memory-management-program

Summery: 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 simpe assignment of your instance variable to the new value.
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-(void)setString: (NSString *)newString{
string = newString;
}
.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 object, and you must release it before it will be deallocated.
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(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.
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(void)setString:(NSString *)newString{
if(string != newString) {
[string release];
string = [newString copy];
}
}
.weak: use weak references to avoid retain cycles

Reference:

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