How to get unique id in iOS device?

Getting device ID in

Swift 3

let device_id = UIDevice.currentDevice().identifierForVendor?.UUIDString

Swift 4:

let device_id = UIDevice.current.identifierForVendor!.uuidString

You can no longer get a unique ID per device. identifierForVendor is the best you're going to get. Apple has systematically disabled identifying a specific device so that users' IDs can't be passed around by marketers.

To get the identifier ID as a string, you can use

let deviceId = UIDevice.current.identifierForVendor?.uuidString

UPDATE

If you want a universal ID then you have the option to use advertisingIdentifier. However, if the user has limited ad tracking in their device settings then this value will simply return all zeroes.

import AdSupport

let identifierManager = ASIdentifierManager.shared()
if identifierManager.isAdvertisingTrackingEnabled {
    let deviceId = identifierManager.advertisingIdentifier.uuidString
}

N.B. Apple recommends that this code only be used by companies building advertisement frameworks, rather than the app developers themselves. If you use this within your code for non-ad-related purposes then prepare to have your app rejected.

The best way to get the device id is identifierForVendor

UIDevice *device = [UIDevice currentDevice];

NSString  *currentDeviceId = [[device identifierForVendor]UUIDString];

UPDATE

For Swift 4.1 use

UIDevice.current.identifierForVendor?.uuidString