StackOverflow2013

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copied!<p>It is not possible with a pure NSMutableDictionary, and in most cases you want to convert nil values into <code>[NSNull null]</code> or just omit them from the dictionary. Sometimes (very seldom), though, it is convenient to allow nil values, and in those cases you can use CFMutableDictionary with custom callbacks.</p> <p>If you go this way, I recommend that you use CoreFoundation API for all accesses, e.g. <code>CFDictionarySetValue</code> and <code>CFDictionaryGetValue</code>.</p> <p>However, if you know what you're doing, you can use toll-free bridging and cast that CFMutableDictionary to NSMutableDictionary or NSDictionary. This may be useful if you have a bunch of helpers that accept NSDictionary, and you want to use them on your modified nil-capable dictionary. (Of course, make sure that the helpers aren't surprised by nil values.)</p> <p>If you do the bridging, note that:</p> <p>1) NSMutableDictionary setter raises errors on nil values before bridging, so you need to use CFDictionarySetValue to set values that are potentially nil.</p> <p>2) technically, we're violating a contract of NSMutableDictionary here, and things may break (e.g. in future OS updates)</p> <p>3) a lot of code will be very surprised to find nil values in a dictionary; you should only pass the bridged frankendictionaries to the code that you control</p> <p>See <a href="http://ridiculousfish.com/blog/posts/bridge.html">ridiculousfish's post on toll-free bridging</a> for an explanation of why a bridged CFDictionary behaves differently from NSDictionary.</p> <p>Example:</p> <pre><code>#import <Foundation/Foundation.h> const void *NullSafeRetain(CFAllocatorRef allocator, const void *value) { return value ? CFRetain(value) : NULL; } void NullSafeRelease(CFAllocatorRef allocator, const void *value) { if (value) CFRelease(value); } const CFDictionaryValueCallBacks kDictionaryValueCallBacksAllowingNULL = { .version = 0, .retain = NullSafeRetain, .release = NullSafeRelease, .copyDescription = CFCopyDescription, .equal = CFEqual, }; int main(int argc, const char * argv[]) { @autoreleasepool { CFMutableDictionaryRef cfdictionary = CFDictionaryCreateMutable(NULL, 0, &kCFTypeDictionaryKeyCallBacks, &kDictionaryValueCallBacksAllowingNULL); CFDictionarySetValue(cfdictionary, @"foo", @"bar"); CFDictionarySetValue(cfdictionary, @"boz", nil); NSMutableDictionary *dictionary = CFBridgingRelease(cfdictionary); NSLog(@"dictionary[foo] = %@", dictionary[@"foo"]); NSLog(@"dictionary[foo] = %@", dictionary[[@"fo" stringByAppendingString:@"o"]]); NSLog(@"dictionary[boz] = %@", dictionary[@"boz"]); NSLog(@"dictionary = %@", dictionary); NSLog(@"(dictionary isEqualTo: dictionary) = %d", [dictionary isEqualToDictionary:dictionary]); } return 0; } </code></pre> <p>outputs:</p> <pre><code>dictionary[foo] = bar dictionary[foo] = bar dictionary[boz] = (null) dictionary = { boz = (null); foo = bar; } (dictionary isEqualTo: dictionary) = 1 </code></pre>

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