What is a bridge between Toll-free and Swift?

The bridge between Toll-free and Swift is a mechanism that allows the use of certain Foundation types (from Objective-C) that are "Toll-free bridged" with their counterparts in Core Foundation (from C) directly in Swift, and vice versa, without explicitly creating new objects or conversions. This ensures interoperability between Swift and older Objective-C/C frameworks, many of which use Core Foundation.

Toll-free bridging means you can freely pass instances of one type where an instance of another type is expected, and vice versa, without overhead of creating a new object. In the context of Swift, this applies to some Foundation types such as NSString, NSArray, NSDictionary, NSDate, which can be used as their Swift counterparts: String, Array, Dictionary, Date.

Examples of Toll-free bridged types:

• CFStringRef <=> NSString <=> String
• CFArrayRef <=> NSArray <=> Array
• CFDictionaryRef <=> NSDictionary <=> Dictionary
• CFDateRef <=> NSDate <=> Date

Swift "automatically" performs this bridge when interacting with APIs written in Objective-C, making the code cleaner and more convenient.

import Foundation // Includes Foundation and thus allows the use of NS types

let nsString: NSString = "Hello from Swift" // NSString object
let swiftString: String = nsString as String // NSString automatically converts to String

let swiftArray: [Int] = [1, 2, 3] // Swift array
let nsArray: NSArray = swiftArray as NSArray // Swift array automatically converts to NSArray

// Example of using NSString in a function expecting String (and vice versa)
func printString(_ str: String) {
    print(str)
}

printString(nsString as String) // Passing NSString expecting String

func printNSString(_ nsStr: NSString) {
    print(nsStr)
}

printNSString(swiftString as NSString) // Passing String expecting NSString

The bridge between Toll-free and Swift is part of Apple's overall strategy to ensure compatibility between Swift and legacy Objective-C and C code. It greatly simplifies migration and integration of new Swift code into existing projects.