在我的iOS5应用程序中，我有NSObject States类，并试图初始化它:
states = [states init];
这是init方法在各州:
- (id) init
{
if ((self = [super init]))
{
pickedGlasses = 0;
}
return self;
}
但是在lines states = [states init]中有错误;
接收端类型为“States”的实例消息是前向声明
这是什么意思?我做错了什么?
当前回答
I got this sort of message when I had two files that depended on each other. The tricky thing here is that you'll get a circular reference if you just try to import each other (class A imports class B, class B imports class A) from their header files. So what you would do is instead place a forward (@class A) declaration in one of the classes' (class B's) header file. However, when attempting to use an ivar of class A within the implementation of class B, this very error comes up, merely adding an #import "A.h" in the .m file of class B fixed the problem for me.
其他回答
I got this sort of message when I had two files that depended on each other. The tricky thing here is that you'll get a circular reference if you just try to import each other (class A imports class B, class B imports class A) from their header files. So what you would do is instead place a forward (@class A) declaration in one of the classes' (class B's) header file. However, when attempting to use an ivar of class A within the implementation of class B, this very error comes up, merely adding an #import "A.h" in the .m file of class B fixed the problem for me.
你正在使用
States states;
你应该在哪里使用
States *states;
init方法应该是这样的
-(id)init {
if( (self = [super init]) ) {
pickedGlasses = 0;
}
return self;
}
最后，当你要为States类创建对象时你应该这样做。
State *states = [[States alloc] init];
我并不是说这是最好的方法。但是它可以帮助您理解初始化对象的基本用法。
有两个相关的错误消息可能告诉您声明和/或导入有问题。
第一个是您所引用的，它可以通过在.m(或.pch文件)中不放置#import而在.h中声明@类来生成。
第二个你可能会看到，如果你在States类中有一个方法:
- (void)logout:(NSTimer *)timer
添加#import后是这样的:
“state”没有可见的@接口声明选择器“logout:”
如果看到这种情况，您需要检查是否在您正在导入或转发的类的.h文件中声明了“注销”方法(在本例中)。
所以在你的例子中，你需要一个:
- (void)logout:(NSTimer *)timer;
在States类的.h中使一个或两个相关错误消失。
检查是否导入了抛出此错误的类的头文件。
FWIW，当我在现有项目中实现核心数据时，我得到了这个错误。结果我忘了把CoreData.h链接到我的项目。我已经在我的项目中添加了CoreData框架，但是通过在我的预编译头中链接到框架来解决这个问题，就像苹果的模板一样:
#import <Availability.h>
#ifndef __IPHONE_5_0
#warning "This project uses features only available in iOS SDK 5.0 and later."
#endif
#ifdef __OBJC__
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>
#endif
