Avoid Excessive Retaining Mistake

When I ran "Analyze" on my project, I got a lot of "potential leak" warnings. I thought I was carefully managing object retaining but I did stupid mistakes everywhere.

part of the sample header file

@property (nonatomic, retain) ClassA objA;

part of the sample source file

@synthesize objA;
...
self.objA = [[ClassA alloc] init];
...
// done using objA instance
[self.objA release];

Above example seems good and there is no places for mistake. But actually retaining is happened twice.

• objA has "retain" attribute
• it will retain anything with assigning operator ("=")
• "alloc" method will increase retain count too

Corrected code

Class *tmpA = [[ClassA alloc] init];
self.objA = tmpA;
[tmpA release];
...
// done using objA instance
[self.objA release];

Objective-C Singleton Class

Singleton class is really awesome when I need to share some data throughout many places. If I use normal way which is passing object pointer around, I have to care about a lot of object assigning tasks. With singleton class, it is really simple to use shared class object.

• no instanciation
• one class method do the all necessary job
• first time of the class method call > create single instance
• other class method calls > return already created instance
• how to use?
• include singleton class's header file
• use class method to obtain class instance

Sample Class's header file

@interface SampleClass : NSObject

+ (SampleClass*)sharedInstance;

@end

Sample Class's source file

@implementation SampleClass

static SampleClass *sharedInstance = nil;   // shared instance

+ (SampleClass*)sharedInstance
{
    @synchronized(self)
    {
        if (sharedInstance == nil)
            sharedInstance = [[super allocWithZone:nil] init];

        return sharedInstance;
    }
}

+ (id)allocWithZone:(NSZone*)zone
{
    return [[self sharedInstance] retain];
}

- (id)copyWithZone:(NSZone*)zone
{
    return self;
}

- (id)retain
{
    return self;
}

- (NSUInteger)retainCount
{
    return NSUIntegerMax;  // object cannot be released
}

- (oneway void)release
{
    // do nothing
}

- (id)autorelease
{
    return self;
}