Guides and Sample Code


The Swift Programming Language (Swift 4)

On This Page

The Basics

Swift is a new programming language for iOS, macOS, watchOS, and tvOS app development. Nonetheless, many parts of Swift will be familiar from your experience of developing in C and Objective-C.

Swift provides its own versions of all fundamental C and Objective-C types, including Int for integers, Double and Float for floating-point values, Bool for Boolean values, and String for textual data. Swift also provides powerful versions of the three primary collection types, Array, Set, and Dictionary, as described in Collection Types.

Like C, Swift uses variables to store and refer to values by an identifying name. Swift also makes extensive use of variables whose values cannot be changed. These are known as constants, and are much more powerful than constants in C. Constants are used throughout Swift to make code safer and clearer in intent when you work with values that do not need to change.

In addition to familiar types, Swift introduces advanced types not found in Objective-C, such as tuples. Tuples enable you to create and pass around groupings of values. You can use a tuple to return multiple values from a function as a single compound value.

Swift also introduces optional types, which handle the absence of a value. Optionals say either “there is a value, and it equals x” or “there isn’t a value at all”. Using optionals is similar to using nil with pointers in Objective-C, but they work for any type, not just classes. Not only are optionals safer and more expressive than nil pointers in Objective-C, they are at the heart of many of Swift’s most powerful features.

Swift is a type-safe language, which means the language helps you to be clear about the types of values your code can work with. If part of your code expects a String, type safety prevents you from passing it an Int by mistake. Likewise, type safety prevents you from accidentally passing an optional String to a piece of code that expects a nonoptional String. Type safety helps you catch and fix errors as early as possible in the development process.

Constants and Variables

Constants and variables associate a name (such as maximumNumberOfLoginAttempts or welcomeMessage) with a value of a particular type (such as the number 10 or the string "Hello"). The value of a constant cannot be changed once it is set, whereas a variable can be set to a different value in the future.

Declaring Constants and Variables

Constants and variables must be declared before they are used. You declare constants with the let keyword and variables with the var keyword. Here’s an example of how constants and variables can be used to track the number of login attempts a user has made:

  1. let maximumNumberOfLoginAttempts = 10
  2. var currentLoginAttempt = 0

This code can be read as:

“Declare a new constant called maximumNumberOfLoginAttempts, and give it a value of 10. Then, declare a new variable called currentLoginAttempt, and give it an initial value of 0.”

In this example, the maximum number of allowed login attempts is declared as a constant, because the maximum value never changes. The current login attempt counter is declared as a variable, because this value must be incremented after each failed login attempt.

You can declare multiple constants or multiple variables on a single line, separated by commas:

  1. var x = 0.0, y = 0.0, z = 0.0

Type Annotations

You can provide a type annotation when you declare a constant or variable, to be clear about the kind of values the constant or variable can store. Write a type annotation by placing a colon after the constant or variable name, followed by a space, followed by the name of the type to use.

This example provides a type annotation for a variable called welcomeMessage, to indicate that the variable can store String values:

  1. var welcomeMessage: String

The colon in the declaration means “…of type…,” so the code above can be read as:

“Declare a variable called welcomeMessage that is of type String.”

The phrase “of type String” means “can store any String value.” Think of it as meaning “the type of thing” (or “the kind of thing”) that can be stored.

The welcomeMessage variable can now be set to any string value without error:

  1. welcomeMessage = "Hello"

You can define multiple related variables of the same type on a single line, separated by commas, with a single type annotation after the final variable name:

  1. var red, green, blue: Double

Naming Constants and Variables

Constant and variable names can contain almost any character, including Unicode characters:

  1. let π = 3.14159
  2. let 你好 = "你好世界"
  3. let 🐶🐮 = "dogcow"

Constant and variable names cannot contain whitespace characters, mathematical symbols, arrows, private-use (or invalid) Unicode code points, or line- and box-drawing characters. Nor can they begin with a number, although numbers may be included elsewhere within the name.

