Why does this NSString created from an NSData object fail to show it has contents?

Question

Why does the following code produce the logging at the bottom ? Here is the anomaly- my second NSLog should print the chrStr but produces nothing, empty, which is verified by this debug command:

(gdb) po chrStr

object returns empty description

However, the third NSString where I re-convert the NSString back to NSData object DOES display the the data, the same value as in the first NSLog, as it should. This would indicate to me that chrStr must have actual contents. But it seems not to be so from the NSLOG or the po command. Why ?

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NSString *login;   
NSString *pass;


// Purpose: NSString *loginString = [NSString stringWithFormat:@"\000%@\000%@", login, pass];    
login = @"Loginname";       // text string1
pass = @"Password";         // text string2

// convert text strings to data objects
NSData *subData1 = [login dataUsingEncoding:NSUTF8StringEncoding];  
NSData *subData2 = [pass dataUsingEncoding:NSUTF8StringEncoding];   

// embed a NULL into new NSData object
NSMutableData *data = [NSMutableData data];
unsigned char zeroByte = 0;
[data appendBytes:&zeroByte length:1];

// append string1, NULL, string2 to data object
[data appendData:subData1];
[data appendBytes:&zeroByte length:1];
[data appendData:subData2];
NSLog(@"1.NSData:  %@", data);                  // print data object

// create a character string from data object
NSString *chrStr = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSLog(@"2.NSString:  %@", chrStr);              // print character string

// create data object from string object
NSData *chrData = [chrStr dataUsingEncoding:NSUTF8StringEncoding]; 
NSLog(@"3.NSDATA:  %@", chrData);               // print data object

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Produces: [1071:207] 1.NSData: 004c6f67 696e6e61 6d650050 61737377 6f7264

[1071:207] 2.NSString:

[1071:207] 3.NSDATA: 004c6f67 696e6e61 6d650050 61737377 6f7264

---

This is a real mystery to me. If chrStr is empty then 3-NSDATA could not display its info, but it does !

What am I trying to accomplish ? Well, check my very first comment line: // purpose:

That line when uncommented produces a warning, even though it actually works, so I was trying to do it another way that allowed me to have a clean compile. If you see a better way to accomplish that objective, I all eyes and ears. But please don't dwell on why that @"\000%@\000%@" string is necessary, start out accepting that it is. Thanks.

Answer 1

In C (and therefore objective-c), a null byte is used to represent the end of a string. When you create the string object, it takes all of the data you have given it without parsing, which is why you can convert it back to data successfully. However, when you display the string, the system reads the string up to the first null byte, which is the first byte. Therefore, the string contains data, but any system functions which read byte by byte instead of using the strings returned length will think it is empty. When you work with non-displayable characters, you should try to use data objects over string objects as often as possible.