When should I add @synthesize explicitly to my code?
Generally, if it's required: You will probably never hit a case where it's needed.
There's one case you might find it useful, though.
Say you're writing both a custom getter and setter, but want an instance variable to back it. (For an atomic property, this is as simple as wanting a custom setter: the compiler will write a getter if you specify a setter for a monatomic property, but not an atomic property.)
Consider this:
@interface MyObject:NSObject
@property (copy) NSString *title;
@end
@implementation MyObject
- (NSString *)title {
return _title;
}
- (void)setTitle:(NSString *)title {
_title = [title copy];
}
@end
This will not work, because _title doesn't exist. You've specified both a getter or setter, so Xcode (correctly) doesn't create a backing instance variable for it.
![enter image description here]()
You have two choices for making it exist. You can either change the @implementation to this:
@implementation MyObject {
NSString *_title;
}
- (NSString *)title {
return _title;
}
- (void)setTitle:(NSString *)title {
_title = [title copy];
}
@end
Or change it to this:
@implementation MyObject
@synthesize title = _title;
- (NSString *)title {
return _title;
}
- (void)setTitle:(NSString *)title {
_title = [title copy];
}
@end
In other words, although synthesize is for practical purposes never necessary*, it can be used to define property-backing instance variables when you're providing a getter/setter. You can decide which form here you want to use.
In the past, I've favoured specifying the instance variable in the @implementation {}, but I now think the @synthesize route is a better choice as it removes the redundant type and explicitly ties the backing variable to the property:
- Change the property's type, and the instance variable's type changes.
- Change its storage qualifier (for instance, make it weak instead of strong or strong instead of weak) and the storage qualifier changes.
- Remove or rename the property, and the
@synthesize will generate a compiler error. You won't end up with stray instance variables.
*-I know one case where it was necessary, relating to splitting functionality across categories in multiple files. And I wouldn't be surprised if Apple fixes this, or even already has.
@sythesize. If the code still works, then it isn't necessary. – Virginiavirginie