Once you’ve declared a constant or variable of a certain type, you can’t redeclare it again with the same name, or change it to store values of a different type. Nor can you change a constant into a variable or a variable into a constant.

You can change the value of an existing variable to another value of a compatible type. In this example, the value of friendlyWelcome is changed from "Hello!" to "Bonjour!":

  1. var friendlyWelcome = "Hello!"
  2. friendlyWelcome = "Bonjour!"
  3. // friendlyWelcome is now "Bonjour!"

Unlike a variable, the value of a constant cannot be changed once it is set. Attempting to do so is reported as an error when your code is compiled:

  1. let languageName = "Swift"
  2. languageName = "Swift++"
  3. // This is a compile-time error: languageName cannot be changed.

Printing Constants and Variables

You can print the current value of a constant or variable with the print(_:separator:terminator:) function:

  1. print(friendlyWelcome)
  2. // Prints "Bonjour!"

The print(_:separator:terminator:) function is a global function that prints one or more values to an appropriate output. In Xcode, for example, the print(_:separator:terminator:) function prints its output in Xcode’s “console” pane. The separator and terminator parameter have default values, so you can omit them when you call this function. By default, the function terminates the line it prints by adding a line break. To print a value without a line break after it, pass an empty string as the terminator—for example, print(someValue, terminator: ""). For information about parameters with default values, see Default Parameter Values.

Swift uses string interpolation to include the name of a constant or variable as a placeholder in a longer string, and to prompt Swift to replace it with the current value of that constant or variable. Wrap the name in parentheses and escape it with a backslash before the opening parenthesis:

  1. print("The current value of friendlyWelcome is \(friendlyWelcome)")
  2. // Prints "The current value of friendlyWelcome is Bonjour!"


Use comments to include nonexecutable text in your code, as a note or reminder to yourself. Comments are ignored by the Swift compiler when your code is compiled.

Comments in Swift are very similar to comments in C. Single-line comments begin with two forward-slashes (//):

  1. // This is a comment.

Multiline comments start with a forward-slash followed by an asterisk (/*) and end with an asterisk followed by a forward-slash (*/):

  1. /* This is also a comment
  2. but is written over multiple lines. */

Unlike multiline comments in C, multiline comments in Swift can be nested inside other multiline comments. You write nested comments by starting a multiline comment block and then starting a second multiline comment within the first block. The second block is then closed, followed by the first block:

  1. /* This is the start of the first multiline comment.
  2. /* This is the second, nested multiline comment. */
  3. This is the end of the first multiline comment. */

Nested multiline comments enable you to comment out large blocks of code quickly and easily, even if the code already contains multiline comments.


Unlike many other languages, Swift does not require you to write a semicolon (;) after each statement in your code, although you can do so if you wish. However, semicolons are required if you want to write multiple separate statements on a single line:

  1. let cat = "🐱"; print(cat)
  2. // Prints "🐱"


Integers are whole numbers with no fractional component, such as 42 and -23. Integers are either signed (positive, zero, or negative) or unsigned (positive or zero).

Swift provides signed and unsigned integers in 8, 16, 32, and 64 bit forms. These integers follow a naming convention similar to C, in that an 8-bit unsigned integer is of type UInt8, and a 32-bit signed integer is of type Int32. Like all types in Swift, these integer types have capitalized names.

Integer Bounds

You can access the minimum and maximum values of each integer type with its min and max properties:

  1. let minValue = UInt8.min // minValue is equal to 0, and is of type UInt8
  2. let maxValue = UInt8.max // maxValue is equal to 255, and is of type UInt8

The values of these properties are of the appropriate-sized number type (such as UInt8 in the example above) and can therefore be used in expressions alongside other values of the same type.


In most cases, you don’t need to pick a specific size of integer to use in your code. Swift provides an additional integer type, Int, which has the same size as the current platform’s native word size:

  • On a 32-bit platform, Int is the same size as Int32.

  • On a 64-bit platform, Int is the same size as Int64.

