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    <p>In Objective-C, just like in C, <code>&amp;</code> is the "address-of operator" and it returns the address of its argument. To find out more about it, I recommend you read <a href="http://publications.gbdirect.co.uk/c_book/chapter5/arrays_and_address_of.html" rel="nofollow">this short chapter</a> from The C Book.</p> <p>Here's an example of how the operator is used, to get a better idea:</p> <pre><code>#include &lt;stdio.h&gt; // define a function that takes a pointer to an integer as argument void change_value_of_int(int* int_to_change) { // change the value to which the argument points *int_to_change = 5; } int main() { // create a stack variable int test_int = 10; // pass the address of test_int to the function defined earlier change_value_of_int(&amp;test_int); // now the value of test_int is 5 printf("%d\n", test_int); return 0; } </code></pre> <p>Note that the <code>change_value_of_int()</code> function expects the first parameter to be a <em>pointer</em> to an int, not an int, so you can't call it with <code>change_value_of_int(test_int)</code>. You must send it the <em>address</em> of the <code>test_int</code> variable, not the variable itself (because if you send a copy of the variable, it can't change it).</p> <p>Same thing with the <code>NSError*</code> example. <code>jsonParser</code> expects the <em>address</em> of a <code>NSError*</code>, not a <code>NSError*</code>, therefore the method is defined as:</p> <pre><code>- (id)objectWithString:(NSString*)jsonrep error:(NSError**)error; </code></pre> <p>Take a look at <a href="http://code.google.com/p/json-framework/source/browse/trunk/Source/SBJSON.h?r=549#51" rel="nofollow">the header file</a> and at <a href="http://code.google.com/p/json-framework/source/browse/trunk/Source/SBJSON.m?r=549#82" rel="nofollow">the implementation</a> to see how it is used. The value of your <code>error</code> (<code>*error</code> = the value of the thing pointed to by the <code>error</code> argument) becomes the return value of <code>[errorTrace lastObject]</code>.</p>
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    1. COThanks, this totally explains it for me. I appreciate your time.
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