@property retain OR copy

Question

First I read this article

I think I should use "copy" in my programe. Problem is using NSMutableDictionary copy it will terminate.

***** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFDictionary removeAllObjects]: mutating method sent to immutable object'**

I have no idea about "mutating method sent to immutable object". I didn't set NSDictionary to NSMutabledictionary pointer.

Here is my code

.h file

@interface Button : NSObject {

@private
    NSString*               gID;                                 
    NSString*               gBackColor;                          
    NSString*               gIconImage;                          
    int                     gIndex;                              
    BOOL                    gEnable;                            
    BOOL                    gVisible;
    NSString*               gText;
    
    NSMutableDictionary*    gEvents;
    
    
    BOOL                    gUseCircle;                 
}

@property (nonatomic,copy) NSString                 *ID;
@property (nonatomic,copy) NSString                 *BackColor;
@property (nonatomic,copy) NSString                 *IconImage;
@property int Index;
@property BOOL Enable;
@property BOOL Visible;
@property (nonatomic,copy) NSString                 *Text;
@property (nonatomic,getter=getEvents,retain) NSMutableDictionary       *Events;
@property BOOL UseCircle;

@end

.m file

@implementation Button
@synthesize ID = gID;
@synthesize BackColor = gBackColor;
@synthesize IconImage = gIconImage;
@synthesize Index = gIndex;
@synthesize Enable = gEnable;
@synthesize Visible = gVisible;
@synthesize Text = gText;
@synthesize Events = gEvents;
@synthesize UseCircle = gUseCircle;

-(NSMutableDictionary*) getEvents
{
    if (!gEvents) 
    {
        gEvents = [[NSMutableDictionary alloc] initWithCapacity:20];
    }
    return gEvents;
}

- (id) init
{
    self = [super init];
    if (self != nil) 
    {
        gID = @"";
        gBackColor = @"";
        gIconImage = @"";
        gIndex = 0;
        gText = @"";
        
        gUseCircle = NO;
    }
    return self;
}

- (void) dealloc
{
    [gID release];
    [gBackColor release];
    [gIconImage release];
    [gText release];
    
    [gEvents removeAllObjects];
    [gEvents release];
    gEvents = nil;
    
    [super dealloc];
}

And implement

tBtnXML.Events = [self SplitEvents:tNode];

SplitEvents function:

-(NSMutableDictionary*) SplitEvents:(NSDictionary*)pEvents
{
    NSMutableDictionary *tEvents = [[NSMutableDictionary alloc] initWithCapacity:5];
    // code blabla
    //.
    //.
    //.
    [tEvents setObject:tEvent forKey:[NSNumber numberWithInt:tEventName]];
    [tEvent release];
            
            

            return [tEvents autorelease];
}

But I chage NSMutableDictionary* gEvents property from copy to retain , it execute normal.

Colud anyone tell me what's wrong with my code?

If my code is incorrect with dealloc,please tell me.

Thank you appriciate.

Yes, So I fixed my setter:

-(void) setEvents:(NSMutableDictionary*) pEvents
{
    NSMutableDictionary* tNewDict = [pEvents mutableCopy];
    [gEvents removeAllObjects];
    [gEvents release];
    gEvents = tNewDict;
}

This work with no error.

It helps me a lot.

But I can't vote up >"<~

So thank you Bavarious :)

score 6 · Accepted Answer · answered Apr 11 '11 at 02:29

In general, mutable properties should be retain instead of copy. When you declare a property as being copy, the synthesised setter method sends -copy to the object that’s being assigned to the property. In the case of mutable objects (e.g. NSMutableDictionary), sending -copy to them makes an immutable copy, effectively creating an object of immutable type (e.g. NSDictionary) instead.

So in:

tBtnXML.Events = [self SplitEvents:tNode];

the synthesised setter sends -copy to [self SplitEvents:tNode], thus creating an immutable copy of that dictionary (i.e., an NSDictionary instance), and assign it to gEvents. This is the cause of your error: gEvents is declared as NSMutableDictionary but points to an NSDictionary instead.

For the record, mutable classes usually declare a -mutableCopy method that does make a mutable copy. It is not used by declared properties, though. If you do not want to use retain, you need to implement a custom setter that uses -mutableCopy.

@wei `NSMutableDictionary` does implement `-mutableCopy` — in fact, it conforms to the `NSMutableCopying` protocol. The problem is that, when synthesising Objective-C declared properties, setter methods **do not** use `-mutableCopy`. — , Apr 11 '11 at 03:13
Thank you for reply :) Is it strange `NSMutableDictionary` copy return an immutable object ? I thik I understand my error. — wei, Apr 11 '11 at 03:31
Ok~~~ I got it~ >"<~~~~ The problem is that, when synthesising Objective-C declared properties, setter methods do not use -mutableCopy — wei, Apr 11 '11 at 03:34

@property retain OR copy

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