Unless you need to work with a specific size of integer, always use Int for integer values in your code. This aids code consistency and interoperability. Even on 32-bit platforms, Int can store any value between -2,147,483,648 and 2,147,483,647, and is large enough for many integer ranges.


Swift also provides an unsigned integer type, UInt, which has the same size as the current platform’s native word size:

  • On a 32-bit platform, UInt is the same size as UInt32.

  • On a 64-bit platform, UInt is the same size as UInt64.

Floating-Point Numbers

Floating-point numbers are numbers with a fractional component, such as 3.14159, 0.1, and -273.15.

Floating-point types can represent a much wider range of values than integer types, and can store numbers that are much larger or smaller than can be stored in an Int. Swift provides two signed floating-point number types:

  • Double represents a 64-bit floating-point number.

  • Float represents a 32-bit floating-point number.

Type Safety and Type Inference

Swift is a type-safe language. A type safe language encourages you to be clear about the types of values your code can work with. If part of your code expects a String, you can’t pass it an Int by mistake.

Because Swift is type safe, it performs type checks when compiling your code and flags any mismatched types as errors. This enables you to catch and fix errors as early as possible in the development process.

Type-checking helps you avoid errors when you’re working with different types of values. However, this doesn’t mean that you have to specify the type of every constant and variable that you declare. If you don’t specify the type of value you need, Swift uses type inference to work out the appropriate type. Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.

Because of type inference, Swift requires far fewer type declarations than languages such as C or Objective-C. Constants and variables are still explicitly typed, but much of the work of specifying their type is done for you.

Type inference is particularly useful when you declare a constant or variable with an initial value. This is often done by assigning a literal value (or literal) to the constant or variable at the point that you declare it. (A literal value is a value that appears directly in your source code, such as 42 and 3.14159 in the examples below.)

For example, if you assign a literal value of 42 to a new constant without saying what type it is, Swift infers that you want the constant to be an Int, because you have initialized it with a number that looks like an integer:

  1. let meaningOfLife = 42
  2. // meaningOfLife is inferred to be of type Int

Likewise, if you don’t specify a type for a floating-point literal, Swift infers that you want to create a Double:

  1. let pi = 3.14159
  2. // pi is inferred to be of type Double

Swift always chooses Double (rather than Float) when inferring the type of floating-point numbers.

If you combine integer and floating-point literals in an expression, a type of Double will be inferred from the context:

  1. let anotherPi = 3 + 0.14159
  2. // anotherPi is also inferred to be of type Double

The literal value of 3 has no explicit type in and of itself, and so an appropriate output type of Double is inferred from the presence of a floating-point literal as part of the addition.

Numeric Literals

Integer literals can be written as:

  • A decimal number, with no prefix

  • A binary number, with a 0b prefix

  • An octal number, with a 0o prefix

  • A hexadecimal number, with a 0x prefix

All of these integer literals have a decimal value of 17:

  1. let decimalInteger = 17
  2. let binaryInteger = 0b10001 // 17 in binary notation
  3. let octalInteger = 0o21 // 17 in octal notation
  4. let hexadecimalInteger = 0x11 // 17 in hexadecimal notation

Floating-point literals can be decimal (with no prefix), or hexadecimal (with a 0x prefix). They must always have a number (or hexadecimal number) on both sides of the decimal point. Decimal floats can also have an optional exponent, indicated by an uppercase or lowercase e; hexadecimal floats must have an exponent, indicated by an uppercase or lowercase p.

For decimal numbers with an exponent of exp, the base number is multiplied by 10exp:

  • 1.25e2 means 1.25 x 102, or 125.0.

  • 1.25e-2 means 1.25 x 10-2, or 0.0125.

For hexadecimal numbers with an exponent of exp, the base number is multiplied by 2exp:

  • 0xFp2 means 15 x 22, or 60.0.

  • 0xFp-2 means 15 x 2-2, or 3.75.

