比较一个空的UITextField与nil字符串

Question

I have a UITextField that I am trying to compare to a string password in a class. The problem happens when the password is nil. According to the debugger an empty UITextField returns @"" so I tried a length based appproach.

 NSString *pw;
 if([[alertView textFieldAtIndex:1].text length] > 0)
     pw = room.password;

 if([pw isEqualToString:room.password])
  //do some code

But this still isn't working for nil passwords...ive even verified with the debugger that both pw and room.password are nil (see below)...whats going on?

Answer 1

The problem is that you're sending a message to nil. That's never going to return TRUE.

pw can't respond to the isEqualToString: selector because it's nil.

Answer 2

You can send any message to a nil object.

"If you expect a return value from a message sent to nil, the return value will be nil for object return types, 0 for numeric types, and NO for BOOL types. Returned structures have all members initialized to zero." - Source: https://developer.apple.com/library/ios/documentation/cocoa/conceptual/ProgrammingWithObjectiveC/WorkingwithObjects/WorkingwithObjects.html

Since pw is nil keep in mind, that there are no isEqual methods declared for that object.

Therefore, what you should do is:

if([pw isEqualToString:room.password] || (pw == nil && room.password == nil))

Answer 3

Try this:

UITextField *textField = [alertView textFieldAtIndex:1];
if (textField.text == nil) {
    //empty textField
    DoSomething();
} else {
    //textField contains text
    DoSomethingElse();
}

Answer 4

如果强为零，则该强不存在，因此您无法进行比较。