Correct bridging for ARC?

问题:

I have a category class for NSString.

@implementation NSString (URLEncode)

- (NSString *)URLEncodedString
{
    __autoreleasing NSString *encodedString;

    NSString *originalString = (NSString *)self;    
    encodedString = (__bridge_transfer NSString * )
            CFURLCreateStringByAddingPercentEscapes(NULL,
                                (__bridge CFStringRef)originalString,
                                NULL,
                                (CFStringRef)@"!*'();:@&=+$,/?%#[]",
                                kCFStringEncodingUTF8);
    return encodedString;
}

Am I using the correct bridge transfers for ARC and the new LLVM?

The original code:

- (NSString *)URLEncodedString
    NSString *encodedString = (NSString *)CFURLCreateStringByAddingPercentEscapes(NULL,
                                (CFStringRef)self,
                                NULL,
                                (CFStringRef)@"!*'();:@&=+$,/?%#[]",
                                kCFStringEncodingUTF8);
    return [encodedString autorelease];
}

回答1:

As mentioned in the comments, I think it's fine to talk about ARC and the contents of Automatic Reference Counting here.

__autoreleasing is not meant to be used like that. It's used for passing indirect object references (NSError**, etc). See 4.3.4 Passing to an out parameter by writeback.

According to 3.2.4 Bridged casts, the __bridge_transfer is correct as the CFURLCreateStringByAddingPercentEscapes function returns a retained object (it has "create" in its name). You want ARC to take ownership of the returned object and insert a release (or autorelease in this case) to balance this out.

The __bridge cast for originalstring is correct too, you don't want ARC to do anything special about it.

回答2:

This is a correct, not leaking version. As you say in the comments: __bridge_transfer transfer the ownership to NSObject (NSString) and assume that the object is retained by CF Framework (the method CFURLCreateStringByAddingPercentEscapes return a retained object so this is what we need) than on the self object we don't want to perform any memory management. Hope it helps Fra

-(NSString *)urlEncodeUsingEncoding:(NSStringEncoding)encoding {
    return (__bridge_transfer NSString *)CFURLCreateStringByAddingPercentEscapes(NULL,
           (__bridge CFStringRef)self,
           NULL,
           (CFStringRef)@"!*'\"();:@&=+$,/?%#[]% ",
           CFStringConvertNSStringEncodingToEncoding(encoding));
}

回答3:

-(NSString *) urlEncoded
{
    CFStringRef encodedCfStringRef = CFURLCreateStringByAddingPercentEscapes(NULL,(CFStringRef)self,NULL,(CFStringRef)@"!*'\"();@+$,%#[]% ",kCFStringEncodingUTF8 );
    NSString *endcodedString = (NSString *)CFBridgingRelease(encodedCfStringRef);
    return endcodedString;
}

回答4:

No __autoreleasing necessary. The correct ARC syntax is simply:

- (NSString *)URLEncodedString
{
    return CFBridgingRelease(CFURLCreateStringByAddingPercentEscapes(NULL,
                                                                     (CFStringRef)self,
                                                                     NULL,
                                                                     (CFStringRef)@"!*'();:@&=+$,/?%#[]",
                                                                     kCFStringEncodingUTF8));
}