All of these floating-point literals have a decimal value of 12.1875:

  1. let decimalDouble = 12.1875
  2. let exponentDouble = 1.21875e1
  3. let hexadecimalDouble = 0xC.3p0

Numeric literals can contain extra formatting to make them easier to read. Both integers and floats can be padded with extra zeros and can contain underscores to help with readability. Neither type of formatting affects the underlying value of the literal:

  1. let paddedDouble = 000123.456
  2. let oneMillion = 1_000_000
  3. let justOverOneMillion = 1_000_000.000_000_1

Numeric Type Conversion

Use the Int type for all general-purpose integer constants and variables in your code, even if they are known to be non-negative. Using the default integer type in everyday situations means that integer constants and variables are immediately interoperable in your code and will match the inferred type for integer literal values.

Use other integer types only when they are specifically needed for the task at hand, because of explicitly-sized data from an external source, or for performance, memory usage, or other necessary optimization. Using explicitly-sized types in these situations helps to catch any accidental value overflows and implicitly documents the nature of the data being used.

Integer Conversion

The range of numbers that can be stored in an integer constant or variable is different for each numeric type. An Int8 constant or variable can store numbers between -128 and 127, whereas a UInt8 constant or variable can store numbers between 0 and 255. A number that will not fit into a constant or variable of a sized integer type is reported as an error when your code is compiled:

  1. let cannotBeNegative: UInt8 = -1
  2. // UInt8 cannot store negative numbers, and so this will report an error
  3. let tooBig: Int8 = Int8.max + 1
  4. // Int8 cannot store a number larger than its maximum value,
  5. // and so this will also report an error

Because each numeric type can store a different range of values, you must opt in to numeric type conversion on a case-by-case basis. This opt-in approach prevents hidden conversion errors and helps make type conversion intentions explicit in your code.

To convert one specific number type to another, you initialize a new number of the desired type with the existing value. In the example below, the constant twoThousand is of type UInt16, whereas the constant one is of type UInt8. They cannot be added together directly, because they are not of the same type. Instead, this example calls UInt16(one) to create a new UInt16 initialized with the value of one, and uses this value in place of the original:

  1. let twoThousand: UInt16 = 2_000
  2. let one: UInt8 = 1
  3. let twoThousandAndOne = twoThousand + UInt16(one)

Because both sides of the addition are now of type UInt16, the addition is allowed. The output constant (twoThousandAndOne) is inferred to be of type UInt16, because it is the sum of two UInt16 values.

SomeType(ofInitialValue) is the default way to call the initializer of a Swift type and pass in an initial value. Behind the scenes, UInt16 has an initializer that accepts a UInt8 value, and so this initializer is used to make a new UInt16 from an existing UInt8. You can’t pass in any type here, however—it has to be a type for which UInt16 provides an initializer. Extending existing types to provide initializers that accept new types (including your own type definitions) is covered in Extensions.

Integer and Floating-Point Conversion

Conversions between integer and floating-point numeric types must be made explicit:

  1. let three = 3
  2. let pointOneFourOneFiveNine = 0.14159
  3. let pi = Double(three) + pointOneFourOneFiveNine
  4. // pi equals 3.14159, and is inferred to be of type Double

Here, the value of the constant three is used to create a new value of type Double, so that both sides of the addition are of the same type. Without this conversion in place, the addition would not be allowed.

Floating-point to integer conversion must also be made explicit. An integer type can be initialized with a Double or Float value:

  1. let integerPi = Int(pi)
  2. // integerPi equals 3, and is inferred to be of type Int

Floating-point values are always truncated when used to initialize a new integer value in this way. This means that 4.75 becomes 4, and -3.9 becomes -3.

Type Aliases

Type aliases define an alternative name for an existing type. You define type aliases with the typealias keyword.

