Problem with releasing an object

Question

I've got this code:

Entry.h

#import <Foundation/Foundation.h>

@interface Entry : NSObject {
    id object;
    SEL function;
}

@property (retain) id object;
@property (assign) SEL function;

-(Entry*) initWithObject:(id)object selector:(SEL)function;

@end

Entry.m

#import "Entry.h"

@implementation Entry

@synthesize object;
@synthesize function;

-(Entry*) initWithObject:(id)obj selector:(SEL)sel {
    self = [super init];
    [self setObject:obj];
    [self setFunction:sel];
    return self;
}

-(void) dealloc {
    [super dealloc];
    if ([self object] != nil)
        [[self object] release];
}

@end

And when I do this:

Entry *hej = [Entry alloc];
[hej release];

I get:

objc[2504]: FREED(id): message object sent to freed object=0xf5ecd0
Program received signal:  “EXC_BAD_INSTRUCTION”.

What am I doing wrong?

(And this insert code thing at stack overflow doesnt work, unless I'm doing something wrong and you're not supposed to click "code sample" and then paste.)

Answer 1

+alloc only allocates memory. You need -init to actually create the object in that memory space. Since you are only allocating memory and not creating an object there, calling -release on a chunk of memory is giving you an error. Further, you want your [super dealloc] call to appear at the end of you -dealloc method. Change those two things and the following should work:

Entry *hej = [[Entry alloc] init];
[hej release];

Answer 2

Firstly, you need an init to go with your alloc. Second, in dealloc, you send a message to self after calling [super dealloc]. You can't do that. The final deallocation should go at the end.

Answer 3

there are two problems here:

1) you need to check that self = [super init] does not return nil. Typical usage would be to follow wrap your initialization code with the conditional:

if ((self = [super init]) != nil) {
    // initialize the object...
}

2) but where you are getting stuck is on instantiating your object: you should do it like this:

Entry *hej = [[Entry alloc] initWithObject:myObj selector:mySelector];

(assuming that you want to go through the custom initializer you just defined... else just use the default init method.) but 'alloc' must be followed by an init.

Entry *hej = [[Entry alloc] init]; // will do the trick...

Answer 4

I would also recommend changing:

if ([self object] != nil)
    [[self object] release];

to:

[self setObject:nil];

It's less code and functionally equivalent. =)

Answer 5

There are many things wrong with your code. I'll try to go through them.

First, its better to use a different ivar name to your property name so its clear where you are using each. Apple normally uses an underscore prefix, but any prefix will do.

@interface Entry : NSObject {
    id _object;
    SEL _function;
}

@property (retain) id object;
@property (assign) SEL function;

@synthesize object = _object;
@synthesize function = _function;

Next, you aren't using the standard init template (although this probably wont make any difference normally).

-(Entry*) initWithObject:(id)obj selector:(SEL)sel {
    self = [super init];
if (self != nil) {
    // initializations
}
    return self;
}

Next, Apple (for good reasons) recommends against using getters/setters in your init/dealloc. So your init would be:

-(Entry*) initWithObject:(id)obj selector:(SEL)sel {
    self = [super init];
if (self != nil) {
    _object = [obj retain];
    _object = sel;
}
    return self;
}

Next, after [super dealloc] your object is destroyed, so you cannot reference self (and hence your ivars) after that, so your dealloc should look like:

-(void) dealloc {
    // your deallocations
    [super dealloc];
}

Further, as above, Apple recommends you should not use setters or getters in your dealloc routine, so your deallocation would initially look like:

if (_object != nil)
    [_object release];

But further still, Objective C allows (and Cocoa encourages) that sending a method to nil does nothing. This is in stark contast to most other languages where messaging nil would cause a crash, but it is how Objective C/Cocoa work and you need to get used to it. So your deallocation is actually just:

[_object release];

And finally, alloc only allocates the memory for your object, you have to initialize it, so the initialization would be something like:

Entry *hej = [[Entry alloc] initWithObject:myobj selector:@selector(mymethod)];