When should I use @synthesize explicitly?

Question

As far as I know, since XCode 4.4 the @synthesize will auto-generate the property accessors. But just now I have read a sample of code about NSUndoManage

Answer 1

Property synthesis is required when a property is declared in a protocol. It will not be automatically synthesized in an implementing interface.

Answer 2

If you do not explicitly use @synthesize the compiler will understand your property the same way if you had written

@synthesize undoManager=_undoManager;

then you will be able to write in your code things like :

[_undoManager doSomething]; // iVar
[self.undoManager doSomethingElse]; // Use generated getter

This is the common convention.

if you write

@synthesize undoManager;

you will have :

[undoManager doSomething]; // iVar
[self.undoManager doSomethingElse]; // Use generated getter

Personally I stop using @synthesize, since it's not mandatory any more. For me the only reason to use @synthesize is to link an iVar to a @property. If you want to generate specific getter and setter for it. But in the given piece of code there is no iVar, I think that this @synthesize is useless. But now I think the new question is "When to use iVar ?", and I've no other response than "never" for this one !

Answer 3

Thanks for clarifying that. I had a similar problem.

@synthesize firstAsset, secondAsset, audioAsset;
@synthesize activityView;

So now, having commented them out, I went through and replaced each occurrence with, for example

self.firstAsset It seems I could also use firstAsset, but I find I miss seeing the "" too often.

Answer 4

Xcode doesn't require an explicit @synthesize declaration.

If you don't write @synthesize its the same as doing :

@synthesize manager = _manager;

The sample code might've been old. They'll update it soon.

You can access your properties like :

[self.manager function];

This is Apple's recommended convention. I follow it, and I recommend that you do too!

Answer 5

There's a lot of answers, but also a big confusion. I'll try to put some order (or increase the mess, we'll see...)

1. Let's stop talking about Xcode. Xcode is an IDE. clang is a compiler. This feature we are discussing is called autosynthesis of properties and it's an Objective-C language extension supported by clang, which is the default compiler used by Xcode.
   Just to make it clear, if you switch to gcc in Xcode, you won't benefit from this feature (regardless from the Xcode version.) In the same way if you use a text editor and compile using clang from the command line, you will.

2. Thank to autosynthesis you don't need to explicitly synthesize the property as it will be automatically synthesized by the compiler as

   @synthesize propertyName = _propertyName
   

   However, a few exceptions exist:

   • readwrite property with custom getter and setter

     when providing both a getter and setter custom implementation, the property won't be automatically synthesized

   • readonly property with custom getter

     when providing a custom getter implementation for a readonly property, this won't be automatically synthesized

   • @dynamic

     when using @dynamic propertyName, the property won't be automatically synthesized (pretty obvious, since @dynamic and @synthesize are mutually exclusive)

   • properties declared in a @protocol

     when conforming to a protocol, any property the protocol defines won't be automatically synthesized

   • properties declared in a category

     this is a case in which the @synthesize directive is not automatically inserted by the compiler, but this properties cannot be manually synthesized either. While categories can declare properties, they cannot be synthesized at all, since categories cannot create ivars. For the sake of completeness, I'll add that's it's still possible to fake the property synthesis using the Objective-C runtime.

   • overridden properties (new since clang-600.0.51, shipping with Xcode 6, thanks Marc Schlüpmann)

     when you override a property of a superclass, you must explicitly synthesize it

It's worth noting that synthesizing a property automatically synthesize the backing ivar, so if the property synthesis is missing, the ivar will be missing too, unless explicitly declared.

Except for the last three cases, the general philosophy is that whenever you manually specify all the information about a property (by implementing all the accessor methods or using @dynamic) the compiler will assume you want full control over the property and it will disable the autosynthesis on it.

Apart from the cases that are listed above, the only other use of an explicit @synthesize would be to specify a different ivar name. However conventions are important, so my advice is to always use the default naming.

Answer 6

OK, when you create a property...

@property NSString *name;

Xcode will auto synthesise an iVar as if you had written...

@synthesize name = _name;

This means you can access the property with...

self.name;
// or
_name;

Either will work but only self.name actually uses the accessor methods.

There is only one time that auto synthesise does not work: If you overwrite but the setter AND the getter method then you will need to synthesise the iVar.

You are fine if you just override the setter or if you just override the getter. But if you do both then the compiler won't understand it and you will need to synthesise it manually.

As a rule of thumb though.

Don't make iVars. Just use the property. Don't synthesise it.