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CHAPTER 8 STRINGS AND PRIMITIVE VALUES
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returns the address of the first byte in the array. Once obtained, there is nothing preventing you from modifying any of the values at that address. I prefer to construct NSData objects for memory that I do not intend to modify, and NSMutableData objects for modifiable memory, but there s nothing that dictates this in practice. NSData objects treat their contents as a single contiguous byte array. You will often want to treat it as something else an array of some other type or a structure. Listing 8-2 demonstrates the use of NSMutableArray to store an array of NSPoint structures.
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Listin g 8-2. Using NSMutableData to Wrap an Array of Structures
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NSArray *objects = ... // array of objects with a coordinate unsigned int count = [objects count]; NSMutableData *data = [NSMutableData dataWithLength:count*sizeof(NSPoint)]; NSPoint *points = (NSPoint*)[data bytes]; unsigned int i; [data retain]; for (i=0; i<[objects count]; i++) { points[i] = [[objects objectAtIndex:i] coordinate]; } [data release]; The code in Listing 8-2 allocates an empty array of bytes large enough to contain count number of NSPoint structures. Once allocated, the address of the array is cast as a pointer to an NSPoint structure. C pointers are interchangeable with array variables, so the pointer is used to address the individual elements of the array. This technique will work for a single structure or an array of any type.
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retain] and [data release]. Due to a quirk of Objective-C 2.0 s garbage collector, the variable ( points) that points to the interior of the NSData object is not sufficient to keep the object from being recycled by the garbage collector immediately following the [data bytes] statement. These extraneous statements prevent the NSData object from being collected by keeping its object reference in scope. This is only a problem for transient NSData objects. If the NSData object was returned from this function, stored in a globally reachable variable, or referenced in any other way following the for loop, it wouldn t be a problem. The -retain and -release methods are for traditional memory management and do nothing when running in a garbage collection environment. See 9 for an explanation and for an alternate solution, described in the GC vs. Non-GC Pointers section of that chapter.
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Caution In Listing 8-2 you ll notice two odd statements: [data
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C allows you to define your own memory structures and variable types using struct and typedef declarations. Consequently, there s more that can be wrapped than just the integer and float types supplied by the language. That s the job of NSValue. NSValue wraps any C data type. This includes all non-object types defined by the language or the programmer. The principal method for creating NSValue objects is +[NSValue valueWithBytes:objCType:]. This method is passed the address of the value not the value itself and an Objective-C type value. The type value is generated using the Objective-C @encode() directive. This evaluates to a constant C string that formally identifies the type of the value. From this information,
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CHAPTER 8 STRINGS AND PRIMITIVE VALUES
NSValue determines the size of the value and makes a copy of it. Listing 8-3 shows how an arbitrary C structure is wrapped in an NSValue object and than added to a collection.
Listin g 8-3. Wrapping an Arbitrary Structure in NSValue
typedef struct { int population; long long hectares; BOOL landLocked; } CountryStats; ... CountryStats stats; stats.population = 195450000; stats.hectares = 859250000; stats.landLocked = NO; NSValue *value = [NSValue valueWithBytes:&stats objCType:@encode(CountryStats)]; NSMutableArray *array = [NSMutableArray array]; [array addObject:value]; ... CountryStats lastStats; value = [array lastObject]; [value getValue:&lastStats]; return (lastStats.population); The key points in Listing 8-3 are as follows: An NSValue is constructed by passing it the address of the value, not the value itself. The size of the value is implied by the Objective-C type generated by the @encode() directive. Once encapsulated, NSValue can be treated like any other object. To examine the value stored in NSValue, send it the -getValue: message again passing the address of the variable to be overwritten with a copy of the value stored in the object.
NSValue has several convenience constructors for commonly used structures, such as +valueWithPoint:(NSPoint)point, +valueWithRange:(NSRange)range, +valueWithSize:(NSSize)size, and so on. Note that these all take a copy of the original value, not an address. NSNumber is actually a subclass of NSValue. In a way, everything in NSNumber can be considered convenience methods for dealing with native data types. Internally, NSNumber uses NSValue to store its value. You can send an NSNumber or NSValue object an -objCType message to determine the type of its original value.
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