Question: I am getting 

```
yld: Symbol not found: _OBJC_CLASS_$_UIUserNotificationSettings

```

and here's the function that is causing the error ***when the application is running on an iOS7 device*** and without even calling the function at all in the code.

> 
> 
> ```
> func reigsterForRemoteUserNotifications(notificationTypes: UIUserNotificationType, categories: NSSet) {
>         let userNotificationSettings = UIUserNotificationSettings(forTypes: notificationTypes, categories: categories)
>         (UIApplication.sharedApplication()).registerUserNotificationSettings(userNotificationSettings)
>         UIApplication.sharedApplication().registerForRemoteNotifications()
>     }
> 
> ```
> 
> 

I don't want this method to be accessible at all when running on an iOS7 device. I do not want a select check inside of it because that means the method is available for use to begin with.

What I want is a build config parameter to check the version :
 I can't figure out a way to write a swift equivalent preprocessor macro to check for the correct iOS version and neglect the new and undeclared iOS 8 library functions. 

```
#if giOS8OrGreater
// declare the functions that are iOS 8 specific
#else 
// declare the functions that are iOS 7 specific

#endif

```

In the documentation apple is suggesting functions and generics to substitute for complex macros but in this case I need a build config precompile check to avoid processing undeclared functions. Any suggestions.

Answer: 
The other answers fail to mention proper ways of checking the system version. You should absolutely never utilize: `Device.systemVersion`
You shouldn't make custom macros to check version numbers, and you shouldn't dig beyond the libraries that Apple has specifically defined for this task. 

There's a great article detailing this out [here](https://pspdfkit.com/blog/2016/efficient-iOS-version-checking/).

Note that Swift 2.0 allows you to directly check if an OS version number is available via:

```
if #available(iOS 10.0, *) {
    // modern code
} else {
    // Fallback on earlier versions
}

```

Prior to Swift 2.0, the recommended approach was via the system macros provided:

```
    if (NSFoundationVersionNumber > NSFoundationVersionNumber_iOS_9_0) {
    // do stuff for iOS 9 and newer
} else {
    // do stuff for older versions than iOS 9
}

```

or via:

```
    if NSProcessInfo().isOperatingSystemAtLeastVersion(NSOperatingSystemVersion(majorVersion: 10, minorVersion: 0, patchVersion: 0)) {
    // modern code
}

```

For anything missing beyond the system macros.

Any other approach has been downplayed as unreliable and not recommended by Apple. There's actually an approach that will break in iOS 10. 

Note that if you need macro like functionality in a check and you'd like to use `#available` you can use `@available` defined in [this article](https://www.hackingwithswift.com/new-syntax-swift-2-availability-checking) as such:

```
 @available(iOS 7, *)
func iOS7Work() {
    // do stuff

    if #available(iOS 8, *) {
        iOS8Work()
    }
}

@available(iOS 8, *)
func iOS8Work() {
    // do stuff
    if #available(iOS 9, *) {
        iOS9Work()
    }
}

@available(iOS 9, *)
func iOS9Work() {
    // do stuff
}

```

For further information on attributes in Swift, you can reference [Apple's documentation](https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Attributes.html).
var systemVersion = UIDevice.currentDevice().systemVersion