NSMutableDictionary instance variable not retaining key values outside of loop

sorry if this question has already come up but I couldn't find anything that addressed the specific problem I'm having here.

I'm working in objective-c on iOS and I have declared an NSMutableDictionary instance variable, a property for it, synthesized it, and allocated space for it in my init method:

SettingsViewController.h

@interface SettingsViewController : UITableViewController <UIAlertViewDelegate> {
    NSMutableDictionary *settingsData;
    UISwitch *emailSwitch;
    UISwitch *phoneSwitch;
    NSMutableData *receivedData;
}

@property (nonatomic, retain) NSMutableDictionary *settingsData;
@property (nonatomic, retain) UISwitch *emailSwitch;
@property (nonatomic, retain) UISwitch *phoneSwitch;
@property (nonatomic, retain) NSMutableData *receivedData;

SettingsViewController.m init method

@synthesize settingsData, emailSwitch, phoneSwitch, receivedData;

# the initialization method, where settingsData is allocated
- (id)initWithStyle:(UITableViewStyle)style {
    self = [super initWithStyle:style];
    if (self) {
        self.navigationItem.title = @"Settings";
        [[self navigationItem] setRightBarButtonItem:[[[UIBarButtonItem alloc]     initWithTitle:@"Done" style:UIBarButtonItemStyleDone target:self action:@selector(doneEditing:)] autorelease]];
        settingsData = [[NSMutableDictionary alloc] init];
    }
    return self;
}

The potion of the method in SettingsViewController.m that uses settingsData (it's just a very simple parser for a block of data formatted key=value&):

NSMutableString *element = [NSMutableString string];
NSMutableString *value = [NSMutableString string];
bool cap_element = true;
char index;
for (int i = 0; i < [response length]; i++) {
    index = [response characterAtIndex:i];
    if (cap_element && index != '=') {
        [element appendFormat:@"%c", index];
        continue; // skip back to the top
    }
    if (index == '=') {
        cap_element = false;
        continue;
    }
    if (!cap_element && index != '&') {
        [value appendFormat:@"%c", index];
        continue;
    }
    if (index == '&') {
        // store the value in the dict and move onto the next element
        # this output is always correct...
        NSLog(@"Key:%@, Value:%@", element, value);
        [self.settingsData setObject:value forKey:element];
        # ...as is this (the value printed above matches the result here)
        NSLog(@"Result:%@", [settingsData objectForKey:element]);
        [value setString:@""];
        [element setString:@""];
        cap_element = true;
    }
    # but this will output an empty string (username prints correctly above)
    NSLog(@"%@", [self.settingsData objectForKey@"username"]);

The first 2 NSLog() comments output as expected, but as soon as I leave the for loop all of the keys remain and their values become empty strings (so output reads username = "" instead of username = "tester"). I would understand this if the dictionary was not i开发者_高级运维nitialized, but I take care of that in the init method shown above. What am I missing?

In the dictionary you are storing a reference to the "value" and "element" variables, not a copy. When you set them to empty string:

[value setString:@""];
[element setString:@""];

you also are updating the values in the dictionary as well.

EDIT: To solve, change this line:

[self.settingsData setObject:value forKey:element];

to this to create new strings (stringWithString should give you autoreleased values):

[self.settingsData setObject: [NSString stringWithString: value] forKey: [NSString stringWithString: element]];

You are setting the values of the two string to "" here:

[value setString:@""];
[element setString:@""];

The NSMutableDictionary only has references to the original objects so changes you make are reflected where ever there is a reference to that object. It is common to copy mutable objects if you want to ensure that there values are not changed underneath you.

The problem lies here:

[value setString:@""];
[element setString:@""];

You're setting element/value to empty strings after you've given the dictionary references to both. The dictionary is holding onto those references, not their values. Because of this, when you change the element/value values, you're unexpectedly (at least in this case) changing the behavior of your dictionary in that it no longer will respond to the key you've set and it will not return the value that was initially defined.

If you don't want this to happen, then you need to copy the strings before you reset them.