What is 'defer' in Swift? How does it work and what is it used for, what are the non-standard scenarios of its use?

Answer.

The defer statement postpones the execution of code until the current scope is exited, usually from a function. It is useful for resource cleanup, closing files, and freeing memory — so-called automated cleanup.

Features:

• Defer blocks are executed in reverse order of their declaration (LIFO).
• The call always occurs — both on a normal function exit and when throwing an error.
• It helps avoid duplication of cleanup code and supports a clean function architecture.

Example:

func processFile() {
    let file = openFile()
    defer { closeFile(file) }
    // Working with the file
    // file will be closed even in case of throw or return
}

Unconventional use: partial imitation of constructs like 'finally', grouping code for logging and monitoring execution times.

Trick question.

Will defer execute if the application terminates during the execution of the function?

— No. Defer executes only on normal exit from the scope (return, throw or normal exit). If the application crashes (e.g., due to a SIGKILL signal or fatalError), the defer blocks will not be executed.

Example:

func foo() {
    defer { print("Clean up") }
    fatalError("Crash!") // defer will not execute
}

Examples of real errors due to ignorance of the topic's nuances.

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Story

A function working with sockets did not clean up the connection in case of throw. Data remained hanging, connections were not released. After introducing defer, everything worked correctly: resources were always closed.

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Story

A developer tried to rely on defer for releasing global state. When a fatalError occurred, the resource was not released, leading to service hangs and the need for a restart.

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Story

In a function, several defer statements were declared, assuming they would execute in the order of declaration. As a result, resources were closed in the wrong order (for example, a file descriptor was closed first, and then access was attempted to it). Solution: remember the LIFO order of defer block execution.