Question

I have a couple strings that I’m synthesizing:

.h

@property(nonatomic,copy) NSString* bio;

.m

@synthesize       bio;

//connectionDidFinish method
bio = [[[profile objectForKey:@"user"] valueForKey:@"profile"] valueForKey:@"bio"];

When my tableview first loads I get an error -[NSNull length]: unrecognized selector sent to instance 0x11d45e8 in cellForRowAtIndexPath:

case kBioSectionDescriptionRow:                 
                    if ([bio length]==0 ||bio == nil) {
                        cell.textLabel.text = @"bio";   
                        cell.selectionStyle = UITableViewCellSelectionStyleNone;
                        cell.detailTextLabel.numberOfLines = 5;
                        cell.detailTextLabel.font = [UIFont fontWithName:@"Helvetica Neue" size:(14.0)];
                        cell.detailTextLabel.lineBreakMode = UILineBreakModeWordWrap;
                        cell.detailTextLabel.text = @"None"; 
                    }
                    else{
                        cell.textLabel.text = @"bio";   
                        cell.selectionStyle = UITableViewCellSelectionStyleNone;
                        cell.detailTextLabel.numberOfLines = 5;
                        cell.detailTextLabel.font = [UIFont fontWithName:@"Helvetica Neue" size:(14.0)];
                        cell.detailTextLabel.lineBreakMode = UILineBreakModeWordWrap;
                        cell.detailTextLabel.text = [NSString stringWithFormat: @"%@", bio]; 
                    }

                    break;

How can I make sure that my bio is allocated and not null?

Answer 1

If you @synthesize your bio property, why do you use the variable instead of the property?

Instead of affecting the variable bio = [[[profile objectForKey:@"user"] valueForKey:@"profile"] valueForKey:@"bio"]; directly, you need instead to affect the property itself: self.bio = [[[profile objectForKey:@"user"] valueForKey:@"profile"] valueForKey:@"bio"];.

---

To be more precise, writing self.bio = xx means that it calls the setter method of the bio property. This setter will manage the memory for you, meaning it will release the previous value of the bio property and copy the new value.

If you write bio = xx instead, thus directly affecting the instance variable and not the property, no release or copy is done, so the objet you affect to the bio variable is not retained nor copied and will be destroyed at the end of the current RunLoop.

This is why your code crash, because you then try to access the bio variable, which does not point to anything anymore (actually it points to garbage, in your case to sthg it erroneously believes to be the [NSNull null] object) as the real object has been destroyed since!

---

Actually, @synthesize bio just asks the compiler to generate the code for the property’ setter and getter, and as your property is defined with the nonatomic,copy attributes, the generated setter will look like this:

-(void)setBio:(NSString*)value {
  if (value == bio) return; // if already the exact same object (same pointer), return

  [self willChangeValueForKey:@"bio"]; // For KVO
  [bio release]; // release previous value
  bio = [value copy]; // copy new value
  [self didChangeValueForKey:@"bio"]; // For KVO
}

Note: Don’t forget to release the bio variable in your dealloc method (or to set the self.bio property to nil) to avoid leaking memory