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August 3, 2012

3 Tips on Auto Synthesized Properties

by Dan

Ha ha! You fool! You fell victim to one of the classic blunders, the most famous of which is “never get involved in a land war in Asia.” But only slightly less well-known is this: “Never have an auto-synthesized property and an instance variable with the same name when death is on the line!”


At work, my team decided to drop support for iOS 4. Most of our customers have upgraded to iOS 5.1. Surprisingly, very few are still using iOS 5.0. I guess over-the-air upgrades are working really well for people.

Our app is old. In some places we still checked if the user had upgraded to iOS 3 yet. :) My team spent the week on cleanup and paying down technical debt. My job was to:

  1. Convert to using object literals for arrays, dictionaries and numbers.
  2. Remove @synthesize calls.
  3. Switch all of our instance variables to properties.

I love shortening and removing code without losing functionality. The first two tasks went fairly quickly. But I…uh…learned a lot while tackling number three.

You can declare an instance variable in an @interface block — either the public interface in the .h file or the private interface in the .m file. You can also declare one in the @implementation block in the .m file. Or all three at once.

You can mix instance variables, properties and auto-synthesized properties. But auto-synthesized properties are so much nicer: less code, less maintenance, less boilerplate typing. Here are 3 tips to help you use properties more effectively.

1. Deal With Read-Only Properties

An auto-synthesized property generates an instance variable with the same name as the property except prefixed with an underscore. This is great because it’s clear whether you’re using the property (self.myprop) or the instance variable (_myprop). Using the bare name (myprop) doesn’t even compile, so you limit the confusion and bugs caused by accidental mistypes.

Auto-synthesized properties create their own getter and setter methods (avoiding some unnecessary typing). You can implement your own getter and/or setter, in which case those methods are not automatically generated. The instance variable (_myprop) is still synthesized.

However, if your property is readonly AND you implement your own getter method, then an instance variable is not synthesized. I suspect this is to allow for a named property that is dynamically calculated. In any case, if you would like an instance variable, you can force it to be synthesized by either:

  1. Using @synthesize myprop = _myprop; in your @implementation, or
  2. Redeclaring the property as readwrite in your private @interface

The second option works like inheritance in that you can redeclare a property as less-restrictive but not the other way. Redeclaring a public readwrite property as privately readonly isn’t possible, though why you’d want to do that remains a mystery.

2. Avoid Instance Variables

It is a Bad Thing™ to have both an auto-synthesized property and an instance variable declared with the same name.

  1. self.myprop = 1 uses the auto-synthesized instance variable
  2. myprop = 1 uses the declared instance variable
  3. _myprop = 1 uses the auto-synthesized instance variable

Since you end up with two instance variables, when you probably intended to have only one, doing this can cause lots of unexpected bugs and problems. Better to avoid declaring instance variables for your synthesized properties.

It is a Worse Thing™ to have two auto-synthesized properties of the same name with one prefixed with an underscore.

    @property myprop;
    @property _myprop;

This causes problems because self._myprop is different than _myprop when you really should have used __myprop, making you start to wonder about your own sanity. This doesn’t seem like it would happen much, but it can if you are importing several .h files, one of which uses underscores in its property names.

So don’t do that. :)

3. Use Instance Variables Correctly

In most cases, it’s best to stick with using the property (self.myprop) in your code. However, it’s important to use the instance variable (_myprop) in two cases:

  1. In your init: methods to avoid possible side-effects if you implement your own setter later, and
  2. In your custom getter and setter methods to avoid infinite recursion (always nice).

Auto-synthesized properties are great. Not quite as awesome as ARC, but close.

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