Question

//viewController.h file
//---------------------
#import <UIKit/UIKit.h>

@interface ItemClass : NSObject 
{
    NSString* name;
}
@property (nonatomic, retain) NSString* name;
@end



@interface PlaceClass : ItemClass 
{
    NSString* coordinates;
}
@property (nonatomic, retain) NSString* coordinates;
@end




@interface viewController : UIViewController {
    NSMutableArray* placesMutArray;
    PlaceClass* currentPlace;
}

@end

//viewController.m file
//------------------------

#import "viewController.h"
@implementation ItemClass
@synthesize name;
@end


@implementation PlaceClass
@synthesize coordinates;
@end


@implementation viewController

- (void)viewDidLoad {
    [super viewDidLoad];

    placesMutArray = [[NSMutableArray alloc] init];
    currentPlace = [[PlaceClass alloc] init];

    // at some point in code the properties of currentPlace are set
    currentPlace.name = [NSString stringWithFormat:@"abc"];
    currentPlace.coordinates = [NSString stringWithFormat:@"45.25,24.22"];
    // currentPlace added to mutable array
    [placesMutArray addObject:currentPlace];

    //now the properties of currentPlace are changed
    currentPlace.name = [NSString stringWithFormat:@"def"];
    currentPlace.coordinates = [NSString stringWithFormat:@"45.48,75.25"];
    // again currentPlace added to mutable array
    [placesMutArray addObject:currentPlace];

    for(PlaceClass* x in placesMutArray)
    {
        NSLog(@"Name is : %@", x.name);
    }

}
@end

输出我得到：

Name is : def
Name is : def

期望的输出：

Name is : abc
Name is : def

我希望placesMutArray有两个独立的对象（每个对象分配不同的内存空间），每个对象都有自己的“name”和“coordinates”属性。但是上面的代码显然只是改变了同一个对象'currentPlaces'的属性，并且它的引用被添加到数组中两次。暗示我只在内存中分配了一个对象。当我使用快速枚举和NSlog遍历数组时，两个元素的name属性我将获得两次最后设置值。

可以采用NSCopying协议解决问题吗？

[placesMutArray addObject:[currentPlace copy]];

如果是，那我该怎么办呢？我尝试了但是我遇到了很多错误。

Answer 1

您只创建了一个PlaceClass对象（currentPlace），然后向该数组添加了对此PlaceClass对象的2个引用。您必须创建第二个对象

secondPlace = [[PlaceClass alloc] init];
secondPlace.name = [NSString stringWithFormat:@"def"];
secondPlace.coordinates = [NSString stringWithFormat:@"45.48,75.25"];
[placesMutArray addObject:secondPlace];

或

secondPlace = [currentPlace mutableCopy];
secondPlace.name = [NSString stringWithFormat:@"def"];
secondPlace.coordinates = [NSString stringWithFormat:@"45.48,75.25"];
[placesMutArray addObject:secondPlace];

无论哪种方式，记得在使用alloc，copy或retain

时释放对象

[secondPlace release];

Answer 2

你是对的，因为你正在使用相同的实例。你只需要制作一个新的。

试试这个：

// Create a second PlaceClass before setting it's properties to 'def'
currentPlace = [[PlaceClass alloc] init];
currentPlace.name = [NSString stringWithFormat:@"def"];
currentPlace.coordinates = [NSString stringWithFormat:@"45.48,75.25"];

将对象添加到数组不会为您复制对象 - 它只是意味着数组知道对象。将第一个currentPlace添加到数组后，currentPlace变量仍然指向第一个对象，因此在开始设置新名称和坐标时，您正在更新第一个，而不是创建新的。

如何使用与前一个相同的对象引用在可变数组中添加新对象实例

2 个答案: