Question

I’m trying to write a class that I can subclass to have an instant singleton. Here’s what I have so far. It works until one of its subclasses calls another via sharedInstance which causes a huge loop that eventually runs out of memory.

Any ideas?

static NSMutableDictionary *sharedInstances = nil;

@implementation Singleton

+ (Singleton*)sharedInstance
{
    [Singleton initSharedInstances];
    Class myClass = [self class];
    Singleton * sharedInstance = [sharedInstances objectForKey:myClass];
    @synchronized(myClass)
    {
        if (sharedInstance == nil)
        {
            sharedInstance = [[myClass alloc] init];
            [sharedInstances setObject:sharedInstance forKey:myClass];
        }
    }
    return sharedInstance;
}

+ (void) initSharedInstances
{
    if (sharedInstances == nil)
    {
        sharedInstances = [[NSMutableDictionary alloc] init];
    }
}

@end

Answer 1

I’m trying to write a class that I can subclass to have an instant singleton. Here’s what I have so far. It works until one of its subclasses calls another via sharedInstance which causes a huge loop that eventually runs out of memory.

This sounds like you are describing mutual-recursion. If subclass1 calls subclass2 and subclass2 calls subclass1 then you need to break the loop somewhere, just as with simple self-recursion.

Your sharedInstance itself should not cause infinite recursion unless the init method you invoke itself invokes sharedInstance…