Is it necessary to use autoreleasepool in a Swift program?

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The autoreleasepool pattern is used in Swift when returning autorelease objects (created by either your Objective-C code or using Cocoa classes). The autorelease pattern in Swift functions much like it does in Objective-C. For example, consider this Swift rendition of your method (instantiating NSImage/UIImage objects):

func useManyImages() {
    let filename = pathForResourceInBundle
    
    for _ in 0 ..< 5 {
        autoreleasepool {
            for _ in 0 ..< 1000 {
                let image = NSImage(contentsOfFile: filename)
            }
        }
    }
}

If you run this in Instruments, you’ll see an allocations graph with 5 small hills (because outer for-loop), like the following:

with autoreleasepool

But if you do it without the autorelease pool, you’ll see that peak memory usage is higher:

without autoreleasepool

The autoreleasepool allows you to explicitly manage when autorelease objects are deallocated in Swift, just like you were able to in Objective-C.

Note: When dealing with Swift native objects, you generally will not receive autorelease objects. This is why the presentation mentioned the caveat about only needing this when “working with Objective-C”, though I wish Apple was more clear on this point. But if you’re dealing with Objective-C objects (including Cocoa classes), they may be autorelease objects, in which case this Swift rendition of the Objective-C @autoreleasepool pattern is still useful.

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