When should I use @synthesize explicitly?

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.