iOS: pointer reference to an array

Question

I have a table view with a search, and search scope buttons with two possible scopes. The table is empty until a search is executed. Each scope has it's own mutable array for the table's data source, we'll say scopeA_array and scopeB_array. To simplify some methods, I'm trying to create a generic pointer reference to whichever array is the currently active scope. So I tried this:

@property (nonatomic, assign) NSMutableArray *tableDataArray;

In viewDidLoad, I assign it to the default selected scope.

_tableDataArray = _scopeA_array;

I can log the memory address of each array, they're both the same.

However, if I execute a search, _scopeA_array gets populated. Then in my numberOfRowsInSection method, I take the count of _tableDataArray but it's empty. I log the addresses again, both are different.

How do I create an array property that just references an array, and always points to the same object in memory even if it changes?

EDIT: A simplified way to test this, with the following lines of code, would like a way for tableDataArray to have the contents of testArray, even though the contents of testArray are assigned after:

NSArray *testArray = [NSArray new];
NSArray *tableDataArray = [testArray copy];
testArray = [NSArray arrayWithObjects:@"my", @"test", @"array", nil];
NSLog(@"table data array: %@", tableDataArray);
// logs empty array

You're not incrementing the retain count of _scopeA_array ... does it get deallocated before your numberOfRowsInSection call? — stevesliva
– stevesliva, Commented Jan 29, 2015 at 5:53
Well, I think that's your problem. To keep the same reference for _scopeA_array you should instead use [_scopeA_array addObjectsFromArray:request_result], asuming request_result is an array — marosoaie
– marosoaie, Commented Jan 29, 2015 at 6:04

dminones · Accepted Answer · 2015-01-29 06:19:40Z

3

I think the best approach is use a method to return conditionally the array for the current scope. So you just always use this method to populate your UITableView

- (NSMutableArray*) tableArray
{
    return [self isScopeA] ? _scopeA_array : _scopeB_array;
}

answered Jan 29, 2015 at 6:19

dminones

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5 Comments

inorganik Over a year ago

This is what I'm doing now, but the point (and the question) was to use a pointer reference, if that's even possible.

dminones Over a year ago

I can't see why would you like to do that rather this. But anyway you should could assign like this self.tableDataArray = _scopeA_array; when you change your scope

inorganik Over a year ago

That wouldn't work, because the array gets populated after a search is made, after the scope is changed.

dminones Over a year ago

I think that will work because once you assing to self.tableDataArray, this array is pointing to _scopeA_array and therefore the changes made on _scopeA_array are available through self.tableDataArray

inorganik Over a year ago

I misunderstood the answer before, you are right. This is the best way to do it.

Darren · Accepted Answer · 2015-01-30 18:33:16Z

1

How do I create an array property that just references an array, and always points to the same object in memory even if it changes?

If you want to track changes to a variable then you use a pointer to the variable rather than a pointer to a single array instance. E.g.:

@implementation MyController
{
    __strong NSArray* *_currentDataPtr;
    NSArray* _dataA;
    NSArray* _dataB;
}

- (id)init
{
    if (self = [super init])
    {
        _currentDataPtr = &_dataA; // Ensure _currentDataPtr is never NULL
    }
    return self;
}

- (void)setSearchScope:(NSInteger)searchScope
{
    switch (searchScope)
    {
        default :
            NSAssert(NO, @"");
        case 0 :
            _currentDataPtr = &_dataA;
            break;
        case 1 :
            _currentDataPtr = &_dataB;
            break;
    }
}

- (NSInteger)tableView:(UITableView*)tableView numberOfRowsInSection:(NSInteger)section
{
    return [*_currentDataPtr count];
}

If you want it to be a property then implement a property getter that dereferences the pointer:

@property (nonatomic, readonly) NSArray* currentData;

- (NSArray*)currentData { return *_currentDataPtr; }

edited Jan 30, 2015 at 18:33

answered Jan 29, 2015 at 16:34

Darren

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2 Comments

inorganik Over a year ago

Tried to implement this but xcode doesn't like the double asterisk in NSArray* *_currentDataPtr;, and complains "Pointer to non-const type 'NSArray *' with no explicit ownership." Is that a typo?

Darren Over a year ago

@inorganik It looks like you have to explicitly declare the ownership in that case. Local variables and instance variables are __strong by default. I've edited my answer.

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