NSAssert in Singleton : why this code is valid?

Question

i don't understand the use of NSAssert in +alloc, as when +alloc is called from +sharedGameManager, the static _sharedGameManager variable is nil (so NSAssert should stop execution the first time [self alloc] init] is called...)

+(GameManager*)sharedGameManager {
    @synchronized([GameManager class])                             
    {
        if(!_sharedGameManager)                                    
            [[self alloc] init]; 
        return _sharedGameManager;                                 
    }
    return nil; 
}

+(id)alloc 
{
    @synchronized ([GameManager class])                            
    {
        NSAssert(_sharedGameManager == nil,
                 @"Attempted to allocated a second instance of the Game Manager singleton");
        _sharedGameManager = [super alloc];
        return _sharedGameManager;                                 
    }
    return nil;  
}

Thanks for your answer

hooleyhoop · Accepted Answer · 2012-04-19T23:21:31.350

3

Are you thinking of NSAssert the wrong way round?

NSAssert( _sharedGameManager==nil, @"Attempted to …");

Will throw an Exception if _sharedGameManager is not nil. It Asserts that the expression is TRUE, it says "I assert that this must be the case", therefore _sharedGameManager must be nil, or an exception is raised. This could only happen if you tried to create 2 instances of this Class.

edited Apr 19 '12 at 23:21

answered Apr 19 '12 at 23:09

hooleyhoop

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oh yes true, i don't why i was sure it was the contrary :) thanks for the answer! – Paul Apr 19 '12 at 23:25

score 0 · Answer 2 · answered Apr 19 '12 at 21:48

0

This looks like a couple of snippets glued together; alloc is an instance method, not a class method (static). If you want to perform initialization of a singleton class, use +(void)initialize

The Objective-C runtime claims to guarantee this class method only gets executed once, so it is an effective mechanism for setting up a singleton. For further reading, hit up Mike Ash's blog post on the topic.

answered Apr 19 '12 at 21:48

wobbals

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8

1

`+alloc` _is_ a class method. Otherwise, you're right - it does seem Paul just has two unrelated snippets put together. Everyone really needs to use `dispatch_once` to create singletons, and not the pattern given by Apple. – Itai Ferber Apr 19 '12 at 22:15
@wobbals : thanks wobbals, alright thanks for the tips, i found this code in a book, what do you think about this NSAssert? is it a mistake? can it work like that? if so, i would not understand why – Paul Apr 19 '12 at 22:45
Hmm, it really doesn't look anything like two unrelated snippets. Both methods could only ever work in the presence of each other. – hooleyhoop Apr 19 '12 at 23:16
@hooleyhoop You're right, I wasn't clear in my thinking when I glanced at this code. You _can_ implement the singleton without overriding alloc (as in the Mike Ash example), but there are clearly multiple acceptable approaches. – wobbals Apr 24 '12 at 18:11

Itai Ferber · Answer 3 · 2012-04-19T22:34:02.563

0

There's a better way to guarantee the behavior you want, through Grand Central Dispatch:

+ (GameManager *)sharedGameManager {
    static GameManager *sharedGameManager = nil;
    static dispatch_once_t token;
    dispatch_once(&token, ^{
        sharedGameManager = [[GameManager alloc] init];
    });

    return sharedGameManager;
}

dispatch_once is guaranteed to be run only once, so your game manager won't be over-initialized. As long as you don't release it, it will stay alive, and it will correctly be released at the end of your program (because of its static context).

edited Apr 19 '12 at 22:34

answered Apr 19 '12 at 22:21

Itai Ferber

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@RichardJ.RossIII Oy, you're right. I was reading too quickly, and mixed things up in my head. You're totally right. Whoops. – Itai Ferber Apr 19 '12 at 22:30
1

@RichardJ.RossIII Anyway, updated my answer to remove that bit. – Itai Ferber Apr 19 '12 at 22:34
@Itai Ferber : thanks for the answer, i'll use this way, i was still wondering why the code used NSAssert, which seems (to me) to block the code, do you have any idea about that? – Paul Apr 19 '12 at 22:48

NSAssert in Singleton : why this code is valid?

3 Answers3