In what cases is the defer operator used in programming language?

The defer operator in Swift is used for deferred execution of code — it guarantees that a certain block of code will be executed at the end of the current scope, usually when exiting a function or block.

This is convenient for releasing resources, closing files, releasing locks, or other cleanup operations that should occur regardless of how the function ends (normally or with an error).

Example:

func readFile() {
    let file = openFile()
    defer {
        closeFile(file) // will be executed guaranteed upon exiting the function
    }
    // work with the file
}