In Objective C memory management for iOS or macOS, what is the main difference between the property attributes \"assign\" and \"retain\"?

Introduction / Context:
Before the introduction of Automatic Reference Counting, Objective C developers manually managed object lifetimes using reference counting. Property attributes like assign and retain controlled how setter methods handled incoming values. Understanding these attributes is important for reading legacy code and passing interviews related to Objective C memory management. This question focuses on the key difference between assign and retain.

Given Data / Assumptions:

• We are working with Objective C properties in environments that use manual retain and release or that respect these semantics under ARC.
• The assign attribute simply assigns the value without changing reference counts.
• The retain attribute calls retain on the new object and typically releases the old one.
• We are considering object pointers, not only primitive types.

Concept / Approach:
The assign keyword generates a simple setter that assigns the pointer value directly. For object pointers, this does not affect the object's reference count, which can lead to dangling pointers if the object is deallocated elsewhere. The retain keyword, by contrast, calls retain on the incoming object and usually releases the previous value, creating strong ownership and keeping the object alive as long as needed. In modern Objective C, retain is conceptually similar to strong. Therefore, the correct option must state that assign does not modify reference counts, while retain increments the count and maintains ownership.

Step-by-Step Solution:
Step 1 Recall that Objective C uses reference counting, where retain increments and release decrements the count. Step 2 Understand that a property declared with assign generates a setter that simply assigns the pointer without calling retain. Step 3 Understand that a property declared with retain generates a setter that calls retain on the new value and release on the old value, thereby managing ownership. Step 4 Choose the option that clearly states this difference in reference count behavior between assign and retain.

Verification / Alternative check:
Apple documentation for Objective C properties explains that assign is suitable for primitive types and weak references, while retain (and later strong) is appropriate for owning object references. Code examples show that retain guarantees the object stays in memory as long as the property refers to it, while assign does not. These descriptions align with the explanation provided in the correct option.

Why Other Options Are Wrong:
The statement that assign automatically releases the old object is incorrect; assign does not interact with reference counting at all. Saying that assign is only for objects and retain only for primitives reverses the real recommendation. The idea that assign creates strong ownership while retain creates a weak reference is wrong; in fact, retain provides strong ownership. Claiming there is no difference between assign and retain contradicts both documentation and actual behavior.

Common Pitfalls:
Developers sometimes misuse assign for object properties and end up with dangling pointers when the object is deallocated elsewhere. Another pitfall is forgetting to release the previous object when using retain manually, which can cause memory leaks. Modern ARC simplifies this, but understanding assign and retain remains valuable for maintaining older code and for conceptual clarity.

Final Answer:
The main difference is that assign does not change the reference count, while retain increases the reference count and keeps the object alive.