Objective-C ARC: strong vs retain and weak vs assign

Question

There are two new memory management attributes for properties introduced by ARC, strong and weak.

Apart from copy, which is ob

Answer 1

Clang's document on Objective-C Automatic Reference Counting (ARC) explains the ownership qualifiers and modifiers clearly:

There are four ownership qualifiers:

• __autoreleasing
• __strong
• __*unsafe_unretained*
• __weak

A type is nontrivially ownership-qualified if it is qualified with __autoreleasing, __strong, or __weak.

Then there are six ownership modifiers for declared property:

• assign implies __*unsafe_unretained* ownership.
• copy implies __strong ownership, as well as the usual behavior of copy semantics on the setter.
• retain implies __strong ownership.
• strong implies __strong ownership.
• *unsafe_unretained* implies __*unsafe_unretained* ownership.
• weak implies __weak ownership.

With the exception of weak, these modifiers are available in non-ARC modes.

Semantics wise, the ownership qualifiers have different meaning in the five managed operations: Reading, Assignment, Initialization, Destruction and Moving, in which most of times we only care about the difference in Assignment operation.

Assignment occurs when evaluating an assignment operator. The semantics vary based on the qualification:

• For __strong objects, the new pointee is first retained; second, the lvalue is loaded with primitive semantics; third, the new pointee is stored into the lvalue with primitive semantics; and finally, the old pointee is released. This is not performed atomically; external synchronization must be used to make this safe in the face of concurrent loads and stores.
• For __weak objects, the lvalue is updated to point to the new pointee, unless the new pointee is an object currently undergoing deallocation, in which case the lvalue is updated to a null pointer. This must execute atomically with respect to other assignments to the object, to reads from the object, and to the final release of the new pointee.
• For __*unsafe_unretained* objects, the new pointee is stored into the lvalue using primitive semantics.
• For __autoreleasing objects, the new pointee is retained, autoreleased, and stored into the lvalue using primitive semantics.

The other difference in Reading, Init, Destruction and Moving, please refer to Section 4.2 Semantics in the document.