How to store an array of nil pointers in objC?

Question

I would like to use an array of pointers to instances of objects, but only want to create instances of those objects when required (i.e. lazily). The array corresponds to a table in the UI, so each array index corresponds to a table row.

I would like to use an NSMutableArray to hold pointers to the object instances as they are created (which occurs when the user selects the corresponding row in the UI).

If a row in the table is selected, the corresponding array entry is checked. If the pointer value is nil, the instance hasn't yet been created, and so it is created at that point, and the object pointer is stored in the corresponding indexed array entry.

Obviously, this requires that I initially start with an array of nil pointers, but objC won't let me put a nil pointer in an NSArray.

I can't just add objects to the array as they are created, as the array index would not correspond to the table row.

What's the best objC solution here?

Community · Accepted Answer · 2017-05-23 12:23:29Z

7

The idiomatic solution in Objective C is to use NSNull:

The NSNull class defines a singleton object used to represent null values in collection objects (which don’t allow nil values).

Create your NSMutableArray, and fill it up with [NSNull null] objects:

NSMutableArray *array = [NSMutableArray arrayWithCapacity:N];
for (int i = 0 ; i != 10 ; [a addObject:[NSNull null]], i++);

When you check for the presence or absence of an object in your NSMutableArray, compare the object at index to [NSNull null]: if they are the same, replace with a real object; otherwise, the real object is already there.

if ([array objectAtIndex:index] == [NSNull null]) {
    MyRealObject realObject = [[MyRealObject alloc] init];
    [array replaceObjectAtIndex:index withObject:realObject];
}

** edit summary ** edited to initialize the array using a loop (thanks bbum).

edited May 23, 2017 at 12:23

CommunityBot

11 silver badge

answered Jul 21, 2012 at 11:00

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

bbum Over a year ago

Won't work; the first parameter to arrayWithObjects:count: must be an (id*) pointing to a language array of objects and N must not be greater than the number of objects.

bbum · Accepted Answer · 2012-07-21 18:17:34Z

3

NSMutableArray doesn't support sparse arrays. Thus, you could pre-seed the array with NSNull instances (or some other "no object" marker). Something like:

a = [NSMutableArray array];
for(int i = 0; i<numberNeeded; i++) [a addObject:[NSNull null]];

Or, if your array is going to be truly sparse, consider the use of some kind of map instead. NSMutableDictionary will work, but requires objects for keys and all that boxing/un-boxing is painful in some cases. Alternatively, a CFDictionary can easily be configured to use integer keys with object values.

While a dictionary is obviously slower for lookup-by-index, that performance difference is unlikely to cause a problem in most cases (but not all).

answered Jul 21, 2012 at 18:17

bbum

163k23 gold badges276 silver badges360 bronze badges

Comments

Francois · Accepted Answer · 2012-07-21 10:49:12Z

0

What about [NSMutableArray arrayWithCapacity:numberOfRows] ?

https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSMutableArray_Class/Reference/Reference.html

answered Jul 21, 2012 at 10:49

Francois

11k9 gold badges53 silver badges81 bronze badges

1 Comment

Snips Over a year ago

Thanks for your response. Unfortunately 'arrayWithCapacity' reserves memory for that potential capacity, the array is still expanded as objects are added (and is initially empty). Also, it is not possible to insert objects beyond the existing value of [array count], which is initially 0, even with a specified capacity. Also, it is not possible to insert values of nil.

Collectives™ on Stack Overflow

How to store an array of nil pointers in objC?

3 Answers 3

1 Comment

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related