C/Obj-C interop at a glance (no iOS specifics)

Interop map

C imports & pointers

// Import the platform C module: Darwin (Apple) or Glibc (Linux)
#if canImport(Darwin)
import Darwin
#else
import Glibc
#endif

// Use C getenv (returns C pointer) and strlen
if let cPtr = getenv("USER") {
    let name = String(cString: cPtr)   // bridge C char* -> Swift String
    print("Hello,", name)
} else {
    print("Hello, anonymous")
}

// strlen works on C strings:
let cString: [CChar] = Array("swift".utf8).map { CChar($0) } + [0]
let len = strlen(cString)
print("strlen(s) =", len)

C random example

#if canImport(Darwin)
import Darwin
let r = arc4random_uniform(100)      // 0..99
print("random:", r)
#else
import Glibc
let r = UInt32(random() % 100)       // 0..99 (demo)
print("random:", r)
#endif

Foundation bridges

import Foundation

// String ↔ NSString bridge is seamless
let swiftString: String = "hello"
let ns: NSString = swiftString as NSString
print(ns.uppercased)           // via NSString API
let back: String = ns as String
print(back + " world")

// NSArray / NSDictionary bridge to Swift collections
let nsArray: NSArray = ["a", "b", "c"]
let swiftArray = nsArray as! [String]
print(swiftArray.joined(separator: ","))

let nsDict: NSDictionary = ["lang": "Swift", "year": 2014]
let swiftDict = nsDict as! [String: Any]
print(swiftDict["lang"] as? String ?? "-")

NSError → throws

import Foundation

// Many Obj-C methods that take NSError** are imported as throwing in Swift.
let fm = FileManager.default

// Create a temporary file, then remove it with "try".
let tmp = (NSTemporaryDirectory() as NSString).appendingPathComponent("demo.txt")
FileManager.default.createFile(atPath: tmp, contents: Data("hi".utf8), attributes: nil)

do {
    try fm.removeItem(atPath: tmp)          // was NSError** in Obj-C → throws in Swift
    print("removed ok")
} catch {
    print("remove failed:", error)
}

Expose Swift to Obj-C

import Foundation

@objcMembers
class Adder: NSObject {
    dynamic var last: Int = 0
    @objc func add(_ a: Int, _ b: Int) -> Int { last = a + b; return last }
}

let a = Adder()
print(a.add(2, 3))  // 5
print(a.last)       // 5
// In mixed projects, Obj-C can call Adder via the generated Swift header.

NSError bridging answer

Recap