NSObject description and custom summaries in Xcode

lldb, objective-c, xcode

Solution

I would suggest at least:

- (NSString*)description {
    return [NSString stringWithFormat:@"%@; x=%f, y=%f", [super description], _x, _y];
}

So that you're not manually replicating the `NSObject` default and thereby blocking any non-default behaviour your superclass may have opted to include.

Beyond that, "summary string parsing error" is an lldb error. It's being reported by the debugger only. Per its documentation, `po` is correct for Objective-C objects; `p` is for C or C++ objects. So you needn't heed that error — it's essentially just telling you that you used the wrong lldb command.

EDIT: for what it's worth, the method used by `CFArray` is open source and looks like:

static CFStringRef __CFArrayCopyDescription(CFTypeRef cf) {
    CFArrayRef array = (CFArrayRef)cf;
    CFMutableStringRef result;
    const CFArrayCallBacks *cb;
    CFAllocatorRef allocator;
    CFIndex idx, cnt;
    cnt = __CFArrayGetCount(array);
    allocator = CFGetAllocator(array);
    result = CFStringCreateMutable(allocator, 0);
    switch (__CFArrayGetType(array)) {
    case __kCFArrayImmutable:
    CFStringAppendFormat(result, NULL, CFSTR("<CFArray %p [%p]>{type = immutable, count = %u, values = (%s"), cf, allocator, cnt, cnt ? "\n" : "");
    break;
    case __kCFArrayDeque:
    CFStringAppendFormat(result, NULL, CFSTR("<CFArray %p [%p]>{type = mutable-small, count = %u, values = (%s"), cf, allocator, cnt, cnt ? "\n" : "");
    break;
    }
    cb = __CFArrayGetCallBacks(array);
    for (idx = 0; idx < cnt; idx++) {
    CFStringRef desc = NULL;
    const void *val = __CFArrayGetBucketAtIndex(array, idx)->_item;
    if (NULL != cb->copyDescription) {
        desc = (CFStringRef)INVOKE_CALLBACK1(cb->copyDescription, val);
    }
    if (NULL != desc) {
        CFStringAppendFormat(result, NULL, CFSTR("\t%u : %@\n"), idx, desc);
        CFRelease(desc);
    } else {
        CFStringAppendFormat(result, NULL, CFSTR("\t%u : <%p>\n"), idx, val);
    }
    }
    CFStringAppend(result, CFSTR(")}"));
    return result;
}

As with the other comments above, I'm willing to gamble that the answer is: Xcode's debugger isn't smart in any sense and definitely isn't smart enough to use the correct `po` means of getting an Objective-C description; if your object is an uninflected Objective-C object then the debugger isn't going to be able to figure it out.

Problem

I override object's `-(NSString*)description` however Xcode always displays `error: summary string parsing error` in summary field in variables view. My current implementation is the following: ``` - (NSString*)description { return [NSString stringWithFormat:@"<%@ %p> x=%f, y=%f", self.class, self, _x, _y]; } ``` If I type `po objectName` in console, LLDB shows a fine output as expected, however Xcode and command `p objectName` always indicate error, so what's the proper debug description format to make summary field work? Worth to notice that the output of "p" command is the same as a summary message that you see in Xcode for instances of Foundation classes. Update: As far as I can see from "WWDC 2012 session Debugging in Xcode", custom summaries can be implemented using Custom python script only. `-(NSString*)description` or `-(NSString*)debugDescription` methods are not connected anyhow to summary messages. I thought they are because I got an error displayed, but it seems it's a standard message for classes that do not have their own formatters.

Original source