Why am I getting an isEqualToString error in this Cocoa code?

Question

I keep getting this error:

alt text http://img514.imageshack.us/img514/2203/help.tif

What is it? I never even called \"isEqualToString\".

Here Is my Joke

Answer 1

The stack trace will help you find exactly what is calling isEqualToString:. Unfortunately, it's not giving you any symbols, so you'll have to do a little digging.

In Objective-C methods, there are two hidden parameters which are always passed as the first two arguments: self, a pointer to the object to which the message is being sent, and _cmd, a pointer to a C string containing the name of the message being sent. Examining the _cmd arguments in the stack frames will help you debug the problem.

The first thing you want to do is set a breakpoint right before the exception is thrown. Open up the debugger console (Cmd+Shift+R) and add a breakpoint to the function on the top of the stack trace by typing:

break 2438463755

Now run your app, and the debugger should break right before throwing the exception. It should also give you a full symbolic backtrace; if not, you'll have to walk the stack yourself. You can walk the stack and print out the value of the various _cmd parameters.

Answer 2

You're shadowing a ivar name with the function argument. Try changing your jokeWithValue: method to this:

In joke.h:

+ (id)jokeWithValue:(NSString *)aJoke;

in joke.m:

+ (id)jokeWithValue:(NSString *)aJoke {
     Joke *j = [[Joke alloc] init];
     j.joke = aJoke;
     return [j autorelease];
}

Notice that the NSString variable name has changed so it no longer shadows the iVar joke.

EDIT:

On seeing how joke is called, it looks as if you're assigning a joke object to cell.text, which I believe is expecting an NSString not a joke.

try setting:

cell.text = [[jokes objectAtIndex:index] joke];

in - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath

Answer 3

Replace this line:

cell.text = [jokes objectAtIndex:indexPath.row];

with these lines:

Joke *j = (Joke *)[jokes objectAtIndex:indexPath.row];
if( j )
  cell.text = j.joke;

Answer 4

The error is not in the definition of the Joke class, but somewhere it's being used. In most cases with errors like this, it's the result of a memory management error — some object (presumably a string) gets deallocated and another gets allocated in its old memory location. Try running with NSZombieEnabled and see if it turns up a message to a dealloced object.

Answer 5

your error message isn't being displayed but I'm assuming you've run into a statement which behind the scenes is calling isEqualToString and one of the objects isn't a string