Objective-C / C Convert UTF8 Literally to Real string

Question

Im wondering how to convert

NSString = "\xC4"; ....

to real NSString represented in normal format

Answer 1

Fundamentally related to xcode UTF-8 literals. Of course, it is ambiguous what you actually mean by "\xC4" - without an encoding specified, it means nothing.

If you mean the character whose Unicode code point is 0x00C4 then I would think (though I haven't tested) that this will do what you want.

NSString *s = @"\u00C4";

Answer 2

First are you sure you have \xC4 in your string? Consider:

NSString *one = @"\xC4\x80";
NSString *two = @"\\xC4\\x80";
NSLog(@"%@ | %@", one, two);

This will output:

Ā | \xC4\x80

If you are certain your string contains the four characters \xC4 are you sure it is UTF-8 encoded as ASCII? Above you will see I added \x80, this is because \xC4 is not valid UTF-8, it is the first byte of a two-byte sequence. Maybe you have only shown a sample of your input and the second byte is present, if not you do not have UTF-8 encoded as ASCII.

If you are certain it is UTF-8 encoded as ASCII you will have to convert it yourself. It might seem the Cocoa string encoding methods would handle it, especially as what you appear to have is a string as it might be written in Objective-C source code. Unfortunately the obvious encoding, NSNonLossyAsciiStringEncoding only handles octal and unicode escapes, not the hexadecimal escapes in your string.

You can use any algorithm you like to convert it. One choice would be a simple finite state machine which scans the input a byte at a time and recognises the four byte sequence: \, x, hex-digit, hex-digit; and combines the two hex-digits into a single byte. NSString is not the best choice for byte-at-time string processing, you may be better off converting to C strings, e.g.:

// sample input, all characters should be ASCII
NSString *input = @"\\xC4\\x80"; 

// obtain a C string containing the ASCII characters
const char *cInput = [input cStringUsingEncoding:NSASCIIStringEncoding]; 

// allocate a buffer of the correct length for the result
char cOutput[strlen(c2a)+1];

// call your function to decode the hexadecimal escapes
convertAsciiEncodedUTF8(cInput, cOutput);

// create a NSString from the result
NSString *output = [NSString stringWithCString:cOutput encoding:NSUTF8StringEncoding];

You just need to write the finite state machine, or other algorithm, for convertAsciiEncodedUTF8.

(If you write an algorithm and it fails ask another question showing your code, somebody will probably help you. But don't expect someone to write it for you.)

HTH