Simple Question

[tooom]

I'm starting with objective c. I wrote the following stuff but I do not understand what I'm doing wrong

MyClass.h

#import <Cocoa/Cocoa.h>


@interface MyClass : NSObject {}

+(MyClass*) initMyClass;

@end

MyClass.m

#import "MyClass.h"

@implementation MyClass

+(MyClass*) initMyClass {
	self = [super init];
	return self;
}

@end

And Prog1.m

#import <Foundation/NSObject.h> 
#import <Foundation/NSAutoreleasePool.h> 
#import <Foundation/NSString.h> 
#import <Foundation/NSArray.h> 
#import <Foundation/NSValue.h>
#import <stdio.h>
#import "MyClass.h"

int main (int argc, const char * argv[]) {
    NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
	
	printf("1\n");
	MyClass* myClass = [MyClass initMyClass];
	printf("2\n");
	[myClass autorelease];
	printf("3\n");
	MyClass* myClass2 = myClass;
	printf("4\n");
	[myClass2 retain];
	printf("5\n");
	[myClass release];
	printf("6\n");	
	
	
	[pool release];
	return 0;
}

There are some includes from XCode that are not necessary (i know). The program terminates and i do not understand why. I get the following error message on the console:

Program loaded.
run
[Switching to process 20067]
Running…
1
2010-09-07 21:10:49.667 Prog1[20067:a0f] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** +[MyClass<0x100001080> init]: cannot init a class object.'
*** Call stack at first throw:
(
	0   CoreFoundation                      0x00007fff83a27cc4 __exceptionPreprocess + 180
	1   libobjc.A.dylib                     0x00007fff86ef70f3 objc_exception_throw + 45
	2   CoreFoundation                      0x00007fff83a80f59 +[NSObject(NSObject) init] + 137
	3   Prog1                               0x0000000100000e67 +[MyClass initMyClass] + 57
	4   Prog1                               0x0000000100000d83 main + 103
	5   Prog1                               0x0000000100000d14 start + 52
	6   ???                                 0x0000000000000001 0x0 + 1
)
terminate called after throwing an instance of 'NSException'
Program received signal:  “SIGABRT”.
sharedlibrary apply-load-rules all

What's wrong here?

[paulperry]

Without compiling your code, it looks like the problem is in your initMyClass static method:

+(MyClass*) initMyClass {
self = [super init];
return self;
}

self is an instance variable, and since you have used the plus (+) sign to signify a static method, self is not available (Although, I would think the compiler would at least warn you about this, if not give an error, but maybe not). You could probably get this to work like this:

+(MyClass*) initMyClass {
MyClass* mc;
mc = [super init];
return mc;
}

Which is what the error message is describing:

reason: '*** +[MyClass<0x100001080> init]: cannot init a class object.'

However, I think there are further issues, in that you shouldn't release anything in the autorelease pool. I am not at a Mac to confirm either of these. Although, I believe that is what you are running into.

[smasher]

Your issue is that only an *instance* of a class has an init method. The class itself does not. A class is like a template for creating objects; you might have a class called "MyClass" and then several instances of that class.

This is not valid:

MyClass* myClass = [MyClass init];

Your initMyClass method doesn't help because it's a class method too. You can't call [self init] or [super init] inside that method because self and super still refer to the class, not an instance.

The usually way to get an instance from class is to call "alloc" on the class. This is more what you want:

// reserve some memory for an instance
MyClass *instance = [MyClass alloc];
// initialize that instance - we assign because there are special cases
// where we get a different address back
instance = [instance init];

That's usually written in shorthand like this:

MyClass *instance = [[MyClass alloc] init];

If you did want to write a class method that returns an object, it should alloc and init an object, autorelease it, and then return it.

EXTRA CREDIT: Classes are also objects in objective-C, but I don't want to confuse the issue.