Type aliases are useful when you want to refer to an existing type by a name that is contextually more appropriate, such as when working with data of a specific size from an external source:

  1. typealias AudioSample = UInt16

Once you define a type alias, you can use the alias anywhere you might use the original name:

  1. var maxAmplitudeFound = AudioSample.min
  2. // maxAmplitudeFound is now 0

Here, AudioSample is defined as an alias for UInt16. Because it is an alias, the call to AudioSample.min actually calls UInt16.min, which provides an initial value of 0 for the maxAmplitudeFound variable.


Swift has a basic Boolean type, called Bool. Boolean values are referred to as logical, because they can only ever be true or false. Swift provides two Boolean constant values, true and false:

  1. let orangesAreOrange = true
  2. let turnipsAreDelicious = false

The types of orangesAreOrange and turnipsAreDelicious have been inferred as Bool from the fact that they were initialized with Boolean literal values. As with Int and Double above, you don’t need to declare constants or variables as Bool if you set them to true or false as soon as you create them. Type inference helps make Swift code more concise and readable when it initializes constants or variables with other values whose type is already known.

Boolean values are particularly useful when you work with conditional statements such as the if statement:

  1. if turnipsAreDelicious {
  2. print("Mmm, tasty turnips!")
  3. } else {
  4. print("Eww, turnips are horrible.")
  5. }
  6. // Prints "Eww, turnips are horrible."

Conditional statements such as the if statement are covered in more detail in Control Flow.

Swift’s type safety prevents non-Boolean values from being substituted for Bool. The following example reports a compile-time error:

  1. let i = 1
  2. if i {
  3. // this example will not compile, and will report an error
  4. }

However, the alternative example below is valid:

  1. let i = 1
  2. if i == 1 {
  3. // this example will compile successfully
  4. }

The result of the i == 1 comparison is of type Bool, and so this second example passes the type-check. Comparisons like i == 1 are discussed in Basic Operators.

As with other examples of type safety in Swift, this approach avoids accidental errors and ensures that the intention of a particular section of code is always clear.


Tuples group multiple values into a single compound value. The values within a tuple can be of any type and do not have to be of the same type as each other.

In this example, (404, "Not Found") is a tuple that describes an HTTP status code. An HTTP status code is a special value returned by a web server whenever you request a web page. A status code of 404 Not Found is returned if you request a webpage that doesn’t exist.

  1. let http404Error = (404, "Not Found")
  2. // http404Error is of type (Int, String), and equals (404, "Not Found")

The (404, "Not Found") tuple groups together an Int and a String to give the HTTP status code two separate values: a number and a human-readable description. It can be described as “a tuple of type (Int, String)”.

You can create tuples from any permutation of types, and they can contain as many different types as you like. There’s nothing stopping you from having a tuple of type (Int, Int, Int), or (String, Bool), or indeed any other permutation you require.

You can decompose a tuple’s contents into separate constants or variables, which you then access as usual:

  1. let (statusCode, statusMessage) = http404Error
  2. print("The status code is \(statusCode)")
  3. // Prints "The status code is 404"
  4. print("The status message is \(statusMessage)")
  5. // Prints "The status message is Not Found"

If you only need some of the tuple’s values, ignore parts of the tuple with an underscore (_) when you decompose the tuple:

  1. let (justTheStatusCode, _) = http404Error
  2. print("The status code is \(justTheStatusCode)")
  3. // Prints "The status code is 404"

Alternatively, access the individual element values in a tuple using index numbers starting at zero:

  1. print("The status code is \(http404Error.0)")
  2. // Prints "The status code is 404"
  3. print("The status message is \(http404Error.1)")
  4. // Prints "The status message is Not Found"

You can name the individual elements in a tuple when the tuple is defined:

  1. let http200Status = (statusCode: 200, description: "OK")

If you name the elements in a tuple, you can use the element names to access the values of those elements:

  1. print("The status code is \(http200Status.statusCode)")
  2. // Prints "The status code is 200"
  3. print("The status message is \(http200Status.description)")
  4. // Prints "The status message is OK"

Tuples are particularly useful as the return values of functions. A function that tries to retrieve a web page might return the (Int, String) tuple type to describe the success or failure of the page retrieval. By returning a tuple with two distinct values, each of a different type, the function provides more useful information about its outcome than if it could only return a single value of a single type. For more information, see Functions with Multiple Return Values.


