iPhone自动释放字符串的内存泄漏

我正在尝试用泄漏仪器清除我的应用程序中的漏洞。 它向我展示了XML解析器(tbxml)上的漏洞。

下面是我将在解析时创建的类:

@interface GraphPoint : NSObject {
    NSString* x;
    NSString* y;
}


@property (nonatomic, copy) NSString* x;
@property (nonatomic, copy) NSString* y;

@end

@implementation GraphPoint

@synthesize x, y;

... some calculations

- (void) dealloc
{
    [x release];
    [y release];
    [super dealloc];
}

@end

在解析器中:

… //根据元素找到时:

        NSString    *str;
        GraphPoint  *aPoint = [[GraphPoint alloc] init];

        TBXMLElement *item = [TBXML childElementNamed:kX_Item parentElement:pntItem];
        str = [TBXML textForElement:item];  
        aPoint.x = [str stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];     

        item = [TBXML childElementNamed:kY_Item parentElement:pntItem];
        str = [TBXML textForElement:item];  
        aPoint.y = [str stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];


        [points addObject:aPoint];
        [aPoint release];

Leaks Instrument在tbxml的textferement函数中显示泄漏,该函数提供自动释放的字符串:

+ (NSString*) textForElement:(TBXMLElement*)aXMLElement {
    if (nil == aXMLElement->text) return @"";
    return [NSString stringWithCString:&aXMLElement->text[0] encoding:NSUTF8StringEncoding];
}

由于我们有时谈论数百甚至数千个点,这些泄漏会变得很大。我不明白为什么自动释放的字符串会产生泄漏?

有什么想法吗?

谢谢