Objective-C and Cocoa : crash when calling a class function without entering the function

Question

I have a class function (declared and implemented) in a class MyUtils. When I call this function my app crashes. In the debugger I have a breakpoint on the first action of the "theFunction" function. And this breakpoint is never reached.

Here is the code :

// =================================================================================================
// MyUtils.m
// =================================================================================================
+ (NSString*) changeDateFormat_fromFormat:(NSString*)sourceFormat sourceDateString:(NSString*)sourceDateString destFormat:(NSString*)destFormat {
    if (sourceDateString == nil) return (nil); **<-- breakpoint here**

    NSDate* aDate = [NSDate dateFromString:sourceFormat theDateString:sourceDateString];
    return ([aDate stringValueWithFormat:destFormat]);
}

// ===================================================================
// MyUtils.h
// ===================================================================
@interface MyUtils
+ (NSString*) changeDateFormat_fromFormat:(NSString*)sourceFormat sourceDateString:(NSString*)sourceDateString destFormat:(NSString*)destFormat;
+ (void) simpleAlert_ok:(NSString*)alertTitle message:(NSString*)alertMessage;

@end


// ===================================================================
// Elsewhere.m
// ===================================================================
- (void) aFunction:(SomeClass*)someParam {
    SomeOtherClass* val = nil;
    NSString* intitule = nil;


    intitule = [MyUtils changeDateFormat_fromFormat:@"yyyyMMdd" sourceDateString:@"toto" destFormat:@"EEEE dd MMMM yyyy"]; **<-- crash here**

The console says :

2011-01-03 02:05:07.188 Learning Project[1667:207] *** NSInvocation: warning: object 0xe340 of class 'MyUtils' does not implement methodSignatureForSelector: -- trouble ahead
2011-01-03 02:05:07.188 Learning Project[1667:207] *** NSInvocation: warning: object 0xe340 of class 'MyUtils' does not implement doesNotRecognizeSelector: -- abort

If I replace the call by NSString *item = @"youyou"; then everything is ok.

Forcing a retain on aPreviousNSString before the call does not change anything. Do you have an idea of what is happening ?

Answer 1

You declared MyUtils without a superclass, so the runtime is complaining that it doesn't implement certain basic behaviors (rightfully so). You probably meant to inherit from NSObject:

@interface MyUtils : NSObject {
}

+ (NSString*) changeDateFormat_fromFormat:(NSString*)sourceFormat sourceDateString:(NSString*)sourceDateString destFormat:(NSString*)destFormat;
+ (void) simpleAlert_ok:(NSString*)alertTitle message:(NSString*)alertMessage;
@end

Answer 2

You do not have a superclass declared on your MyUtils class. To fix it, just change @interface MyUtils to @interface MyUtils : NSObject. If you do not declare a superclass, you have to provide all of the required methods yourself.

Answer 3

Your class needs to be of some object type for it to be compilable. The base object in Objective-C for iOS is NSObject, all classes inherit from it.

You want to change the line that says:

@interface MyUtils

to this:

@interface MyUtils : NSObject { 


}

  + (NSString *) ... ... ...

For more information on NSObject, see the NSObject Class reference in the Apple Developer Library.