Question

I’m trying to pick up Objective-C but ran into a roadblock experimenting with the object syntax. When I try to call a method from a class I’ve defined, though, I’m getting this as output.

11:29:19 $ /Users/rsmith/Projects/Objective-C/Expedition/a.out; exit
Loading command-line wrapper...load successful!
2010-08-26 11:29:19.070 a.out[1619] *** _NSAutoreleaseNoPool(): Object 0x303ff0 of class NSCFString autoreleased with no pool in place - just leaking
2010-08-26 11:29:19.071 a.out[1619] *** -[NSCFArray getChild]: selector not recognized [self = 0x302d70]
2010-08-26 11:29:19.071 a.out[1619] *** _NSAutoreleaseNoPool(): Object 0x304bd0 of class NSCFString autoreleased with no pool in place - just leaking
2010-08-26 11:29:19.071 a.out[1619] *** _NSAutoreleaseNoPool(): Object 0x3050f0 of class NSCFString autoreleased with no pool in place - just leaking
2010-08-26 11:29:19.071 a.out[1619] *** _NSAutoreleaseNoPool(): Object 0x3050d0 of class NSException autoreleased with no pool in place - just leaking
2010-08-26 11:29:19.071 a.out[1619] *** Uncaught exception: <NSInvalidArgumentException> *** -[NSCFArray getChild]: selector not recognized [self = 0x302d70]
Trace/BPT trap
logout
[Process completed]

Memory leaks aside, it seems to be not recognizing my method because it thinks the object is a NSCFArray. What am I doing wrong here? The full wrapper code and class definitions are as follows:

wrapper.m

#import "AbstractController.h"

int main() {
    fprintf(stdout, "Loading command-line wrapper...");
    fprintf(stdout, "load successful!\n");

    AbstractController * controller = [[AbstractController alloc] init];
    [controller getChild];

    return 0;
} // end function main

AbstractController.h

#import <Foundation/NSObject.h>
#import <Foundation/NSArray.h>
#import <Foundation/NSString.h>

@interface AbstractController : NSObject
{
    AbstractController * parent;
    NSMutableArray * children;
}
// instance methods
- (id)init;

- (AbstractController *)getAncestor;
- (AbstractController *)getChild;
- (AbstractController *)getHeir;
- (AbstractController *)getParent;
@end // interface AbstractController

AbstractController.m

#import <Foundation/NSObject.h>
#import <Foundation/NSArray.h>
#import <Foundation/NSString.h>
#import "AbstractController.h"

@implementation AbstractController : NSObject
- (id)init {
    children = [[NSMutableArray alloc] init];
} // end method init

- (AbstractController *)getParent {
    return parent;
} // end method getParent
- (AbstractController *)getChild {
    AbstractController * child = ([children count] > 0) ? [children objectAtIndex:0] : self;
    return child;
} // end method getChild
- (AbstractController *)getAncestor {
    AbstractController * ancestor;
    ancestor = (parent) ? [parent getAncestor] : self; 
    return ancestor;
} // end method getAncestor
- (AbstractController *)getHeir {
    AbstractController * child = [self getChild];
    AbstractController * heir;
    heir = (child) ? [child getHeir] : self;
    return heir;
} // end method getHeir
@end // implementation AbstractController

Answer 1

First, put an autorelease pool in your main() to shut up all the autorelease problems.

Next, pay attention to compiler warnings. If there is a warning, there is a bug or potential bug in your code. And there should definitely be a warning that documents exactly what is wrong.

Namely, your -init method is wrong:

- (id)init {
    children = [[NSMutableArray alloc] init];
} // end method init

You don’t end it with return self;. Or, more specifically, you don’t:

- init {
    self = [super init];
    if (self) {
        children = [[NSMutableArray alloc] init];
    }
    return self;
}

Also, you should try and follow the same patterns as Apple’s frameworks and APIs. Namely, don’t prefix those methods with get. Finally, make sure you add a dealloc method, too.