Challenge

When employees is set to nil, the array no longer has an owner. So it is deallocated, which sets up an even larger chain reaction of memory clean-up and deallocation when, suddenly, none of the employees has an owner.

Tidy, right? As the objects become unnecessary, they are being deallocated. This is good. When unnecessary objects don’t get deallocated, you are said to have a memory leak. Typically a memory leak causes more and more objects to linger unnecessarily over time. The memory footprint of your application just gets bigger and bigger. On iOS, the operating system will eventually kill your application. On Mac OS X, the performance of the entire system will suffer as the machine spends more and more time swapping data out of memory and onto disk.

Challenge

Using the StockHolding class from a previous challenge, make a tool that creates an instance of a Portfolio class and fills it with stock holdings. A portfolio can tell you what its current value is.

123

This page intentionally left blank

20

Preventing Memory Leaks

It is pretty common to have relationships that go in two directions. For example, maybe an asset should know which employee is currently holding it. Let’s add that relationship. The new object diagram would look like this:

Figure 20.1 Adding holder relationship

From a design standpoint, you would say that we are adding a pointer from the child (an instance of Asset) back to its parent (the instance of Employee that is holding it).

In Asset.h, add a pointer instance variable to hold on to the holding employee:

#import <Foundation/Foundation.h>

@class Employee;

@interface Asset : NSObject

{

NSString *label;

Employee *holder; unsigned int resaleValue;

}

@property (strong) NSString *label;

@property (strong) Employee *holder;

125

Chapter 20 Preventing Memory Leaks

@property unsigned int resaleValue; @end

In Asset.m, synthesize the accessors and extend the description method to display the holder:

#import "Asset.h"

#import "Employee.h"

@implementation Asset

@synthesize label, resaleValue, holder;

- (NSString *)description

{

// Is holder non-nil? if ([self holder]) {

return [NSString stringWithFormat:@"<%@: $%d, assigned to %@>", [self label], [self resaleValue], [self holder]];

} else {

return [NSString stringWithFormat:@"<%@: $%d unassigned>", [self label], [self resaleValue]];

}

}

- (void)dealloc

{

NSLog(@"deallocing %@", self);

}

@end

This brings us to a style question: When people use the Asset class and Employee class together, how do we make sure that the two relationships are consistent? That is, an asset should appear in an employee’s assets array if and only if the employee is the asset’s holder. There are three options:

• Set both relationships explicitly:

[vicePresident addAssetsObject:townCar]; [townCar setHolder:vicePresident];

• In the method that sets the child’s pointer, add the child to the parent’s collection.

- (void)setHolder:(Employee *)e

{

holder = e;

[e addAssetsObject:self];

}

This approach is not at all common.

• In the method that adds the child to the parent’s collection, set the child’s pointer.

In this exercise, you will take this last option. In Employee.m, extend the addAssetsObject: method to also set holder:

126