Reading data with a known encoding

To read from a file or URL for which you know the encoding, you use stringWithContentsOfFile:encoding:error: or stringWithContentsOfURL:encoding:error:, or the corresponding init... method, as illustrated in the following example.

NSURL *URL = ...;

NSError *error;

NSString *stringFromFileAtURL = [[NSString alloc]

                                      initWithContentsOfURL:URL

                                      encoding:NSUTF8StringEncoding

                                      error:&error];

if (stringFromFileAtURL == nil) {

    // an error occurred

    NSLog(@"Error reading file at %@\n%@",

              URL, [error localizedFailureReason]);

    // implementation continues ...

You can also initialize a string using a data object, as illustrated in the following examples. Again, you must specify the correct encoding.

NSURL *URL = ...;

NSData *data = [NSData dataWithContentsOfURL:URL];

// Assuming data is in UTF8.

NSString *string = [NSString stringWithUTF8String:[data bytes]];

// if data is in another encoding, for example ISO-8859-1

NSString *string = [[NSString alloc]

            initWithData:data encoding: NSISOLatin1StringEncoding];

Reading data with an unknown encoding

If you find yourself with text of unknown encoding, it is best to make sure that there is a mechanism for correcting the inevitable errors. For example, Apple's Mail and Safari applications have encoding menus, and TextEdit allows the user to reopen the file with an explicitly specified encoding.

If you are forced to guess the encoding (and note that in the absence of explicit information, it is a guess):

1. Try stringWithContentsOfFile:usedEncoding:error: or initWithContentsOfFile:usedEncoding:error: (or the URL-based equivalents).

   These methods try to determine the encoding of the resource, and if successful return by reference the encoding used.

2. If (1) fails, try to read the resource by specifying UTF-8 as the encoding.

3. If (2) fails, try an appropriate legacy encoding.

   "Appropriate" here depends a bit on circumstances; it might be the default C string encoding, it might be ISO or Windows Latin 1, or something else, depending on where your data are coming from.

4. Finally, you can try NSAttributedString's loading methods from the Application Kit (such as initWithURL:options:documentAttributes:error:).

   These methods attempt to load plain text files, and return the encoding used. They can be used on more-or-less arbitrary text documents, and are worth considering if your application has no special expertise in text. They might not be as appropriate for Foundation-level tools or documents that are not natural-language text.