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Compiler Warnings for Objective-C Developers by Ole Begemann

Ole Begemann on compiler warning:

You should always strive for a project that builds with zero warnings. A code base that leaves compiler warnings unfixed is a sign of carelessness on the part of the developer.

Because the compiler knows more about machine architecture, because he is less lazy than me, because he is clever, because he is rentless, I always care when he complains.

A recent example of mine: I’ve declared an IBAction in a subclass of UITableViewCell

in GotoCell.h:

@interface GotoCell : UITableViewCell 

    - (IBAction)select:(id)sender;


in GotoCell.m:

@implementation GotoCell 

    - (IBAction)select:(id)sender{
        NSLog(@"Cell has been selected")


In Xcode 4.6.2, these apparently innocuous lines started to give warnings:

/Users/jc/Documents/Dev/Meon/iOS-Meon/Classes/GotoCell.m:58:1: warning: attributes on method implementation and its declaration must match [-Wmismatched-method-attributes]
/Users/jc/Documents/Dev/Meon/iOS-Meon/Classes/GotoCell.h:28:1: note: method ’select:’ declared here
- (IBAction)select:(id)sender;
1 warning generated.

How the hell this basic Objective-C code could raise warning? Command+click on the selector name in the implementation file gives me the answer:

Warning popup

In UIResponder.h, there is a category on NSObject called UIResponderStandardEditActions, that already declared select:. The difference is just in the returned type void instead of IBAction, which are exactly the same and NS_AVAILABLE_IOS(3_0) which decorates the selector

@interface NSObject(UIResponderStandardEditActions)   // these methods are not implemented in NSObject

- (void)cut:(id)sender NS_AVAILABLE_IOS(3_0);
- (void)copy:(id)sender NS_AVAILABLE_IOS(3_0);
- (void)paste:(id)sender NS_AVAILABLE_IOS(3_0);
- (void)select:(id)sender NS_AVAILABLE_IOS(3_0);

In this case, I thanks the compiler to warm me about this because I didn’t want to override the select selector of NSObject(UIResponderStandardEditActions), I just wanted to have my own selector.

So I rename my methods to selectCell:. (You can arguably say that there is a small risk that selectCell: is also declared somewhere… If only we could have namespace in Objective-C!)

Zen of Developer: always strive for 0 warning

A last note: Xcode 4.6 brings new warnings, more information at

From jc.

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