What is the autoclosure attribute?

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

The @autoclosure attribute can be applied to a closure parameter for a function, and automatically creates a closure from an expression you pass in. When you call a function that uses this attribute, the code you write isn't a closure, but it becomes a closure, which can be a bit confusing – even the official Swift reference guide warns that overusing autoclosures makes your code harder to understand.

To help you understand how it works, here's a trivial example:

func printTest1(_ result: () -> Void) {
    print("Before")
    result()
    print("After")
}

printTest1({ print("Hello") })

That code creates a printTest() method, which accepts a closure and calls it. As you can see, the print("Hello") is inside a closure that gets called between "Before" and "After", so the final output is "Before", "Hello", "After".

If we used @autoclosure instead, it would allow us to rewrite the printTest() call so that it doesn't need braces, like this:

func printTest2(_ result: @autoclosure () -> Void) {
    print("Before")
    result()
    print("After")
}

printTest2(print("Hello"))

These two pieces of code produce identical results thanks to @autoclosure. In the second code example, the print("Hello") won't be executed immediately because it gets wrapped inside a closure for execution later.

The @autoclosure attribute is used inside Swift wherever code needs to be passed in and executed only if conditions are right. For example, the && operator uses @autoclosure to allow short-circuit evaluation, and the assert() function uses it so that the assertion isn’t checked outside of debug mode.