72

I'm just trying to do my usual data transfert. I define my NSMutableURLRequest then call

[[NSURLConnection alloc] initWithRequest:request delegate:self];

This used to be ok with Xcode 3 but Xcode 4 warns me about "Expression result unused" on that line. The request does work but I would like to find a way to get rid of the warning.

I suppose I could store the connection in a variable but I don't really need it and I can't see the point of setting it to nil the next line (although this would remove the warning)

Please note: I'm not 100% sure if it's Xcode 4 or the fact ARC is enabled.

Cœur
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Johann
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3 Answers3

182

When a function returns a result that you don't need you can cast it to void to eliminate the compiler warning:

(void) [[NSURLConnection alloc] initWithRequest:request delegate:self];

I haven't used ARC yet so I can't say if this is a good idea, before ARC you would need to keep this pointer result somewhere so you could release it.

progrmr
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    This did remove the warning and the function is still working as required so all good! It's the first time I see this cast to void. Is it good practice? Thanks! – Johann Oct 27 '11 at 12:36
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    Yes, because it documents the fact that you know there is a result being returned but you don't need that result. – progrmr Oct 27 '11 at 12:40
  • I still get the warning when I am attempting to use a #define based logging macro: `#if SHOW_DEBUG_LOG # define DEBUG_LOG(x,...) NSLog(@"Debug Log: "x, __VA_ARGS__) #else # define DEBUG_LOG(x,...) (void)(x); do { (void)(__VA_ARGS__); } while (0) #endif` – Dan F Jun 06 '14 at 13:45
34

progrmr's answer is correct, but here's an even cleaner way to do it:

[NSURLConnection connectionWithRequest:request delegate:self];

This doesn't cause a warning, even if you don't cast the result to void.

Elliot
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3

Someone should be responsible for that NSURLConnection. It is not needed to store the connection but it is better coding if you do. The problem is that after you created our NSURLConnection no one has a pointer to that created instance which should not be the case.

Let's assume the following example:

  1. your instance of ClassA is creating an instane of NSURLConnection
  2. your instance of ClassA is beeing released and dealloced
  3. NSURLConnection is still alive and will fire the delegate to your deallocated instance.

To solve that problem you should store the instance of NSURLConnection and should release that connection if your instance of ClassA is being dealloced which results in deallocating the instance of NSURLConnection as well.

thomas
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  • I can't release it as ARC is on. Is it not what ARC is supposed to do automatically, deallocating the pointer? – Johann Oct 27 '11 at 11:17
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    but the problem still exist semantically: `NSURLConnection` knows your instance of `ClassA` but nobody knows your instance. There is a relationship between both instances and this relationship needs to be managed ;) But if you dont save the pointer to `NSURLConnection` then nobody has a pointer to it and either ARC is deallocating the connection immediately or never. If you save the resulting pointer then the warning will not appear and you have a better coding-design and you are then able write more reliable code:`connection:(NSURLConnection*)con didSth{if(con != self.curCon) return;}` – thomas Oct 27 '11 at 14:32