Build and Analyze false positive on leak detection?

Question

I have this code:

- (CGImageRef)createImageWithContext:(CGContextRef)context
{
    return CGBitmapContextCreateImage(context);
}

- (void)fooWithContext:(CGContextRef)context
{
    CGImageRef imgRef = [self createImageWithContext:context];
    CGImageRelease(imgRef);
}

This is an Objective-C project building in Xcode with ARC enabled. Build and Analyze reports two errors: One on the CGBitmapContextCreateImage line identifying a potential leak, and one on the CGImageRelease noting an "Incorrect decrement of the reference count of an object that is not owned at this point by the caller".

If I combine these two functions into one:

- (void)fooWithContext:(CGContextRef)context
{
    CGImageRef imgRef = CGBitmapContextCreateImage(context);
    CGImageRelease(imgRef);
}

I get no warnings.

Static code analysis bug? Or am I missing something here?

Answer 1

By the standard Cocoa naming conventions, a method that starts with the word create should return a non-owned reference. You're returning a retained object, but you're expected to return a non-retained object. Thus, the when the analyzer looks at -createImageWithContext:, it sees that it's supposed to return a non-retained object, but is actually returning a retained object. Hence the first warning.

In -fooWithContext:, it looks at your code and says "Hey, according to my naming conventions, createImageWithContext: should return a non-owning reference. But then they're releasing something they don't own! That's BAD!" Hence the second warning.

You could fix this by changing the name of -createImageWithContext: to something that starts with new, such as -newImageWithContext:. Or you could annotate the method with the cf_returns_retained macro to indicate to the static analyzer that the method is returning an owning reference.