Why is the return value of String.addingPercentEncoding() optional?

Question

The signature of the String method for percent-escaping is:

func addingPercentEncoding(withAllowedCharacters: CharacterSet)
    -> String?
         


        

Answer 1

Based on Paul Cantrell answer, small demonstration that it's also possible for the same method to also return null in Objective-C, despite String and NSString being different beasts when it comes to encodings:

uint8_t bytes[2] = { 0xD8, 0x00 };
NSString *string = [[NSString alloc] initWithBytes:bytes length:2 encoding:NSUTF16BigEndianStringEncoding];
// \ud800
NSLog(@"%@", string);

NSString *escapedString = [string stringByAddingPercentEncodingWithAllowedCharacters:NSCharacterSet.URLHostAllowedCharacterSet];
// (null)
NSLog(@"%@", escapedString);

---

_{For fun, https://r12a.github.io/app-conversion/ will percent escape the same as:}

Error%20in%20convertUTF162Char%3A%20low%20surrogate%20expected%2C%20b%3D0%21%00