NSString no 'assign', 'retain', or 'copy' attribute is specified

Question

I\'m declaring an NSString property in a class and objective-c is complaining that:

NSString no \'assign\', \'retain\', or \'copy\' attribute is specifie

Answer 1

Cocoa uses reference counting to manage memory. Objects with a reference count of 0 are deleted.

• assign - does nothing to reference count simply points your variable to the data
• retain - points your variable to data and adds 1 to reference count, data is guaranteed to be there while your variable is still alive
• copy - makes a copy of data, points your variable at it and makes the retain count 1

More detail here, at Apple's own documentation.

Answer 2

I think it is drawing your attention to the fact that a assign is being used, as opposed to retain or copy. Since an NSString is an object, in a reference-counted environment (ie without Garbage Collection) this can be potentially "dangerous" (unless it is intentional by design).

However, the difference between assign, retain and copy are as follows:

• assign: In your setter method for the property, there is a simple assignment of your instance variable to the new value, eg:

  - (void)setString:(NSString*)newString
  {
      string = newString;
  }
  

  This can cause problems since Objective-C objects use reference counting, and therefore by not retaining the object, there is a chance that the string could be deallocated whilst you are still using it.

• retain: this retains the new value in your setter method. For example:

  - (void)setString:(NSString*)newString
  {
      [newString retain];
      [string release];
      string = newString;
  }
  

  This is safer, since you explicitly state that you want to maintain a reference of the object, and you must release it before it will be deallocated.

• copy: this makes a copy of the string in your setter method:

  - (void)setString:(NSString*)newString
  {
      if(string!=newString)
      {
          [string release];
          string = [newString copy];
      }
  }
  

  This is often used with strings, since making a copy of the original object ensures that it is not changed whilst you are using it.

Answer 3

assign - the ivar is set by doing a simple assignment. Implementation:

- (void) setFoo:(NSString *)newFoo {
  foo = newFoo;
}

retain - the ivar is sent the retain message before doing the assignment. Implementation:

- (void) setFoo:(NSString *)newFoo {
  if (foo != newFoo) {
    [foo release];
    foo = [newFoo retain];
  }
}

copy - the ivar is sent the copy message before doing the assignment. Implementation:

- (void) setFoo:(NSString *)newFoo {
  if (foo != newFoo) {
    [foo release];
    foo = [newFoo copy];
  }
}