Swift bug when formatting CGFloats

CGFloatFormattingBug.swift

import Cocoa

// Doesn't work for CGFloats
let a: CGFloat = 1.0
let b: CGFloat = 2.0
String(format: "%f %f", a, b)   // "0.000000 0.000000"

// Works for Floats
let c: Float = 1.0
let d: Float = 2.0
String(format: "%f %f", c, d)   // "1.000000 2.000000"

// Works for Doubles
let e: Double = 1.0
let f: Double = 2.0
String(format: "%f %f", e, f)   // "1.000000 2.000000"

I’m assuming that initialiser is actually NSString’s, whose signature is

convenience init(format: NSString, _ args: CVarArgType...)

It’s interesting that CGFloat ends up being converted to CVarArgType even though it doesn’t explicitly conform to CVarArgType. For example,

a.encode()

is a compiler error whereas

c.encode()

returns a word array representation suitable for variadic arguments.

Since CGFloat is not explicitly CVarArgType, there is some conversion path that results in CVarArgType. I am tempted to say it is a conversion to NSNumber because of the code below (appended to your original code):

func format1(value: CVarArgType) -> String {
    return String(format: "%f", value)
}

func format2(value: CVarArgType) -> String {
    return String(format: "%@", value)
}

format1(a)    // "0.000000"
format2(a)    // "1"

format1(c)    // "1.000000"
format2(c)    // "(null)"

format1(NSNumber(double: 1.0))    // "0.000000"
format2(NSNumber(double: 1.0))    // "1"

@bavarious Yep, it is using the NSString initializer. The public interface for Swift's String does not have that method defined on it, it just uses the bridging. Here's another interesting bit:

// The following code crashes
let numbers = [a, b]
String(format: "%f %f", arguments: numbers)

(This is using the version of the initializer that allows passing the arguments as an array as opposed to a variadic argument list)

Swift's String is extended in Foundation to include most of NSString's initializers—you can see all of them by command-clicking on an import Foundation statement or on the SwiftDoc.org String page marked with [Foundation].