Objective c what is nonatomic

You would also see greater differences in contested cases. Although I pay close attention to performance, I still say Semantics First! Meanwhile, performance is a low priority for many projects. However, knowing execution details and costs of technologies you use certainly doesn't hurt. You should use the right technology for your needs, purposes, and abilities. Hopefully this will save you a few hours of comparisons, and help you make a better informed decision when designing your programs.

Instance variables are thread-safe if they behave correctly when accessed from multiple threads, regardless of the scheduling or interleaving of the execution of those threads by the runtime environment, and with no additional synchronization or other coordination on the part of the calling code.

If a thread changes the value of the instance the changed value is available to all the threads, and only one thread can change the value at a time. Not as fast as nonatomic because nonatomic doesn't require any watchdog work on that from runtime. If the instance variable is not gonna be changed by multiple threads you can use it. It improves the performance. After reading so many articles, Stack Overflow posts and making demo applications to check variable property attributes, I decided to put all the attributes information together:.

In the article Variable property attributes or modifiers in iOS you can find all the above-mentioned attributes, and that will definitely help you. I found a pretty well put explanation of atomic and non-atomic properties here.

Here's some relevant text from the same:. Just in case you didn't know: since the CPU can only do one thing at a time, the OS rotates access to the CPU to all running processes in little time-slices, to give the illusion of multitasking. The CPU scheduler can and does interrupt a process at any point in its execution - even in mid function call. So for actions like updating shared counter variables where two processes could try to update the variable at the same time, they must be executed 'atomically', i.

Because the atomic variables can not be interrupted, the value contained by them at any point is thread-lock guaranteed to be uncorrupted , although, ensuring this thread lock makes access to them slower. To sum it up, go with non-atomic when you know your variables won't be accessed by multiple threads simultaneously and speed things up.

Atomic guarantees that access to the property will be performed in an atomic manner. If you imagine the following function occurring on two threads at once you can see why the results would not be pretty. Pros : Return of fully initialised objects each time makes it best choice in case of multi-threading. Easiest answer first: There's no difference between your second two examples. By default, property accessors are atomic. Atomic accessors in a non garbage collected environment i. Atomic means only one thread accesses the variable static type.

Atomic is thread-safe, but it is slow. Nonatomic means multiple threads access the variable dynamic type. Nonatomic is thread-unsafe, but it is fast. Atomic is thread safe , it is slow and it well-assures not guaranteed that only the locked value is provided no matter how many threads are attempting access over the same zone. When using atomic, a piece of code written inside this function becomes the part of the critical section, to which only one thread can execute at a time. It only assures the thread safety; it does not guarantee that.

What I mean is you hire an expert driver for you car, still it doesn't guarantees car won't meet an accident. However, probability remains the slightest. Atomic - it can't be broken down, so the result is expected. With nonatomic - when another thread access the memory zone it can modify it, so the result is unexpected.

An atomic property is guaranteed that if you try to read from it, you will get back a valid value. It does not make any guarantees about what that value might be, but you will get back good data, not just junk memory. What this allows you to do is if you have multiple threads or multiple processes pointing at a single variable, one thread can read and another thread can write.

If they hit at the same time, the reader thread is guaranteed to get one of the two values: either before the change or after the change.

What atomic does not give you is any sort of guarantee about which of those values you might get. Atomic is really commonly confused with being thread-safe, and that is not correct. You need to guarantee your thread safety other ways. However, atomic will guarantee that if you try to read, you get back some kind of value.

If you try to read in the middle of a write, you could get back garbage data. But, on the other hand, you go a little bit faster. Because atomic properties have to do some magic to guarantee that you will get back a value, they are a bit slower. If it is a property that you are accessing a lot, you may want to drop down to nonatomic to make sure that you are not incurring that speed penalty.

The default is atomic , this means it does cost you performance whenever you use the property, but it is thread safe. On the other hand does nonatomic add nothing to your code.

So it is only thread safe if you code security mechanism yourself. Don't forget, this doesn't mean that the property as a whole is thread-safe. But if you use a setter and after that a getter at the same time with 2 different threads, it could be broken too!

Thats why non atomic is called thread unsafe But but it is fast in performance because of parallel execution. That's why atomic is called thread Safe and That's why it is called read-write safe. One reason is so you think about whether per-property atomicity is sufficient for your needs. If you are using your property in multi-threaded code then you would be able to see the difference between nonatomic and atomic attributes. Nonatomic is faster than atomic and atomic is thread-safe, not nonatomic.

Before you begin: You must know that every object in memory needs to be deallocated from memory for a new writer to happen. You can't just simply write on top of something as you do on paper. You must first erase dealloc it and then you can write onto it. If at the moment that the erase is done or half done and nothing has yet been wrote or half wrote and you try to read it could be very problematic!

Atomic and nonatomic help you treat this problem in different ways. First read this question and then read Bbum's answer. In addition, then read my summary. Thread-safe means, at a time only one thread of a particular instance of that class can have the control over that object.

If the thread is performing getter method then other thread cannot perform setter method on that object. It is slow. The setter method will increase retain count of the object, so that it will occupy memory in autorelease pool. Even if a mutable string is set and subsequently changed, the instance captures whatever value it has at the time it is set.

No setter and getter methods will be synthesized. If you need to specify multiple attributes, simply include them as a comma-separated list, like this:. Variable property attributes or Modifiers in iOS. Suppose there is an atomic string property called "name", and if you call [self setName: "A"] from thread A, call [self setName: "B"] from thread B, and call [self name] from thread C, then all operation on different thread will be performed serially which means if one thread is executing setter or getter, then other threads will wait.

Developer should ensure thread safety for such objects. If the property "name" was nonatomic, then all threads in above example - A,B, C and D will execute simultaneously producing any unpredictable result. Objective-C declared property attributes nonatomic, copy, strong, weak Nonatomic Nonatomic will not generate threadsafe routines thru synthesize accessors.

Assign Assign is somewhat the opposite to copy. Retain retain is required when the attribute is a pointer to a reference counted object that was allocated on the heap. The above link contain both Good information regarding Weak and Strong. Please reconnect the device. I just performed a simple test which I think answers my question. I created a new Swift class inherited from NSObject and added a reference type property.

In the generated header for Objective-C it shows that property declared as nonatomic. Search by keywords or tags Submit Search Clear search query Additional information about Search by keywords or tags Supported Searches:. Click again to start watching.