Objective-C 指针类型“NSString *”到 C 指针类型“CFStringRef”（又名“const struct __CFString *”）的转换需要桥接转换

Question

When converting an Objective-C program to a Objective-C ARC, I get the error:

"cast of Objective-C pointer type 'NSString *' to C pointer type 'CFStringRef' (aka 'const struct __CFString *') requires a bridged cast "

The code is as follows:

- (NSString *)_encodeString:(NSString *)string
{
    NSString *result = (NSString *)CFURLCreateStringByAddingPercentEscapes(NULL, 
                                   (CFStringRef)string, // this is line in error
                                   NULL, 
                                   (CFStringRef)@";/?:@&=$+{}<>,",
                                   kCFStringEncodingUTF8);
    return [result autorelease];
}

What is a bridged cast?

Answer 1

Have a look at the ARC documentation on the LLVM website. You'll have to use __bridge or one of the other keywords.

This is because Core Foundation objects (CF*Refs) are not controlled by ARC, only Obj-C objects are. So when you convert between them, you have to tell ARC about the object's ownership so it can properly clean them up. The simplest case is a __bridge cast, for which ARC will not do any extra work (it assumes you handle the object's memory yourself).

Answer 2

Here is a nice ARC tutorial that I found to be easier to understand than Apple's documentation that @jtbandes references.

Take a look at the section titled "Toll free bridging" in particular.

Answer 3

I know this is an old thread, I came across this issue while I need to use

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

So what I did is go to Target > Build phase > Compile sources . There is your file listed, double click on that and add following lines next to your file.

-fno-objc-arc