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The wrapping of a primitive type in an object is often called boxing, and taking the primitive type out is unboxing. Some languages have an autoboxing feature that will automatically converts a primitive to its corresponding wrapped type and back. Objective-C does not support autoboxing.
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NSValue
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NSNumber is actually a subclass of NSValue, which wraps arbitrary values. You can use NSValue to put structures into NSArrays and NSDictionaries. Create a new NSValue using
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this class method:
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+ (NSValue *) valueWithBytes: (const void *) value objCType: (const char *) type;
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You pass the address of the value you want to wrap (such as an NSSize or your own struct). Usually, you take the address (using the & operator in C) of the variable you want to save. You also supply a string describing the type, usually by reporting the types and sizes of the entries in the struct. You don t actually have to write code to build this string yourself. There s a compiler directive called @encode that takes a type name and generates the proper magic for you. So, to put an NSRect into an NSArray, you do something like this:
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NSRect rect = NSMakeRect (1, 2, 30, 40); NSValue *value; value = [NSValue valueWithBytes: &rect objCType: @encode(NSRect)]; [array addObject: value];
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You can extract the value using getValue:
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- (void) getValue: (void *) value;
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When you call getValue:, you pass the address of a variable that you want to hold the value:
value = [array objectAtIndex: 0]; [value getValue: &rect];
CHAPTER 8: A Quick Tour of the Foundation Kit
NOTE
In the getValue: example, you can see the use of get in the name of the method to indicate that we re providing a pointer as the place to store the value the method generates.
Convenience methods are provided for putting common Cocoa structs into NSValues, and we have conveniently listed them here:
+ (NSValue *) valueWithPoint: (NSPoint) point; + (NSValue *) valueWithSize: (NSSize) size; + (NSValue *) valueWithRect: (NSRect) rect; - (NSPoint) pointValue; - (NSSize) sizeValue; - (NSRect) rectValue;
To store and retrieve an NSRect in an NSArray, you do this:
value = [NSValue valueWithRect: rect]; [array addObject: value]; .... NSRect anotherRect = [value rectValue];
NSNull
We ve told you that you can t put nil into a collection, because nil has special meaning to NSArray and NSDictionary. But sometimes you really need to store a value that means there s nothing here at all. For example, let s say you have a dictionary that holds a person s contact information, and under the key @"home fax machine", you store the user s home fax number. If that key holds a phone number value, you know that person has a fax machine. But if there s no value in the dictionary, does it mean that person has no home fax machine or that you don t know if they have one or not By using NSNull, you can eliminate the ambiguity. You can decide that a value of NSNull for the key @"home fax machine" means the person definitely does not have a fax machine, and no value for the key means that you don t know if the person has one or not.
NSNull is probably the simplest of all Cocoa classes. It has but a single method: + (NSNull *) null;
And you add it to a collection like this:
[contact setObject: [NSNull null] forKey: @"home fax machine"];
CHAPTER 8: A Quick Tour of the Foundation Kit
You access it as follows:
id homefax; homefax = [contact objectForKey: @"home fax machine"]; if (homefax == [NSNull null]) { // ... no fax machine. rats. } [NSNull null] always returns the same value, so you can use == to compare it with other
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