Atomic vs Nonatomic: Key Differences Explained

Atomic and nonatomic attributes in Objective-C property declarations primarily differ in their thread safety guarantees and performance characteristics. Atomic properties ensure that property access operations are thread-safe by using synchronization locks, while nonatomic properties provide faster access but without thread safety guarantees. The first declaration explicitly specifies nonatomic, the second explicitly specifies atomic, and the third defaults to atomic since it’s the default behavior in Objective-C.

Contents

• Basic Definition and Purpose
• Thread Safety Differences
• Performance Implications
• Implementation Details
• Practical Usage Guidelines
• The Three Declaration Styles Compared
• Common Misconceptions

Basic Definition and Purpose

In Objective-C property declarations, atomic and nonatomic specify the synchronization behavior of property accessors. As Apple’s documentation explains, properties are atomic by default, meaning synthesized accessors provide robust access to properties in a multithreaded environment.

The key distinction lies in how property values are accessed and modified when multiple threads are involved:

• Atomic properties: Ensure that the getter and setter operations are fully completed regardless of concurrent thread activity
• Nonatomic properties: Provide direct access with no synchronization guarantees

Thread Safety Differences

Atomic properties implement thread safety through synchronization mechanisms. When you declare a property as atomic, the runtime ensures that property access operations cannot be interrupted or corrupted by other threads.

According to Stack Overflow discussions, atomic properties “ensure thread safety by locking the thread using NSLOCK.” This means:

The property is thread-safe at the pointer level - an object is read/write safe (ATOMIC) but not thread safe as another threads can simultaneously send any type of messages to the object.

However, it’s crucial to understand that atomic properties only guarantee the atomicity of the pointer operation itself, not the entire object’s thread safety. As one source explains:

An atomic property is guaranteed to never read back a garbage value due to multiple writer threads.

For nonatomic properties, the situation is different:

Multiple threads can access the properties simultaneously, so there is always a risk of inconsistency between values.

Performance Implications

The thread safety provided by atomic properties comes with performance overhead. As Stack Overflow documentation states:

You can use the nonatomic property attribute to specify 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.

This direct access approach makes nonatomic properties significantly faster than their atomic counterparts, especially in high-frequency access scenarios.

Implementation Details

The actual implementation of atomic properties involves more complex code. As Apple’s documentation reveals:

If you specify strong, copy, or retain and do not specify nonatomic, then in a reference-counted environment, a synthesized get accessor for an object property uses a lock and retains and autoreleases the returned value.

This means atomic properties typically generate code similar to:

- (NSString *)firstName {
    @synchronized(self) {
        return [_firstName retain];
    }
}

In contrast, nonatomic properties generate much simpler code:

- (NSString *)firstName {
    return _firstName;
}

The atomic implementation ensures that the returned object has a valid reference count even if the original object is deallocated by another thread during the getter operation.

Practical Usage Guidelines

When to Use Atomic Properties

• When you need basic thread safety for simple property access
• When the property might be accessed from multiple threads
• When you’re developing a library or framework where thread safety is important
• When you’re unsure about the threading behavior of consumers of your class

When to Use Nonatomic Properties

• When performance is critical and the property is accessed frequently
• When you have explicit thread synchronization in place
• When the property is only accessed from a single thread
• When you’re working with UI elements that are typically accessed from the main thread

As a general rule in iOS development, most UI-related properties are declared as nonatomic since they’re typically accessed from the main thread, and the performance benefits outweigh the thread safety concerns.

The Three Declaration Styles Compared

Let’s examine the three specific property declarations you mentioned:

1. @property(nonatomic, retain) UITextField *userName;

This declaration explicitly specifies:

• nonatomic: No thread synchronization
• retain: Object ownership follows the old reference counting rules
• *UITextField userName: The property name and type

2. @property(atomic, retain) UITextField *userName;

This declaration explicitly specifies:

• atomic: Thread synchronization is used
• retain: Object ownership follows the old reference counting rules
• *UITextField userName: The property name and type

3. @property(retain) UITextField *userName;

This declaration only specifies:

• retain: Object ownership follows the old reference counting rules
• *UITextField userName: The property name and type

The third declaration is functionally identical to the second because atomic is the default behavior in Objective-C. As Repeato’s documentation explains:

The second and third declarations are functionally identical because atomic is the default behavior.

In modern Objective-C with ARC (Automatic Reference Counting), the retain attribute is typically replaced with strong, but the atomic/nonatomic distinction remains the same.

Common Misconceptions

Misconception 1: Atomic properties are fully thread-safe

Many developers believe that atomic properties make objects fully thread-safe. This is not true. As one expert explains:

Atomic does not mean thread safe… An atomic property will guarantee that a valid value will be returned. Notice that valid does not always mean correct.

Misconception 2: Nonatomic properties are always unsafe

While nonatomic properties don’t provide thread safety guarantees, this doesn’t mean they’re inherently “unsafe.” They’re appropriate when:

• The property is only accessed from one thread
• You implement your own synchronization
• The performance benefits outweigh the risks

Misconception 3: Atomic properties prevent race conditions

Atomic properties only guarantee that pointer operations are atomic. They don’t prevent race conditions in complex operations that involve multiple properties or method calls.

Conclusion

The difference between atomic and nonatomic properties in Objective-C comes down to thread synchronization and performance:

• Atomic properties (default behavior) provide thread-safe pointer access through synchronization locks but with performance overhead
• Nonatomic properties offer faster access without thread safety guarantees
• The three declaration styles you provided result in just two distinct behaviors: nonatomic and atomic (with the third being identical to the second)

When choosing between atomic and nonatomic, consider your application’s threading requirements and performance needs. For most modern iOS development, nonatomic is preferred for UI properties due to performance benefits, while atomic may be appropriate for data properties that might be accessed from multiple threads.

Remember that atomic properties only provide basic thread safety at the pointer level - they don’t make your entire object thread-safe. For complex threading scenarios, you’ll need additional synchronization mechanisms beyond just atomic properties.

Sources

1. What’s the difference between the atomic and nonatomic attributes? - Stack Overflow
2. Understanding the Difference Between Atomic and Nonatomic Attributes in Objective-C - Repeato
3. Atomic vs. Non Atomic Properties Crash Course - Medium
4. Declared Properties - Apple Developer Documentation
5. iOS - @property retain, assign, copy, nonatomic in Objective-C - Stack Overflow
6. atomic vs nonatomic properties in Objective-C - Nabeel Writes
7. reference type properties atomic vs nonatomic - Apple Developer Forums