正确初始化后添加NSMutableArray时崩溃

Question

I have an NSMutableArray defined as a property, synthesized and I have assigned a newly created instance of an NSMutableArray. But after this my application always crashes whenever I try adding an object to the NSMutableArray.

@interface Page : NSObject  
{  
    NSString *name;  
    UIImage *image;  
    NSMutableArray *questions;  
}
@property (nonatomic, copy) NSString *name;  
@property (nonatomic, retain) UIImage *image;  
@property (nonatomic, copy) NSMutableArray *questions;  
@end

@implementation Page  
@synthesize name, image, questions;  
@end  

Page *testPage = [[Page alloc] init];  
testPage.image = [UIImage imageNamed:@"Cooperatief  leren Veenman-11.jpg"];  
testPage.name = [NSString stringWithString:@"Cooperatief  leren Veenman-11.jpg"];  
testPage.questions = [[NSMutableArray alloc] init];  
[testPage.questions addObject:[NSNumber numberWithFloat:arc4random()]];  

The debugger reveals that the moment I use testPage.questions = [[NSMutableArray alloc] init]; the type of testPage.questions changes from NSMutableArray* to __NSArrayL* (or __NSArrayI*, not sure). I suspect this to be the problem, but I find it extremely odd. Anyone know what's happening here?

Answer 1

The problem is that you've declared the property as copy . This means your setter is going to be implemented something like this:

- (void) setQuestions:(NSMutableArray *)array {
  if (array != questions) {
    [questions release];
    questions = [array copy];
  }
}

The kicker here is that if you -copy an array (whether immutable or mutable), you will always get an immutable NSArray .

So to fix this, change the property to be retain instead of copy , and also fix this memory leak:

testPage.questions = [[NSMutableArray alloc] init];

It should be:

testPage.questions = [NSMutableArray array];

Answer 2

@property (nonatomic, copy) This setter declaration "copy" probably cast to NSArray why not retain or assign? I would retain anyway

Answer 3

You can also create a mutable copy method like so:

- (void)setQuestions:(NSMutableArray *)newArray
{
    if (questions != newArray) 
    {
        [questions release];
        questions = [newArray mutableCopy];
    }
}