Comma Trick

When failing, functions like NSXMLDocument's initWithContentsOfURL:options:error: give more information via NSError than a BOOL. And you end up with code looking like that …
// Log and return 
if (error)
	// Somehow display the error
	NSLog(@"MyFunction failed with error: %@", error);
	// And return from our function
	return NO;

… opening a brace just to handle the error. This can be compacted with :

// Log and return 
if (error)	return NSLog(@"MyFunction failed with error: %@", error), NO;

The comma operator evaluates expressions (standard arithmetic, function calls, etc. but NOT statements : while, if, …) and returns the value of the last one.

  • 1, 2, 3 yields 3
  • fn(), NO calls fn and yields NO
  • fn1(), fn2(), fn3() calls fn1, then fn2, then fn3, then yields the result of fn3

return will evaluate its following expression (the comma-separated expression list) and return the value of the last one. This allows you to do one quick call to some logging function before returning.

I used to do that in Javascript for logging, it's nice to see it also fits nicely in ObjC. Not that you should pack function calls on your returns (return fn1(), fn2(), fn3(), fn4(), fn5() will work ! ) but I think it's a nice thing to know.

Follow me on Twitter
Planet Cocoa

2011 02 22Distance field
2010 07 202Binding through NSApp
2010 05 122Forwarding invocations
2010 02 272Core Image black fringes
2010 02 21Quickest Way to Shell
2010 02 08Who's calling ?
2009 09 2138 ways to use Blocks in Snow Leopard
2009 08 182Bracket Mess
2009 08 124Taming JavascriptCore within and without WebView
2009 04 15Debugging with Activity Monitor
2009 03 25How Core Image Color Tracking works
2009 03 1510Custom NSThemeFrame
2009 03 10Which framework is running ?
2009 03 074CoreUI can paint pretty big
2009 02 18Localization with functions
2009 01 30Did you forget to nest alloc and init?
2009 01 16JSCocoa on the iPhone
2009 01 11Mixing WebView and JavascriptCore
2009 01 09Badge overflow
2009 01 09Find your Garbage Collection leaks with Instruments

Powered by MediaWiki