@property and @synthesize

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First, note that the latest version of Xcode does not require @synthesize at all anymore. You can (and should) just omit it. That said, here’s what the pieces do.

@property is a declaration of accessors. It is just a declaration. There is very little difference between the following:

@property (nonatomic, readwrite, strong) NSString *something;

vs.

- (NSString *)something;
- (void)setSomething:(NSString)aSomething;

The main difference is that declaring these methods using @property lets the compiler automatically generate (synthesize) the implementations for you. There is no requirement that you let the compiler do it for you. You are absolutely free to implement something and setSomething: by hand, and it is common to do. But, if you don’t implement them by hand, the compiler will automatically create an ivar for you called _something and create a reasonable implementation for the getter and setter.

In older versions of Xcode, you had to explicitly request the auto-generation using the @synthesize keyword. But that is no longer required. Today, the only reason to use @synthesize is if you want the ivar to have a non-standard name (never do that).

A key point here is that the methods something and setSomething: are just methods. There is nothing magical about them. They’re not special “property methods.” They’re just methods that by convention access a piece of state. That piece of state is often stored in an ivar, but does not need to be.

To be even more clear: object.something does not mean “return the ivar named _something from object.” It means “return the result of [object something], whatever that does.” It is common for that to return the value of an ivar.

You should declare all of your state (internal and external) using @property declarations, and you should avoid directly declaring ivars. You should also always access your properties via their accessors (self.something), except in the init and dealloc methods. In init and dealloc, you should directly use the ivar (_something).

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