You use optionals in situations where a value may be absent. An optional represents two possibilities: Either there is a value, and you can unwrap the optional to access that value, or there isn’t a value at all.

Here’s an example of how optionals can be used to cope with the absence of a value. Swift’s Int type has an initializer which tries to convert a String value into an Int value. However, not every string can be converted into an integer. The string "123" can be converted into the numeric value 123, but the string "hello, world" does not have an obvious numeric value to convert to.

The example below uses the initializer to try to convert a String into an Int:

  1. let possibleNumber = "123"
  2. let convertedNumber = Int(possibleNumber)
  3. // convertedNumber is inferred to be of type "Int?", or "optional Int"

Because the initializer might fail, it returns an optional Int, rather than an Int. An optional Int is written as Int?, not Int. The question mark indicates that the value it contains is optional, meaning that it might contain some Int value, or it might contain no value at all. (It can’t contain anything else, such as a Bool value or a String value. It’s either an Int, or it’s nothing at all.)


You set an optional variable to a valueless state by assigning it the special value nil:

  1. var serverResponseCode: Int? = 404
  2. // serverResponseCode contains an actual Int value of 404
  3. serverResponseCode = nil
  4. // serverResponseCode now contains no value

If you define an optional variable without providing a default value, the variable is automatically set to nil for you:

  1. var surveyAnswer: String?
  2. // surveyAnswer is automatically set to nil

If Statements and Forced Unwrapping

You can use an if statement to find out whether an optional contains a value by comparing the optional against nil. You perform this comparison with the “equal to” operator (==) or the “not equal to” operator (!=).

If an optional has a value, it is considered to be “not equal to” nil:

  1. if convertedNumber != nil {
  2. print("convertedNumber contains some integer value.")
  3. }
  4. // Prints "convertedNumber contains some integer value."

Once you’re sure that the optional does contain a value, you can access its underlying value by adding an exclamation mark (!) to the end of the optional’s name. The exclamation mark effectively says, “I know that this optional definitely has a value; please use it.” This is known as forced unwrapping of the optional’s value:

  1. if convertedNumber != nil {
  2. print("convertedNumber has an integer value of \(convertedNumber!).")
  3. }
  4. // Prints "convertedNumber has an integer value of 123."

For more on the if statement, see Control Flow.

Optional Binding

You use optional binding to find out whether an optional contains a value, and if so, to make that value available as a temporary constant or variable. Optional binding can be used with if and while statements to check for a value inside an optional, and to extract that value into a constant or variable, as part of a single action. if and while statements are described in more detail in Control Flow.

Write an optional binding for an if statement as follows:

  • if let constantName = someOptional {
  •     statements
  • }

You can rewrite the possibleNumber example from the Optionals section to use optional binding rather than forced unwrapping:

  1. if let actualNumber = Int(possibleNumber) {
  2. print("\"\(possibleNumber)\" has an integer value of \(actualNumber)")
  3. } else {
  4. print("\"\(possibleNumber)\" could not be converted to an integer")
  5. }
  6. // Prints ""123" has an integer value of 123"

This code can be read as:

“If the optional Int returned by Int(possibleNumber) contains a value, set a new constant called actualNumber to the value contained in the optional.”

If the conversion is successful, the actualNumber constant becomes available for use within the first branch of the if statement. It has already been initialized with the value contained within the optional, and so there is no need to use the ! suffix to access its value. In this example, actualNumber is simply used to print the result of the conversion.

You can use both constants and variables with optional binding. If you wanted to manipulate the value of actualNumber within the first branch of the if statement, you could write if var actualNumber instead, and the value contained within the optional would be made available as a variable rather than a constant.

