NSMutableDictionary is being treated as an NSDictionary

I have a simple class with an NSMutableDictionary member variable. However, when I call setObject:forKey I get an error ('mutating method sent to immutable object'). The source of the problem is obvious from the debugger -- my NSMutableDictionary is actually of type NSDictionary.

I must be missing something incredibly simple but can't seem to fix it. Here is the relevant code:

// Model.h
@interface Model : NSObject {
    NSMutableDictionary *piers;
}
@property (nonatomic,retain) NSMutableDictionary *piers;
@end


// Model.m
@implementation Model
@synthesize piers;

-(id) init {
 if (self = [super init]) {
     self.piers = [[NSMutableDictionary alloc] initWithCapacity:2];
        [self createModel];
    }
    return self;
}

-(void) cr开发者_开发问答eateModel {
 [piers setObject:@"happy" forKey:@"foobar"];  
}
@end

If I put a breakpoint anywhere in the code and investigate self.piers, it is of type NSDictionary. What am I missing so that it is treated as an NSMutableDictionary instead? Thanks!

Your code works for me without any modification. I made a Foundation based command line tool (Mac OS X) with this code:

#import <Foundation/Foundation.h>

// Model.h
@interface Model : NSObject {
    NSMutableDictionary *piers;
}
@property (nonatomic,retain) NSMutableDictionary *piers;

-(void) createModel;

@end


// Model.m
@implementation Model
@synthesize piers;

-(id) init {
    if (self = [super init]) {
        self.piers = [[NSMutableDictionary alloc] initWithCapacity:2];
        [self createModel];
    }
    return self;
}

-(void) createModel {
    [piers setObject:@"happy" forKey:@"foobar"];  
}
@end

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    // insert code here...
    Model *model = [[Model alloc] init];

    NSLog(@"Model: %@", [model.piers objectForKey:@"foobar"]);

    [pool drain];
    return 0;
}

and it gave me the expected output:

2010-04-06 12:10:19.510 Model[3967:a0f] Model: happy

As KennyTM says, your use of self is a little wrong. in your init, the general pattern is

NSMutableDictionary *aPiers = [[NSMutableDictionary alloc] initWithCapacity:2];
self.piers = aPiers;
[aPiers release];

Later on in the code, you should be using self.piers.

Try making a project like mine and see if the problem still exists. You'll probably find that the problem is somewhere else.

Try remove the self..

 piers = [[NSMutableDictionary alloc] initWithCapacity:2];

In ObjC, the notation

obj.prop = sth;

is equivalent to

[obj setProp:sth];

which has totally different semantic than

obj->prop = sth;

Although highly unlikely, probably your mutable dictionary becomes immutable during the -setPiers: procedure. Just say No to self.anything (until you understand how property works).