在附加到NSString对象时，为什么我的程序在3次附加后崩溃了？

Question

这是代码

在.h ...

@interface Calculator2ViewController : UIViewController {

    NSString *currentNumber;
    bool buildingNumber;
    NSMutableArray *arrayExpression;
    IBOutlet UILabel *display;

}

在.m ...

-(NSString *) makeCurrentNumber{

    if(!currentNumber){
        currentNumber = [[NSString alloc] init];
        currentNumber = @"";
    }
    NSLog(@"Inside make makeCurrentNumber. The currentNumber is %@",currentNumber);
    return currentNumber;
}



-(IBAction) digitPressed:    (UIButton *)sender{

    NSString *digit = sender.titleLabel.text;
    NSLog(@"The digit is: %@", digit);
    if(!currentNumber)
      currentNumber = @"";
     currentNumber = [currentNumber stringByAppendingString:digit];
        NSLog (@"The string is: %@",currentNumber);
    buildingNumber = YES;
    }//end of digitPressed

为什么我的程序会在3次追加后崩溃？

Answer 1

currentNumber最初由makeCurrentNumber中的alloc-init保留。稍后，您将使用自动保留值替换其值（stringByAppendingString的结果。使用保留增强该行，您可能会获得更好的结果。

[...]
if(!currentNumber)
{
    [self makeCurrentNumber];
}
currentNumber = [[currentNumber stringByAppendingString:digit] retain];
NSLog (@"The string is: %@",currentNumber);
[...]