You can include as many optional bindings and Boolean conditions in a single if statement as you need to, separated by commas. If any of the values in the optional bindings are nil or any Boolean condition evaluates to false, the whole if statement’s condition is considered to be false. The following if statements are equivalent:

  1. if let firstNumber = Int("4"), let secondNumber = Int("42"), firstNumber < secondNumber && secondNumber < 100 {
  2. print("\(firstNumber) < \(secondNumber) < 100")
  3. }
  4. // Prints "4 < 42 < 100"
  5. if let firstNumber = Int("4") {
  6. if let secondNumber = Int("42") {
  7. if firstNumber < secondNumber && secondNumber < 100 {
  8. print("\(firstNumber) < \(secondNumber) < 100")
  9. }
  10. }
  11. }
  12. // Prints "4 < 42 < 100"

Implicitly Unwrapped Optionals

As described above, optionals indicate that a constant or variable is allowed to have “no value”. Optionals can be checked with an if statement to see if a value exists, and can be conditionally unwrapped with optional binding to access the optional’s value if it does exist.

Sometimes it is clear from a program’s structure that an optional will always have a value, after that value is first set. In these cases, it is useful to remove the need to check and unwrap the optional’s value every time it is accessed, because it can be safely assumed to have a value all of the time.

These kinds of optionals are defined as implicitly unwrapped optionals. You write an implicitly unwrapped optional by placing an exclamation mark (String!) rather than a question mark (String?) after the type that you want to make optional.

Implicitly unwrapped optionals are useful when an optional’s value is confirmed to exist immediately after the optional is first defined and can definitely be assumed to exist at every point thereafter. The primary use of implicitly unwrapped optionals in Swift is during class initialization, as described in Unowned References and Implicitly Unwrapped Optional Properties.

An implicitly unwrapped optional is a normal optional behind the scenes, but can also be used like a nonoptional value, without the need to unwrap the optional value each time it is accessed. The following example shows the difference in behavior between an optional string and an implicitly unwrapped optional string when accessing their wrapped value as an explicit String:

  1. let possibleString: String? = "An optional string."
  2. let forcedString: String = possibleString! // requires an exclamation mark
  3. let assumedString: String! = "An implicitly unwrapped optional string."
  4. let implicitString: String = assumedString // no need for an exclamation mark

You can think of an implicitly unwrapped optional as giving permission for the optional to be unwrapped automatically whenever it is used. Rather than placing an exclamation mark after the optional’s name each time you use it, you place an exclamation mark after the optional’s type when you declare it.

You can still treat an implicitly unwrapped optional like a normal optional, to check if it contains a value:

  1. if assumedString != nil {
  2. print(assumedString)
  3. }
  4. // Prints "An implicitly unwrapped optional string."

You can also use an implicitly unwrapped optional with optional binding, to check and unwrap its value in a single statement:

  1. if let definiteString = assumedString {
  2. print(definiteString)
  3. }
  4. // Prints "An implicitly unwrapped optional string."

Error Handling

You use error handling to respond to error conditions your program may encounter during execution.

In contrast to optionals, which can use the presence or absence of a value to communicate success or failure of a function, error handling allows you to determine the underlying cause of failure, and, if necessary, propagate the error to another part of your program.

When a function encounters an error condition, it throws an error. That function’s caller can then catch the error and respond appropriately.

  1. func canThrowAnError() throws {
  2. // this function may or may not throw an error
  3. }

A function indicates that it can throw an error by including the throws keyword in its declaration. When you call a function that can throw an error, you prepend the try keyword to the expression.

Swift automatically propagates errors out of their current scope until they are handled by a catch clause.

  1. do {
  2. try canThrowAnError()
  3. // no error was thrown
  4. } catch {
  5. // an error was thrown
  6. }

A do statement creates a new containing scope, which allows errors to be propagated to one or more catch clauses.

