a consistent way. This keeps the quality of our programs higher and makes programs easier to read.

Init

Init, convenience

Multiple init

Failable

Multiple `init`