It says "keep this as long as someone else points to it strongly" -the same thing as assign, no retain or release. A "weak" reference is a reference that you do not retain. This works because the child object only needs to exist as long as the parent object does. A weak reference is a reference that does not protect the referenced object from collection by a garbage collector. Weak is essentially assign, a unretained property. Strong pointers are like a leash on the dog.
As long as you have the leash attached to the dog, the dog will not run away. If five people attach their leash to one dog, five strong pointers to one object , then the dog will not run away until all five leashes are detached. Weak pointers, on the other hand, are like little kids pointing at the dog and saying "Look!
A dog! As soon as all the leashes are detached, though, the dog runs away no matter how many little kids are pointing to it. As soon as the last strong pointer leash no longer points to an object, the object will be deallocated, and all weak pointers will be zeroed out.
When we use weak? The only time you would want to use weak, is if you wanted to avoid retain cycles e. It is retained, old value is released and it is assigned retain specifies the new value should be sent retain on assignment and the old value sent release -retain is the same as strong. Is default behavior will ensure the present process is completed by the cpu, before another process access the variable not fast, as it ensures the process is completed entirely. 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. Retain counts are the way in which memory is managed in Objective-C. When you create an object, it has a retain count of 1.
When you send an object a retain message, its retain count is incremented by 1. When you send an object a release message, its retain count is decremented by 1. When you send an object an autorelease message , its retain count is decremented by 1 at some stage in the future. Are multithreading and thread safety different? Multithreading means: multiple threads can read a shared piece of data at the same time and we will not crash, yet it doesn't guarantee that you aren't reading from a non-autoreleased value.
With thread safety, it's guaranteed that what you read is not auto-released. The reason that we don't make everything atomic by default is, that there is a performance cost and for most things don't really need thread safety. A few parts of our code need it and for those few parts, we need to write our code in a thread-safe way using locks, mutex or synchronization.
Allowing to be read right in the middle of a 'not yet finished write or empty value' or not allowing and only allowing to read when the value is fully written. Atomicity property attributes atomic and nonatomic are not reflected in the corresponding Swift property declaration, but the atomicity guarantees of the Objective-C implementation still hold when the imported property is accessed from Swift.
The nonatomic property specifies that synthesized accessors simply set or return a value directly, with no guarantees about what happens if that same value is accessed simultaneously from different threads. Atomic means only one thread can access the variable at a time static type. Nonatomic means multiple threads can access the variable at same time dynamic type.
If you are using atomic, it means the thread will be safe and read-only. If you are using nonatomic, it means the multiple threads access the variable and is thread unsafe, but it is executed fast, done a read and write operations; this is a dynamic type.
Mutex lock, as per the name, locks the mutability of the object. So if the object is accessed by a class, no other class can access the same object. In iOS, sychronise also provides the mutex lock.
Now it serves in FIFO mode and ensures the flow is not affected by two classes sharing the same instance. However, if the task is on main thread, avoid accessing object using atomic properties as it may hold your UI and degrade the performance.
Atomic properties :- When a variable assigned with atomic property that means it has only one thread access and it will be thread safe and will be good in performance perspective, will have default behaviour. Non Atomic Properties :- When a variable assigned with atomic property that means it has multi thread access and it will not be thread safe and will be slow in performance perspective, will have default behaviour and when two different threads want to access variable at same time it will give unexpected results.
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What's the difference between the atomic and nonatomic attributes? Ask Question. Asked 12 years, 8 months ago. Active 4 months ago. Viewed k times. What do atomic and nonatomic mean in property declarations? Improve this question. Chirag Kothiya 3 3 silver badges 18 18 bronze badges. Alex Wayne Alex Wayne k 44 44 gold badges silver badges bronze badges. Add a comment. Active Oldest Votes. Accept Reject Read More. Close Privacy Overview This website uses cookies to improve your experience while you navigate through the website.
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