Ownership of NSString returned by reference from NSScanner

Question

Is the 'reference returned' NSString from the methods:

• scanString:intoString:
• scanCharactersFromSet:intoString:
• scanUpToString:intoString:
• scanUpToCharactersFromSet:intoString:

in the NSScanner class owned by the calling instance (retain count of 1 and NOT in the autorelease pool) or the NSScanner instance (retain count of 1 and in the autorelease pool)?

If it is the later would not the autorelease pool balloon in size if used to iterate over a list?

Answer 1

The value parameter of

- (BOOL)scanString:(NSString *)string intoString:(NSString **)value;

is an "Indirect parameter" in the sense of Indirect parameters in the Clang ARC documentation:

Indirect parameters

If a function or method parameter has type T*, where T is an ownership-unqualified retainable object pointer type, then:

• if T is const-qualified or Class, then it is implicitly qualified with __unsafe_unretained;
• otherwise, it is implicitly qualified with __autoreleasing.

The second case applies here. And __autoreleasing means:

For __autoreleasing objects, the new pointee is retained, autoreleased, and stored into the lvalue using primitive semantics.

So value points to an autoreleased object upon return from that function.

The Xcode autocompletion "knows" this and shows

[scanner scanString:(NSString *) intoString:(NSString *__autoreleasing *)]

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Added: For "manual reference counting", see Memory Management Policy in the "Advanced Memory Management Programming Guide":

You Don’t Own Objects Returned by Reference

... In these cases, the same rules apply as have already been described. When you invoke any of these methods, you do not create the NSError object, so you do not own it. There is therefore no need to release it, ...