Why does declaring pointer makes it contents nil?

Question

Possible Duplicate:
Declared but unset variable evaluates as true?

Why does this code work on Mac OS

int main (int argc, const char * argv[]) 
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSString *str;
    NSLog(@"%@", [str length]);

    NSLog(@"Hello, World!");
    [pool drain];
    return 0;
}

and console output is

2011-09-23 15:37:17.481 Untitled1[80021:903] (null) 
2011-09-23 15:37:17.495 Untitled1[80021:903] Hello, World!

but crashes on iPhone ?

What is the message in the log? – Lou Franco Sep 23 '11 at 12:58 — Lou Franco, Sep 23 '11 at 12:58

score 3 · Answer 1 · answered Sep 23 '11 at 12:59

3

Two mistakes.
First, you declare pointer str, don't set it's value and use it in NSLog.
Second, you use formatting %@, but supply integer value ([str length]), result is undefined. That's why on simulator it prints null, but crashes on device.

answered Sep 23 '11 at 12:59

Valeriy Van

1,851
16
19

score 3 · Answer 2 · answered Sep 23 '11 at 13:04

You were just lucky. Local variables are uninitialised and could contain anything. It's likely that on OS X, str contains 0 (aka nil), sending -length to nil returns 0. Then you are treating the length (which is an NSInteger) as a pointer for the NSLog.

If the length is 0, the NSLog will treat it as a nil pointer if the format specifier is %@ and will print (null).

If str has a random non zero value, either your program will crash when you send length or if it miraculously works and returns a non zero length, it'll probably crash trying to treat it as an object pointer in NSLog

justin · Answer 3 · 2011-09-23T13:08:04.593

1

the value is never initialized (or set). it could be any random value. in debug, it may be zero-iniitalized.

a crash (or some UB) is what you should expect in this case. so... turn up your compiler and static analyzer warnings, fix the issues, and always initialize your values.

edited Sep 23 '11 at 13:08

answered Sep 23 '11 at 12:58

justin

104,054
14
179
226

score 0 · Answer 4 · answered Sep 23 '11 at 12:49

0

try this.

     NSString *str=@"";

NSLog(@"%d", [str length]);

answered Sep 23 '11 at 12:49

Arun

2,271
1
14
18

score -2 · Answer 5 · answered Sep 23 '11 at 13:00

See , You can call only release method over a nil object. other than release method we cannot call any method over a nil.

Here you are calling a method ( length )over an object which is pointing to nil , in this case it should point to nsstring object in general before sending length message .

Why does declaring pointer makes it contents nil?

5 Answers5