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copied!<blockquote> <p>Can someone explain what the compiler does when it sees this?</p> </blockquote> <pre><code> DooDad* aDooDad = nil; </code></pre> <p>If you are really interested in what the compiler does, the answer is: the compiler will reserve some memory on the stack for the local variable <code>aDooDad</code>, which is a pointer type (it is generally 64 or 32 bits in size depending on the processor). That pointer is then initialized to contain <code>nil</code> (usually <code>0x00..00</code>).</p> <p>A statement like this:</p> <pre><code> DooDad* aDooDad = [[DooDad alloc] init...]; </code></pre> <p>makes use of pointer variable <code>aDooDad</code> to store the address in memory of the object that is further allocated (which is the address of memory reserved by <code>alloc</code>).</p> <p>So, in the end,</p> <pre><code> DooDad* aDooDad = nil; </code></pre> <p>is not declaring an object, just a variable whose content is interpreted as the address of an object of <code>DooDad</code> type. Such declaration, therefore, is just like any other declaration you know, e.g. when initializing an <code>int</code> to 0, so that later you can assign it some value in an <code>if</code> statement.</p> <p>A statement like:</p> <pre><code> [aDooDad doSomething]; </code></pre> <p>is interpreted by the Objective-C runtime system like: send message <code>doSomething</code> to the object whose address is stored in <code>aDooDad</code>. If that address is <code>nil</code> no message is sent. On the other hand, if you <em>dereference</em> a nil pointer: <code>*aDooDad</code> you'll get undefined behavior.</p> <p>Pointers are pretty low level stuff. I hope this helps.</p>

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