Build an array of pointers from existing pointers

Question

I have a class which must initialize a group of similar objects the same way.

NSNumber *a = nil;
NSNumber *b = nil;
NSNumber *c = nil;

a, b and c are member variables of an existing object. In my code I use a more complex NSObject subclass, but this example is easier to understand using NSNumbers.

All 3 objects must be initialized in a similar way. Therefore I want to build an array of pointers which I can use within a for-loop like this:

NSPointerArray *objs = [NSPointerArray weakObjectsPointerArray];
[objs addPointer:&a];
[objs addPointer:&b];
[objs addPointer:&c];

Running the above code I get the following error:

error: address doesn't contain a section that points to a section in a object file

How can I build an array which I can explore with a loop like this?

    for (id *o in objs) {
        *o = @2;
    }

Are you by any chance trying to store weak references in your array? Cause that's not supported by NSPointerArray. — TotoroTotoro
– TotoroTotoro, Commented Dec 22, 2012 at 17:58
what do you mean by not supported? The NSPointerArray has weak and strong pointer array constructors. I understand those constructors describe the week/strong relationship of the array pointers, not the pointers I insert. In my example, @[a,b,c] are strong pointers, and that is the reason I use the weakObjtsPointerArray constructor — SystematicFrank
– SystematicFrank, Commented Dec 22, 2012 at 19:49
Hmm. You're right, NSPointerArray can hold weak refereces. But the doc page for NSPointerArray says: "Important: NSPointerArray does not support weak references under Automatic Reference Counting (ARC)." I don't know how to interpret this, to be honest :) — TotoroTotoro
– TotoroTotoro, Commented Dec 22, 2012 at 22:42

Daij-Djan · Accepted Answer · 2012-12-22 21:09:04Z

5

just wrap the pointers in 'NSValue' objects and you're good

id v1 = [NSValue valueWithPointer:p1];
id v2 = [NSValue valueWithPointer:p2];
id v3 = [NSValue valueWithPointer:p3];

id array = @[v1,v2,v3];

for(NSValue *value in array) {
    void *pointer = value.pointerValue;
    NSNumber *n = (__bridge NSNumber*)pointer;
}

demo for what YOU want to do it appears: (NSNumbers are immutable)

#import <Foundation/Foundation.h>

int main(int argc, char *argv[]) {
@autoreleasepool {
    NSNumber *a;
    NSNumber *b;

    NSArray *os = @[ [NSValue valueWithPointer:&a],
                 [NSValue valueWithPointer:&b] ];

    for (NSValue *v in os) {
        void *pointer = [v pointerValue];
        NSNumber **n_ptr = (__bridge NSNumber**)pointer; 
        *n_ptr = @2.0;
    }

    NSLog(@"%@ %@", a,b);
}
}

edited Dec 22, 2012 at 21:09

answered Dec 22, 2012 at 18:01

Daij-Djan

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7 Comments

Daij-Djan Over a year ago

it isnt as convenient as NSPointerArray but has fewer limits I think

SystematicFrank Over a year ago

OMG! so long ago since I had the mindset from C++ to wrap things in Value objects. That answers the first part of my question, but now I am having problems with the second part. I need to cast that (void )pointer into something I can use to change the value of the variable it references. Looking at my example I need something like (NSNumber**) but I cannot get it to compile. I am trying to fix the code at gist.github.com/4360450. I guess I am close to solve it myself, but I jumped to Cocoa after ARC and never had this fight with pointers :(

Daij-Djan Over a year ago

use id n = (__bridge NSNumber*) - added to sample

SystematicFrank Over a year ago

is that the right cast? I get an run-time error. Excuse me for using Gist and not StackOverflow, but over there I have version control of my edits and line numbers. I do what you say at line 9, and it seems wrong because I am assigning an (NSNumber**) to (NSNumber*). The following lines is what I assume it should look like, except that I cannot get Xcode to not freak out with the (NSNumber**) declaration: gist.github.com/4360450/…

SystematicFrank Over a year ago

just solved the ARC compatible version. The key was casting with (__bridge void*). Now finally got your answer ARC compatible

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