On 14 Dec 2009, at 23:01, David Hirsch wrote:
> (After drafting the first version of this email, I found a sort-of solution,
> but I do not understand why it works. I thought that simple properties
> (e.g., object.foo) essentially just called the foo:/setFoo: methods,
> particularly if you used @property (assign). That is apparently not the
> case. See the code for a working and non-working version. Note that I do
> not have custom setters/getters; I've synthesized them.)
>
> I have a Configuration object that has an NSMutableArray of instances (i.e.,
> the class name is "Instance" - this is not the computer science definition of
> "instance"). Each Instance has a course, room, instructor (which are,
> respectively, Course *, Room *, Instructor *). In the newNearbyConfiguration
> method of Configuration, I need to make a slightly modified copy of the
> Configuration object. To do this, I choose an Instance from the array,
> modify it (for example, by changing the room to a different Room *). Oddly,
> however, when I make this change, the array suddenly gets a new copy of the
> original Instance. The new copy has a new memory address. I would have
> expected that I can modify the contents of an object in an NSMutableArray
> without needing to replace the original.
> The Instance * should still point to the same memory, right?
> Also, I put a breakpoint in Instance copyWithZone, but that was apparently
> not called.
> Can anybody shed light on this? Here is some relevant code:
>
> (From Instance.h:)
> @interface Instance : NSObject {
> Course * course; // shallow copy
> WeekPattern * pattern; // shallow copy
> Instructor * instructor; // Shallow copy
> Timing * timing; // Deep copy
> Room * room; // Shallow copy
> NSUInteger quarter; // Simple copy 0=fall, 1=winter,
> 2=spring
> }
>
> @property (assign) Course * course; // weak reference
> @property (assign) WeekPattern * pattern; // weak reference
> @property (assign) Instructor * instructor; // weak
> reference
> @property (assign) Timing * timing; // strong reference - I own this
> @property (assign) Room * room; // weak reference
> @property (assign) NSUInteger quarter;
>
>
> (From Configuration.h:)
> @interface Configuration : NSObject <NSCopying> {
> NSMutableArray *instances; // array of Instances
> ClassSchedulerDoc *myDoc; // weak reference
> }
> @property (retain) NSMutableArray *instances;
> @property (assign) ClassSchedulerDoc *myDoc;
>
> (From Configuration.m, highly edited for brevity)
> - (Configuration *) newNearbyConfiguration {
> // make a new configuration like the current one but with one small
> change
> Configuration *newConfig = [self copy];
Great, assuming you've implemented -copyWithZone: properly, you've made a copy
of the configuration.
>
> short randInstance = rand() % [self.instances count];
> Instance *instanceToAlter = [self.instances objectAtIndex:randInstance];
> Room *newRoom;
> do {
> newRoom = [[instanceToAlter course] randomLowCostRoom];
> } while (newRoom == [instanceToAlter room]);
> [instanceToAlter setRoom: newRoom]; // this works
> // instanceToAlter.room = newRoom; // this displays the problem described
> above
>
> return newConfig;
And as expected, you're returning the new configuration. But, since creating
it, you've not modified it in any way. I suggest looking at all the calls to
self within this method and checking whether you actually meant to send them to
newConfig.
> }
>
> - (NSString *)description {
> NSMutableString *desc = [NSMutableString stringWithCapacity:200];
>
> for (Instance *thisInstance in instances) {
> [desc appendString:[thisInstance briefDescription]];
> }
> return desc;
> }
>
> Also, I do know that I could easily have just made Configuration inherit from
> NSMutableArray. That would appear to be irrelevant to my question.
>
> Thanks,
> Dave
>
> ============================
> Dave Hirsch
> Associate Professor
> Department of Geology
> Western Washington University
> persistent email: [email protected]
> http://www.davehirsch.com
> voice: (360) 389-3583
> aim: [email protected]
> vCard: http://almandine.geol.wwu.edu/~dave/personal/DaveHirsch.vcf
> ============================
>
>
>
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