Use defer Statement for Cleanup in Swift

Written by Team Kodeco

The defer statement in Swift allows you to execute a block of code when the current scope is exited, regardless of how control leaves the scope. This can be useful for performing cleanup tasks, such as closing file handles or freeing resources, that must be executed even if an error occurs.

Here is an example of using defer to ensure that a file handle is closed when a function exits:

import Foundation

func readFile() throws {
  let fileHandle = try FileHandle(forReadingFrom: URL(fileURLWithPath: "file.txt"))
  defer {
    fileHandle.closeFile()
  }
  // read data from file
  let data = fileHandle.readDataToEndOfFile()
  // process data
  // ...
}

In this example, the defer statement is used to close the file handle, regardless of whether the readFile function completes successfully or throws an error.

It’s important to note that the defer statement is executed in reverse order of the order in which they are declared. Which means, if there are multiple defer statement in a scope the last one declared will be executed first.

func someFunc() {
  defer {
    print("1st defer")
  }
  defer {
    print("2nd defer")
  }
}
someFunc()
// Output:
// 2nd defer
// 1st defer

Keep in mind that defer statement isn’t limited to error handling and can be used for any kind of cleanup tasks.