long windowId = kCGNullWindowID;
CGRect *temp = new CGRect;

// get all windows on the current screen
CFArrayRef windowList = CGWindowListCopyWindowInfo(kCGWindowListOptionAll | kCGWindowListOptionOnScreenOnly, kCGNullWindowID);

// find the current window, that will be the first one on layer 0
CFIndex windowNum = CFArrayGetCount(windowList);
for (int i=0;i<(int)windowNum;i++) {
    CFDictionaryRef info = (CFDictionaryRef)CFArrayGetValueAtIndex(windowList, i);

    CFNumberRef currentWindowLayer = (CFNumberRef)CFDictionaryGetValue(info, kCGWindowLayer);

    long layer = LONG_MAX;
    CFNumberGetValue(currentWindowLayer, kCFNumberLongType, &layer);

    if (layer > 0)
        continue;

    // get window title (not necessary, but as reference, don't forget to free the string)
    CFStringRef currentTitle = (CFStringRef)CFDictionaryGetValue(info, kCGWindowName);
    char* bla = MYCFStringCopyUTF8String(currentTitle);

    // get the window number
    CFNumberRef currentWindowNumber = (CFNumberRef)CFDictionaryGetValue(info, kCGWindowNumber);
    CFNumberGetValue(currentWindowNumber, kCFNumberLongType, &windowId);

    // get the window rect
    CFDictionaryRef bounds = (CFDictionaryRef)CFDictionaryGetValue(info, kCGWindowBounds);
    CGRectMakeWithDictionaryRepresentation(bounds, temp);

    // break on the first window, that will be the current
    break;
}

CFStringRef filename = CFSTR("/tmp/test.gif");

CFURLRef urlRef = CFURLCreateWithFileSystemPath( kCFAllocatorDefault, filename, kCFURLPOSIXPathStyle, false );

// create new destination, for an animated gif, change the count variable
int count = 1;

CGImageDestinationRef idst = CGImageDestinationCreateWithURL( urlRef, kUTTypeGIF, count, NULL );

// set the delay in case of animated gif
float delay =   1.0 / (1e6/1e2);
CFNumberRef delayNumber = CFNumberCreate(NULL, kCFNumberFloatType,  &delay);
const void *keys[1] = { kCGImagePropertyGIFDelayTime };
const void *values[1] = { delayNumber };
CFDictionaryRef options = CFDictionaryCreate(NULL, keys, values, 3,
    &kCFTypeDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks);

// create the screenshot of windowId with rect temp
CGImageRef screenShot = CGWindowListCreateImage(*temp, kCGWindowListOptionIncludingWindow, windowId, kCGWindowImageDefault);

// add screenshot to the destination
CGImageDestinationAddImage( idst, screenShot, options );

// release screenshot
CGImageRelease(screenShot);

// finalize the animated gif
CGImageDestinationFinalize( idst );