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The Swift Programming Language (Swift 3.0.1)

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Error Handling

Error handling is the process of responding to and recovering from error conditions in your program. Swift provides first-class support for throwing, catching, propagating, and manipulating recoverable errors at runtime.

Some operations aren’t guaranteed to always complete execution or produce a useful output. Optionals are used to represent the absence of a value, but when an operation fails, it’s often useful to understand what caused the failure, so that your code can respond accordingly.

As an example, consider the task of reading and processing data from a file on disk. There are a number of ways this task can fail, including the file not existing at the specified path, the file not having read permissions, or the file not being encoded in a compatible format. Distinguishing among these different situations allows a program to resolve some errors and to communicate to the user any errors it can’t resolve.

Representing and Throwing Errors

In Swift, errors are represented by values of types that conform to the Error protocol. This empty protocol indicates that a type can be used for error handling.

Swift enumerations are particularly well suited to modeling a group of related error conditions, with associated values allowing for additional information about the nature of an error to be communicated. For example, here’s how you might represent the error conditions of operating a vending machine inside a game:

  1. enum VendingMachineError: Error {
  2. case invalidSelection
  3. case insufficientFunds(coinsNeeded: Int)
  4. case outOfStock
  5. }

Throwing an error lets you indicate that something unexpected happened and the normal flow of execution can’t continue. You use a throw statement to throw an error. For example, the following code throws an error to indicate that five additional coins are needed by the vending machine:

  1. throw VendingMachineError.insufficientFunds(coinsNeeded: 5)

Handling Errors

When an error is thrown, some surrounding piece of code must be responsible for handling the error—for example, by correcting the problem, trying an alternative approach, or informing the user of the failure.

There are four ways to handle errors in Swift. You can propagate the error from a function to the code that calls that function, handle the error using a do-catch statement, handle the error as an optional value, or assert that the error will not occur. Each approach is described in a section below.

When a function throws an error, it changes the flow of your program, so it’s important that you can quickly identify places in your code that can throw errors. To identify these places in your code, write the try keyword—or the try? or try! variation—before a piece of code that calls a function, method, or initializer that can throw an error. These keywords are described in the sections below.

Propagating Errors Using Throwing Functions

To indicate that a function, method, or initializer can throw an error, you write the throws keyword in the function’s declaration after its parameters. A function marked with throws is called a throwing function. If the function specifies a return type, you write the throws keyword before the return arrow (->).

  1. func canThrowErrors() throws -> String
  2. func cannotThrowErrors() -> String

A throwing function propagates errors that are thrown inside of it to the scope from which it’s called.

In the example below, the VendingMachine class has a vend(itemNamed:) method that throws an appropriate VendingMachineError if the requested item is not available, is out of stock, or has a cost that exceeds the current deposited amount:

  1. struct Item {
  2. var price: Int
  3. var count: Int
  4. }
  5. class VendingMachine {
  6. var inventory = [
  7. "Candy Bar": Item(price: 12, count: 7),
  8. "Chips": Item(price: 10, count: 4),
  9. "Pretzels": Item(price: 7, count: 11)
  10. ]
  11. var coinsDeposited = 0
  12. func vend(itemNamed name: String) throws {
  13. guard let item = inventory[name] else {
  14. throw VendingMachineError.invalidSelection
  15. }
  16. guard item.count > 0 else {
  17. throw VendingMachineError.outOfStock
  18. }
  19. guard item.price <= coinsDeposited else {
  20. throw VendingMachineError.insufficientFunds(coinsNeeded: item.price - coinsDeposited)
  21. }
  22. coinsDeposited -= item.price
  23. var newItem = item
  24. newItem.count -= 1
  25. inventory[name] = newItem
  26. print("Dispensing \(name)")
  27. }
  28. }

The implementation of the vend(itemNamed:) method uses guard statements to exit the method early and throw appropriate errors if any of the requirements for purchasing a snack aren’t met. Because a throw statement immediately transfers program control, an item will be vended only if all of these requirements are met.

Because the vend(itemNamed:) method propagates any errors it throws, any code that calls this method must either handle the errors—using a do-catch statement, try?, or try!—or continue to propagate them. For example, the buyFavoriteSnack(person:vendingMachine:) in the example below is also a throwing function, and any errors that the vend(itemNamed:) method throws will propagate up to the point where the buyFavoriteSnack(person:vendingMachine:) function is called.

  1. let favoriteSnacks = [
  2. "Alice": "Chips",
  3. "Bob": "Licorice",
  4. "Eve": "Pretzels",
  5. ]
  6. func buyFavoriteSnack(person: String, vendingMachine: VendingMachine) throws {
  7. let snackName = favoriteSnacks[person] ?? "Candy Bar"
  8. try vendingMachine.vend(itemNamed: snackName)
  9. }

In this example, the buyFavoriteSnack(person: vendingMachine:) function looks up a given person’s favorite snack and tries to buy it for them by calling the vend(itemNamed:) method. Because the vend(itemNamed:) method can throw an error, it’s called with the try keyword in front of it.

