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copied!<p>File this under C NULL pointer type stuff. </p> <p>Here is an example:</p> <pre><code>#import <Foundation/Foundation.h> void test(void){ NSString *test1, *test2, *test3; test1=nil; // Note: NSString would USUALLY be aloc then init before use... [test1 isEqual:@"static object string"]; [test2 isEqual:@"static string"]; [test3 isEqual:test1]; } int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; test(); [pool drain]; return 0; } </code></pre> <p>Now place a breakpoint on <code>test1=nil</code> in xcode and run. On <em>my</em> machine, I get:</p> <pre><code>test1=(NSString *) 0x0 <nil> test2=(NSString *) 0x0 <nil> test3=(NSString *) 0x7fff84db7dd7 Invalid </code></pre> <p>If you run it, you will get <code>EXC_BAD_ACCESS</code> and termination when it tries to execute <code>[test3 isEqual:test1]</code>. You can use the nil values of test1 and (accidental?) nil value of test2 but use the random value in test3: death. </p> <p>Now change that line to <code>test1=test2=test3=nil;</code> Run it again. Works great. </p> <p>An important concept: <a href="http://cocoawithlove.com/2008/06/doing-things-in-cocoa-with.html" rel="nofollow">You can validly send a message to nil</a> in Objective-C.</p> <p>Just as in may be good practice to assign a C pointer to a safe value when not in use, <code>nil</code> is a safe value in Objective-C until it is assigned to something else. For the programmer who wrote the example -- it was probably muscle memory. Unnecessary in the particular case, but safer than a random value. </p>

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