如何将NSValue转换为NSString

Question

Some background... I am writing code that interacts with javascript via a ObjC-JS bridge utilizing UIWebView 's stringByEvaluatingJavaScriptFromString: . The idea is that the "brains" of the app be in JS which tells Objective-C how to behave. There are multiple benefits to this like reduced binary size, flexible updates, etc. However, there is a case where there is some Objective-C only object that the JS needs to have a reference to (JS instructs ObjC when to use/remove the object). This is being done by placing the native object in a dictionary with a unique identifier which can be passed as a string to JS (over the bridge). My problem stems with coming up with a nice identifier for said native Objective-C object.

Thus, I am trying to convert a reference to an object to a string with no luck. This is what I have:

// anObject is a custom class
NSValue *handle = [NSValue valueWithPointer:(__bridge const void *)anObject]; 
NSData *data = [NSData dataWithValue:handle];
NSString *stringHandle = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

The dataWithValue: function ( taken from this SO post ):

+ (NSData *)dataWithValue:(NSValue *)value {
    NSUInteger size;
    const char* encoding = [value objCType];
    NSGetSizeAndAlignment(encoding, &size, NULL);

    void* ptr = malloc(size);
    [value getValue:ptr];
    NSData* data = [NSData dataWithBytes:ptr length:size];
    free(ptr);

    return data;
}

Walking through it in the debugger shows me a nil value for stringHandle :

What am I doing wrong?

Answer 1

What you're doing wrong is trying to treat an address as if it's a UTF-8 encoded string. An address -- or any other chunk of arbitrary data -- isn't very likely to be valid UTF-8 data. (If by chance it were, it still wouldn't be the string you expect.)

If you're trying to get a string containing the pointer value, ie, the address of the original object, that's just [NSString stringWithFormat:@"%p", anObject];

If you really need to do it from the NSValue , then replace anObject with [theValue pointerValue] .

If you want to pretty-print arbitrary data, see How to convert an NSData into an NSString Hex string?

Answer 2

You can get a string representation by calling the NSObject method "description". You can override the "description" method in a subclass if you need.

An NSValue of a pointer will be an object holding the 4 bytes of the 32-bit pointer. It will not hold any of the data pointed to in RAM.