CGImageRef属性保留或不保留

Question

I have a question on how to handle a CGImageRef as a synthesized property of a class. If I define an CGImageRef with

@property (nonatomic, retain) CGImageRef image;

then the compiler complains that "retain" cannot be used here. If I leave out the retain, then I assume "assign" is used instead, and I need to do the retain myself when I set the property:

self.image = CGImageRetain ( cgimage );

then I get an "Potential leak" warning when running Analyze. Can I safely ignore this warning? Or does the synthesize code do an implicit CGRetain anyways, even though no "retain" is specified in the property definition?

Answer 1

What you want to do is add an annotation to the property that the type really can be retained.

Change the property declaration to

@property (nonatomic, retain) CGImageRef image __attribute__((NSObject));

Note that this will only generate the getters and setters for you, the instance variable itself is not ARC controlled. Specifically, this means that you must release it in dealloc , and that you need to use proper retain and release when assigning directly to the instance variable.

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A better approach may be to use a typedef :

typedef CGImageRef CGImageObject __attribute__((NSObject));
@property (nonatomic, retain) CGImageObject image;

In this case, the instance variable is controlled by ARC, and so you must not release it in dealloc , and direct assignments to the instance variable are handled by ARC as well.

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For reference, see the specification , specifically section 4.1.1 :

Applying __attribute__((NSObject)) to a property not of retainable object pointer type has the same behavior it does outside of ARC: it requires the property type to be some sort of pointer and permits the use of modifiers other than assign . These modifiers only affect the synthesized getter and setter; direct accesses to the ivar (even if synthesized) still have primitive semantics, and the value in the ivar will not be automatically released during deallocation.

and section 3 :

A retainable object pointer (or “retainable pointer”) is a value of a retainable object pointer type (“retainable type”). There are three kinds of retainable object pointer types:

• block pointers (formed by applying the caret ( ^ ) declarator sigil to a function type)
• Objective-C object pointers ( id , Class , NSFoo* , etc.)
• typedefs marked with __attribute__((NSObject))

Answer 2

I don't like to instruct the compiler when compiling. I think it's ugly. I'd override the methods myself.

@interface MyClass : NSObject {
    CGImageRef _image;
}
@property (nonatomic, assign) CGImageRef image;
@end

@implementation MyClass
- (void)setImage:(CGImageRef)i {
    if(_image != i) {
        CGImageRelease(_image);
        _image = CGImageRetain(i);
    }
}

- (CGImageRef)image {
    return _image;
}
@end

Answer 3

How about this?

@property (nonatomic, setter=setImage:) CGImageRef image;

(void)setImage:(CGImageRef)image {
    if (_image != image) {
        CGImageRelease(_image);
        _image = CGImageRetain(image);
    }
}