Instantiate an NSArray passing a mutable array changes array to mutable?

Question

If I instantiate an NSArray passing it an NSMutableArray, does it become a NSArray or does it just appear to be one.

i.e. is the mutable array eventually released

- (NSArray *)getObjectivesWithPerspective:(Perspective *)perspective
{
    NSMutableArray *result = [NSMutableArray array];

    for (ObjectiveManagedObject *objective in self.objectives)
    {
        if (objective.perspective.objectID == perspective.objectID) {
            [result addObject:objective];
        }
    }

    return [NSArray arrayWithArray:result];
}

Community · Accepted Answer · 2017-05-23 12:04:53Z

1

See this answer: https://stackoverflow.com/a/1392051/254422

Basically, it copies the objects into a new, autoreleased, immutable NSArray.

The NSMutableArray created using +array is autoreleased.

edited May 23, 2017 at 12:04

CommunityBot

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answered May 21, 2012 at 12:23

hwaxxer

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4 Comments

jlehr Over a year ago

You might want to reread the Advanced Memory Management Programming Guide -- the rule you stated is incorrect. According to the guide, "You create an object using a method whose name begins with “alloc”, “new”, “copy”, or “mutableCopy” (for example, alloc, newObject, or mutableCopy)."

hwaxxer Over a year ago

You might want to read the NSArray class reference. +arrayWithArray: "Creates and returns an array containing the objects in another given array." Just because the particular static method +arrayWithArray: does not begin with alloc, new, copy or mutableCopy, does not mean that any underlying code beneath it does not.

jlehr Over a year ago

Actually, Objective-C doesn't have static methods; it has class methods which are something quite different. Not sure what relevance you think the NSArray class reference has to the naming rules for memory management, but your statement regarding "methods not starting with new/create or init" is flat-out wrong.

hwaxxer Over a year ago

Ah, I thought you were arguing that the NSArray returned from +arrayWithArray: was not a new object just because it wasn't called with alloc, new, copy or mutableCopy. I have updated the answer.

Community · Accepted Answer · 2017-05-23 12:27:59Z

0

You can find some info about this here: what does -arrayWithArray actually DO?. But the array you get back should be immutable. NSArray will copy the references into an immutable array.

If in doubt do a test. Get an array back from the method, cast it as a NSMutableArray and try to add an object. You should get an error when you do this.

edited May 23, 2017 at 12:27

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answered May 21, 2012 at 12:23

Aaron Hayman

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Comments

trojanfoe · Accepted Answer · 2012-05-21 13:02:40Z

0

As NSMutableArray is a subclass of NSArray, you can return result directly without the need to create a new NSArray object:

- (NSArray *)getObjectivesWithPerspective:(Perspective *)perspective
{
    NSMutableArray *result = [NSMutableArray array];

    for (ObjectiveManagedObject *objective in self.objectives)
    {
        if (objective.perspective.objectID == perspective.objectID) {
            [result addObject:objective];
        }
    }

    return result;
}

answered May 21, 2012 at 13:02

trojanfoe

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