Is it possible to remove dispatch_once in Objective-C++?

Question

Since C++11, local static variables are known to be initialized in a thread safe manner (unless the -fno-threadsafe-statics is given), as specified

Answer 1

Generally, Objective-C++, which allows mixing Objective-C-Objects and code with C++ objects and code, is a different language than "pure" C++11. Therefore, I don't think that everything guaranteed for C++11 is automatically guaranteed in Objectiver-C++'s mixed world. And I have been spending some time now investigating apple's documentation whether specific guarantees on static local variables or even block variables are also given in Objective-C++.

As I did not find a statement to this, I tried introducing a race condition on the creation of an object, one with the proposed "new style", i.e. using a static local variable, one with the "old style" with dispatch_once, and one "real" race condition "notOnlyOnce" ignoring any synchronization (just to be sure that the code actually introduces a race condition).

The tests show that both "new style" and "old style" seem to be thread safe, whereas "notOnlyOnce" clearly is not. Unfortunately, such a test could have just proofen that "new style" produces a race condition, but it cannot proof that there will never be a race condition. But as "new style" and "old style" behave the same, but "notOnlyOnce" shows up a race condition in the same setting, we can at least assume that static local variables work as you proposed.

See the following code and the respective outputs.

@interface SingletonClass : NSObject

- (instancetype)init;

@end

@implementation SingletonClass

- (instancetype)init {
    self = [super init];
    std::cout << "Created a singleton object" << std::endl;
    for (int i=0; i<1000000; i++) { i++; }
    return self;
}

@end

@interface TestClassObjCPP : NSObject 

@property (nonatomic) SingletonClass *sc;

+ (SingletonClass *)onlyOnceNewStyle;
+ (SingletonClass *)onlyOnceOldStyle: (TestClassObjCPP*)caller;
+ (SingletonClass *)notOnlyOnce: (TestClassObjCPP*)caller;

@end

@implementation TestClassObjCPP


+ (SingletonClass *)onlyOnceNewStyle {
    static SingletonClass *object = [[SingletonClass alloc] init];
    return object;
}

+ (SingletonClass *)onlyOnceOldStyle: (TestClassObjCPP*)caller {

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        caller.sc = [[SingletonClass alloc] init];
    });

    return caller.sc;
}

+ (SingletonClass *)notOnlyOnce: (TestClassObjCPP*)caller {

    if (caller.sc == nil)
        caller.sc = [[SingletonClass alloc] init];

    return caller.sc;
}

@end


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


    @autoreleasepool {

        std::cout << "Before loop requesting singleton." << std::endl;
        TestClassObjCPP *caller = [[TestClassObjCPP alloc] init];
        caller.sc = nil;
        for (int i=0; i<10000; i++) {
            dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                [TestClassObjCPP onlyOnceNewStyle];  // (1)
                // [TestClassObjCPP onlyOnceOldStyle:caller]; // (2)
                // [TestClassObjCPP notOnlyOnce:caller]; // (3)
            });

        }
        std::cout << "After loop requesting singleton." << std::endl;

        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

Output for onlyOnceNewStyle (1):

Before loop requesting singleton.
Created a singleton object
After loop requesting singleton.

Output for onlyOnceOldStyle (2):

Before loop requesting singleton.
Created a singleton object
After loop requesting singleton.

Output for notOnlyOnce (3):

Before loop requesting singleton.
Created a singleton object
Created a singleton object
Created a singleton object
After loop requesting singleton.

So not a clear yes or no, but I hope it helps in some way.