Throwing initializers can propagate errors in the same way as throwing functions. For example, the initializer for the PurchasedSnack structure in the listing below calls a throwing function as part of the initialization process, and it handles any errors that it encounters by propagating them to its caller.

  1. struct PurchasedSnack {
  2. let name: String
  3. init(name: String, vendingMachine: VendingMachine) throws {
  4. try vendingMachine.vend(itemNamed: name)
  5. self.name = name
  6. }
  7. }

Handling Errors Using Do-Catch

You use a do-catch statement to handle errors by running a block of code. If an error is thrown by the code in the do clause, it is matched against the catch clauses to determine which one of them can handle the error.

Here is the general form of a do-catch statement:

  • do {
  •     try expression
  •     statements
  • } catch pattern 1 {
  •     statements
  • } catch pattern 2 where condition {
  •     statements
  • }

You write a pattern after catch to indicate what errors that clause can handle. If a catch clause doesn’t have a pattern, the clause matches any error and binds the error to a local constant named error. For more information about pattern matching, see Patterns.

The catch clauses don’t have to handle every possible error that the code in its do clause can throw. If none of the catch clauses handle the error, the error propagates to the surrounding scope. However, the error must be handled by some surrounding scope—either by an enclosing do-catch clause that handles the error or by being inside a throwing function. For example, the following code handles all three cases of the VendingMachineError enumeration, but all other errors have to be handled by its surrounding scope:

  1. var vendingMachine = VendingMachine()
  2. vendingMachine.coinsDeposited = 8
  3. do {
  4. try buyFavoriteSnack(person: "Alice", vendingMachine: vendingMachine)
  5. } catch VendingMachineError.invalidSelection {
  6. print("Invalid Selection.")
  7. } catch VendingMachineError.outOfStock {
  8. print("Out of Stock.")
  9. } catch VendingMachineError.insufficientFunds(let coinsNeeded) {
  10. print("Insufficient funds. Please insert an additional \(coinsNeeded) coins.")
  11. }
  12. // Prints "Insufficient funds. Please insert an additional 2 coins."

In the above example, the buyFavoriteSnack(person:vendingMachine:) function is called in a try expression, because it can throw an error. If an error is thrown, execution immediately transfers to the catch clauses, which decide whether to allow propagation to continue. If no error is thrown, the remaining statements in the do statement are executed.

Converting Errors to Optional Values

You use try? to handle an error by converting it to an optional value. If an error is thrown while evaluating the try? expression, the value of the expression is nil. For example, in the following code x and y have the same value and behavior:

  1. func someThrowingFunction() throws -> Int {
  2. // ...
  3. }
  4. let x = try? someThrowingFunction()
  5. let y: Int?
  6. do {
  7. y = try someThrowingFunction()
  8. } catch {
  9. y = nil
  10. }

If someThrowingFunction() throws an error, the value of x and y is nil. Otherwise, the value of x and y is the value that the function returned. Note that x and y are an optional of whatever type someThrowingFunction() returns. Here the function returns an integer, so x and y are optional integers.

Using try? lets you write concise error handling code when you want to handle all errors in the same way. For example, the following code uses several approaches to fetch data, or returns nil if all of the approaches fail.

  1. func fetchData() -> Data? {
  2. if let data = try? fetchDataFromDisk() { return data }
  3. if let data = try? fetchDataFromServer() { return data }
  4. return nil
  5. }

Disabling Error Propagation

Sometimes you know a throwing function or method won’t, in fact, throw an error at runtime. On those occasions, you can write try! before the expression to disable error propagation and wrap the call in a runtime assertion that no error will be thrown. If an error actually is thrown, you’ll get a runtime error.

For example, the following code uses a loadImage(atPath:) function, which loads the image resource at a given path or throws an error if the image can’t be loaded. In this case, because the image is shipped with the application, no error will be thrown at runtime, so it is appropriate to disable error propagation.

  1. let photo = try! loadImage(atPath: "./Resources/John Appleseed.jpg")

Specifying Cleanup Actions

You use a defer statement to execute a set of statements just before code execution leaves the current block of code. This statement lets you do any necessary cleanup that should be performed regardless of how execution leaves the current block of code—whether it leaves because an error was thrown or because of a statement such as return or break. For example, you can use a defer statement to ensure that file descriptors are closed and manually allocated memory is freed.

A defer statement defers execution until the current scope is exited. This statement consists of the defer keyword and the statements to be executed later. The deferred statements may not contain any code that would transfer control out of the statements, such as a break or a return statement, or by throwing an error. Deferred actions are executed in reverse order of how they are specified—that is, the code in the first defer statement executes after code in the second, and so on.

  1. func processFile(filename: String) throws {
  2. if exists(filename) {
  3. let file = open(filename)
  4. defer {
  5. close(file)
  6. }
  7. while let line = try file.readline() {
  8. // Work with the file.
  9. }
  10. // close(file) is called here, at the end of the scope.
  11. }
  12. }

The above example uses a defer statement to ensure that the open(_:) function has a corresponding call to close(_:).