NSMutableArray 的 count 方法导致访问错误？
我看到一些类似的问题，但没有简单的答案。在我在实际项目中实际使用它们之前，我只是尝试使用 NSMutableArray 来感受一下它们。由于某种原因，当我尝试对数组调用 count 时，它给了我一个 EXC_BAD_ACCESS 错误，但我不明白为什么。
- (void) applicationDidFinishLaunching:(UIApplication*)application
{
// Create window and make key
_window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
[_window makeKeyAndVisible];
NSMutableArray* test = [[NSMutableArray alloc] initWithObjects:[NSString stringWithFormat:@"first!"], [NSString stringWithFormat:@"second!"], nil];
[test insertObject:[NSString stringWithFormat:@"inserted"] atIndex:0];
NSLog(@"%@", [test objectAtIndex:0]);
NSLog(@"%@", [test objectAtIndex:1]);
NSLog(@"%@", [test objectAtIndex:2]);
NSLog(@"%@", [test count]); //bad access here
}
除了计数方法之外，所有插入和访问都可以正常工作。我不明白为什么这不起作用，并且非常感谢一些帮助。谢谢！
I see a few similar questions, but no simple answers. I'm just playing around with NSMutableArray's to get a feel for them before I actually use them in my real project. For some reason, it's giving me an EXC_BAD_ACCESS error when I try to call count on the array, and I can't figure out why.
- (void) applicationDidFinishLaunching:(UIApplication*)application
{
// Create window and make key
_window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
[_window makeKeyAndVisible];
NSMutableArray* test = [[NSMutableArray alloc] initWithObjects:[NSString stringWithFormat:@"first!"], [NSString stringWithFormat:@"second!"], nil];
[test insertObject:[NSString stringWithFormat:@"inserted"] atIndex:0];
NSLog(@"%@", [test objectAtIndex:0]);
NSLog(@"%@", [test objectAtIndex:1]);
NSLog(@"%@", [test objectAtIndex:2]);
NSLog(@"%@", [test count]); //bad access here
}
All the inserting and accessing EXCEPT the count method work just fine. I don't see why this isn't working, and would greatly appreciate some help. Thanks!
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- (NSUInteger)count;返回一个NSUInteger。使用这个代替：
- (NSUInteger)count;returns anNSUInteger.Use this instead:
count工作得很好。然而，它确实返回一个NSUInteger原语，而不是指向NSObject子类的指针。%@字符串格式化程序需要一个指向对象的点，并记录从该对象的-description方法返回的NSString。当您传递一个NSUIntegerNSLog时，它假定它是一个对象指针，并尽职地尝试将-description消息发送到内存地址 3，这会导致EXEC_BAD_ACCESS。countis working just fine. It does however return aNSUIntegerprimitive and not a pointer to aNSObjectsubclass. The%@string formatter expects a point to an object and logs theNSStringreturned from that object's-descriptionmethod. When you pass it aNSUIntegerNSLogassumes it to be an object pointer and dutifully tries to send a-descriptionmessage to memory address 3 which causes that EXEC_BAD_ACCESS.%@格式说明符打印对象。-count的返回值只是一个无符号整数。您应该为该类型使用格式说明符%u。The
%@format specifier prints objects. The return value of-countis just an unsigned integer. You should use the format specifier%ufor that type.问题是
[test count]返回一个NSUInteger而不是一个指针（指向NSObject）。请尝试以下操作：请注意，使用
%d也可以，但首选%u。The problem is that
[test count]returns anNSUIntegernot a pointer (to anNSObject). Try this instead:Note that using
%dalso works, but%uis preferred.