Example showing the use of accessing the indexed ivars.

indexedivars.m

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

// This stuff won't work with ARC, so make sure it's turned off.
// clang -g -fno-objc-arc -framework Foundation -o indexedivars indexedivars.m

@interface Stuffage : NSObject {
    int _extraCount;
}

- (id) initWithExtraSpace: (int) extraCount;
- (int) stuffAtIndex: (unsigned int) index;

@end // Stuffage

@implementation Stuffage

- (id) initWithExtraSpace: (int) extraCount  {
    if ((self = [super init])) {
        _extraCount = extraCount;

        int *extra = object_getIndexedIvars (self);
        for (int i = 0; i < extraCount; i++) {
            extra[i] = i * 100;
        }
    }
    return self;
} // initWithExtraSpace


- (int) stuffAtIndex: (unsigned int) index {
    assert (index < _extraCount);

    int *extra = object_getIndexedIvars (self);
    return extra[index];
    
} // stuffAtIndex


@end // Stuffage


int main (void) {
    Stuffage *stuff = class_createInstance ([Stuffage class], sizeof(int) * 30000);
    // Stuffage *stuff = class_createInstance ([Stuffage class], 0);
    stuff = [stuff initWithExtraSpace: 30000];

    printf ("%d %d %d %d\n",
            [stuff stuffAtIndex: 1],
            [stuff stuffAtIndex: 25],
            [stuff stuffAtIndex: 300],
            [stuff stuffAtIndex: 29999]);

    typedef struct ExpandOMatic {
        char *name;
        int footCount;
        float footSizes[0];
    } ExpandOMatic;

    ExpandOMatic *nofoot = malloc (sizeof(ExpandOMatic));
    nofoot->name = "nochan";
    nofoot->footCount = 0;

    int footCount = 30;
    ExpandOMatic *lotsOfFeet = malloc (sizeof(ExpandOMatic) + sizeof(float) * footCount);
    lotsOfFeet->name = "footloose";
    lotsOfFeet->footCount = footCount;
    for (int i = 0; i < footCount; i++) {
        lotsOfFeet->footSizes[i] = random() % 15;
    }
    
    printf ("size of expand-o-matic: %zu\n", sizeof(ExpandOMatic));
    
    return 0;

} // main