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What is NSNumber?

Swift version: 5.1

Paul Hudson    @twostraws   

NSNumber is an Objective-C class designed to store a variety of types of numbers. It was important in Objective-C because its primitive number types – integers, doubles, etc – could not be used in most of Apple’s APIs without wrapping them in an object such as NSNumber, but mostly Swift does a good job of automatically converting its numbers to NSNumber when you need it.

That being said, there are a few times when Swift won’t help you out, and you need to convert to NSNumber by hand. For example, this code is designed to convert numerical numbers like 50 into textual numbers like “fifty”, but it won’t compile:

let number = 50
let formatter = NumberFormatter()
formatter.numberStyle = .spellOut

// this line won't work
// let string1 = formatter.string(from: number) ?? ""

The problem is that the string(from:) method expects an NSNumber and Swift isn’t able to automatically bridge the integer we created in number. The fix here is nice and easy – just add as NSNumber to help Swift bridge the two worlds:

let string2 = formatter.string(from: number as NSNumber) ?? ""
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