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    <p>Defining the variables in the brackets simply declares them instance variables.</p> <p>Declaring (and synthesizing) a property generates getters and setters for the instance variable, according to the criteria within the parenthesis. This is particularly important in Objective-C because it is often by way of getters and setters that memory is managed (e.g., when a value is assigned to an ivar, it is by way of the setter that the object assigned is retained and ultimately released). Beyond a memory management strategy, the practice also promotes encapsulation and reduces the amount of trivial code that would otherwise be required.</p> <p>It is very common to declare an ivar in brackets and then an associated property (as in your example), but that isn't strictly necessary. Defining the property and synthesizing is all that's required, because synthesizing the property implicitly also creates an ivar. </p> <p>The approach currently suggested by Apple (in templates) is:</p> <p>Define property in header file, e.g.:</p> <pre><code>@property (assign, readonly) gameCenter; </code></pre> <p>Then synthesize &amp; declare ivar in implementation:</p> <pre><code>@synthesize gameCenter = __gameCenter; </code></pre> <p>The last line synthesizes the <code>gameCenter</code> property and asserts that whatever value is assigned to the property will be stored in the <code>__gameCenter</code> ivar. Again, this isn't necessary, but by defining the ivar next to the synthesizer, you are reducing the locations where you have to type the name of the ivar while still explicitly naming it.</p>
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    1. CODeclaring a property (not 'defining', by the way -- properties can't be defined in Objective-C) declares a getter and/or setter method, but does *not* generate accessor methods. Accessor methods are synthesized if and only if there's a corresponding `@synthesize` in the implementation, which is optional. Also, it's probably worth mentioning that variables declared inside curly braces are known as *instance variables*.
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    2. COOn points of detail: Yes, it is @synthesize that actually generates the accessors. Yes, you "declare" properties (and then "define" their behaviors in the parenthesis). Given the scope of the question, I think it's fair to assume we know that "variable" refers to an instance variable (the answer uses "ivar" repeatedly).
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    3. COFurther elaboration: ObjC promotes encapsulation as a design strategy. Other classes shouldn't be able to directly mess with the internal state of an object (indeed, these days it's seen as best to show as few implementation details in your class' header file as possible). Instead, they can use getter/setter methods (such as those automatically created by the @property/@synthesize syntax) to access its declared properties. This allows the object to react to requests to return a value (say, by lazily loading it) or to set a new value (say, by also updating some corresponding UI).
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