Updated Answer for Changed Documentation
The information is now spread across several guides in the documentation. Here’s a list of required reading:
- Cocoa Core Competencies: Declared property
- Programming with Objective-C: Encapsulating Data
- Transitioning to ARC Release Notes
- Advanced Memory Management Programming Guide
- Objective-C Runtime Programming Guide: Declared Properties
The answer to this question now depends entirely on whether you’re using an ARC-managed application (the modern default for new projects) or forcing manual memory management.
Assign vs. Weak – Use assign to set a property’s pointer to the address of the object without retaining it or otherwise curating it; use weak to have the property point to nil automatically if the object assigned to it is deallocated. In most cases you’ll want to use weak so you’re not trying to access a deallocated object (illegal access of a memory address – “EXC_BAD_ACCESS“) if you don’t perform proper cleanup.
Retain vs. Copy – Declared properties use retain by default (so you can simply omit it altogether) and will manage the object’s reference count automatically whether another object is assigned to the property or it’s set to nil; Use copy to automatically send the newly-assigned object a -copy message (which will create a copy of the passed object and assign that copy to the property instead – useful (even required) in some situations where the assigned object might be modified after being set as a property of some other object (which would mean that modification/mutation would apply to the property as well).