Delete all keys from a NSUserDefaults dictionary iOS

If you have a look at the NSUserDefaults documentation you will see a method - (NSDictionary *) dictionaryRepresentation. Using this method on the standard user defaults, you can get a list of all keys in the user defaults. You can then use this to clear the user defaults:

- (void)resetDefaults {
    NSUserDefaults * defs = [NSUserDefaults standardUserDefaults];
    NSDictionary * dict = [defs dictionaryRepresentation];
    for (id key in dict) {
        [defs removeObjectForKey:key];
    }
    [defs synchronize];
}

Shortest way to do this with the same results like in Alex Nichol's top answer:

NSString *appDomain = NSBundle.mainBundle.bundleIdentifier;
[[NSUserDefaults standardUserDefaults] removePersistentDomainForName:appDomain];
[[NSUserDefaults standardUserDefaults] synchronize];

One-liner:

[[NSUserDefaults standardUserDefaults] removePersistentDomainForName:NSBundle.mainBundle.bundleIdentifier];

Simple Solution

Objective C:

NSString *appDomain = [[NSBundle mainBundle] bundleIdentifier];
[[NSUserDefaults standardUserDefaults] removePersistentDomainForName:appDomain];

Swift 3.0 to Swift 5.0 :

if let appDomain = Bundle.main.bundleIdentifier {
    UserDefaults.standard.removePersistentDomain(forName: appDomain)
}

Swift version:

if let bid = NSBundle.mainBundle().bundleIdentifier {
    NSUserDefaults.standardUserDefaults().removePersistentDomainForName(bid)
}