Question

I am learning Objective-C and using the Big Nerd Ranch’s Obj-C book to familiarize myself with the language. Everything is going good so far until I reach this one exercise where I am supposed to create 2 classes where each of them inherits the dealloc method. The goal to show when an object actually gets deallocated. So the new dealloc looks like

Asset.h

#import <Foundation/Foundation.h>
@class Employee;

@interface Asset : NSObject{
    NSString * label;
    unsigned int resaleValue;
}

@property (strong) NSString * label;
@property unsigned int resaleValue;

@end

Employee.h

#import "Person.h"
@class Asset;

@interface Employee : Person
{
    int employeeID;   
    NSString * lastName;
    Person * spouse;
    NSMutableArray * children;
    NSMutableArray * assets;
}

@property int employeeID;
- (void) addAssetObject: (Asset *) a;
- (unsigned int) valueOfAssets;

@end

Implementations

@implementation Employee
//omitted
    -(void) dealloc
    {
        NSLog(@"deallocating %@", self);
    }
@end


@implementation Asset
//omitted
    -(void) dealloc
    {
        NSLog(@"deallocating %@", self);
    }
@end

main.m

int main (int argc, const char * argv[])
{

    @autoreleasepool {

        NSMutableArray * employees = [[NSMutableArray alloc] init];
        for(int i = 0; i< 10; i++){
            Employee * person = [[Employee alloc] init];
            [person setWeightInKilos:90+i];
            [person setHeightInMeters:1.8 - i/10.0];
            [person setEmployeeID:i];
            [employees addObject:person];
        }

        for(int i = 0; i< 10; i++){
            Asset * asset = [[Asset alloc] init];
            NSString * currentLabel = [NSString stringWithFormat:@"Laptop %d", i];
            [asset setLabel:currentLabel];
            [asset setResaleValue:i*17];

            NSUInteger * randomIndex = random() % [employees count];
            Employee * randomEmployee = [employees objectAtIndex:randomIndex];

            [randomEmployee addAssetObject:asset];
        }

        NSLog(@"Employees: %@", employees);
        NSLog(@"Giving up ownership of one employee");
        [employees removeObjectAtIndex:5]; // This is supposed to trigger the deallocate method
        NSLog(@"Giving up ownership of array");
        employees = nil;
    }
    return 0;
}

Of course description is already inherited so that %@would work. However, when I ran it. The dealloc is not getting called and I am not getting the print outs that deallocates the object. I am not sure what I am doing wrong here.

Sidenote: Shouldn’t the dealloc do [super dealloc] as well?

Answer 1

From reading your code, I believe you’re using Automatic Reference Counting (ARC). If you’re not, you should start. (ARC is the new default, Apple’s recommendation, and clearly the future.)

You have a circular reference: Your employee has an array of assets, which results in the assets being held in existence by the employee. Meanwhile each asset contains a strong reference to the owning holder employee. This means neither object can ever be released.

To fix this, convert the reference from the asset back to the holding Employee to be a weak reference.

@interface Asset : NSObject{
    NSString * label;
    __weak Employee * holder;
    unsigned int resaleValue;
}

Without this fix:

• Employee holds assets in memory.
• Assets hold employee in memory.
• Even if all other references to the employee and assets disappear, they’ll continue to hold each other in memory.

With this fix:

• Employee holds assets in memory.
• Assets do not hold employee in memory. Rather, they contain a safe, zeroing reference.
• The employee will be released when no longer needed by anything else.
• The holder instance variable will be “magically” cleared when the employee is released.
• Since the assets are no longer referenced, they too will pass.