iOS Dev Nugget 204 Swift @autoclosure


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Swift provides the @autoclosure declaration attribute. When applied to a function parameter type, it converts the call site expression into a no-arg closure with a return value, delaying the evaluation of the expression. For example, in the following code, both normalFunction() and withAutoClosure() are called with the output of foo(), but because withAutoClosure()'s argument is declared as @autoclosure, it doesn't evaluate foo(). Instead the foo() call is wrapped into a closure which is then passed to withAutoClosure().

func foo() -> Int {
    print("foo called")
    return 1

func normalFunction(_ i: Int) {
    print("    Value returned by foo: \(i)\n")

func withAutoClosure(_ fn: @autoclosure ()->Int) {
//  print("value returned: \(fn())")
    print("    Notice foo is not called?")


This will produce the output:

foo called
    Value returned by foo: 1

    Notice foo is not called?

Using @autoclosure, you can decide to skip the evaluating the expression or evaluate it one or more times. The Swift blog gives an example of implementing assert() using @autoclosure, as well as short-circuiting logical operators.

Note that @autoclosure has to be used judiciously because the someone reading the code cannot easily determine that the argument has @autoclosure applied to it.

PS: This code is written in Swift 3. Minor modifications would make it work with Swift 2.2

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