Here’s an example of how error handling can be used to respond to different error conditions:

  1. func makeASandwich() throws {
  2. // ...
  3. }
  4. do {
  5. try makeASandwich()
  6. eatASandwich()
  7. } catch SandwichError.outOfCleanDishes {
  8. washDishes()
  9. } catch SandwichError.missingIngredients(let ingredients) {
  10. buyGroceries(ingredients)
  11. }

In this example, the makeASandwich() function will throw an error if no clean dishes are available or if any ingredients are missing. Because makeASandwich() can throw an error, the function call is wrapped in a try expression. By wrapping the function call in a do statement, any errors that are thrown will be propagated to the provided catch clauses.

If no error is thrown, the eatASandwich() function is called. If an error is thrown and it matches the SandwichError.outOfCleanDishes case, then the washDishes() function will be called. If an error is thrown and it matches the SandwichError.missingIngredients case, then the buyGroceries(_:) function is called with the associated [String] value captured by the catch pattern.

Throwing, catching, and propagating errors is covered in greater detail in Error Handling.

Assertions and Preconditions

Assertions and preconditions are checks that happen at runtime. You use them to make sure an essential condition is satisfied before executing any further code. If the Boolean condition in the assertion or precondition evaluates to true, code execution continues as usual. If the condition evaluates to false, the current state of the program is invalid; code execution ends, and your app is terminated.

You use assertions and preconditions to express the assumptions and expectations that you make while coding, so you can include them as part of your code. Assertions help you find mistakes and incorrect assumptions during development, and preconditions help you detect issues in production. In addition to verifying your expectations at runtime, assertions and preconditions also become a useful form of documentation within the code. Unlike the error conditions discussed in Error Handling above, assertions and preconditions are not used for recoverable or expected errors. Because a failed assertion or precondition indicates an invalid program state, there is no way to catch a failed assertion.

Using assertions and preconditions is not a substitute for designing your code in such a way that invalid conditions are unlikely to arise. However, using them to enforce valid data and state causes your app to terminate more predictably if an invalid state occurs, and helps makes the problem easier to debug. Stopping execution as soon as an invalid state is detected also helps limit the damage caused by that invalid state.

The difference between assertions and preconditions is in when they are checked: Assertions are checked only in debug builds, but preconditions are checked in both debug and production builds. In production builds, the condition inside an assertion isn’t evaluated. This means you can use as many assertions as you want during your development process, without impacting the performance in production.

Debugging with Assertions

You write an assertion by calling the assert(_:_:file:line:) function from the Swift standard library. You pass this function an expression that evaluates to true or false and a message to display if the result of the condition is false. For example:

  1. let age = -3
  2. assert(age >= 0, "A person's age can't be less than zero.")
  3. // This assertion fails because -3 is not >= 0.

In this example, code execution continues if age >= 0 evaluates to true, that is, if the value of age is positive. If the value of age is negative, as in the code above, then age >= 0 evaluates to false, and the assertion fails, terminating the application.

You can omit the assertion message—for example, when it would just repeat the condition as prose.

  1. assert(age >= 0)

If the code already checks the condition, you use the assertionFailure(_:file:line:) function to indicate that an assertion has failed. For example:

  1. if age > 10 {
  2. print("You can ride the roller-coaster or the ferris wheel.")
  3. } else if age > 0 {
  4. print("You can ride the ferris wheel.")
  5. } else {
  6. assertionFailure("A person's age can't be less than zero.")
  7. }

Enforcing Preconditions

Use a precondition whenever a condition has the potential to be false, but must definitely be true for your code to continue execution. For example, use a precondition to check that a subscript is not out of bounds, or to check that a function has been passed a valid value.

You write a precondition by calling the precondition(_:_:file:line:) function. You pass this function an expression that evaluates to true or false and a message to display if the result of the condition is false. For example:

  1. // In the implementation of a subscript...
  2. precondition(index > 0, "Index must be greater than zero.")

You can also call the preconditionFailure(_:file:line:) function to indicate that a failure has occurred—for example, if the default case of a switch was taken, but all valid input data should have been handled by one of the switch’s other cases.