Difference- Copy vs assign vs retain

Question

Tarun Kumar · Answer

Assign is for primitive values like BOOL, NSInteger or double. For objects use retain or copy, depending on if you want to keep a reference to the original object or make a copy of it.

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

(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:

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.

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

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

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.

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

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Difference- Copy vs assign vs retain

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