Objective-C：覆盖超类getter并尝试访问ivar时的编译器错误 [英] Objective-C: Compiler error when overriding a superclass getter and trying to access ivar

问题描述

我正在构建一个iOS 6应用程序。

I'm working on building an iOS 6 app.

我有一个类 TDBeam ，它继承自超类 TDWeapon 。

I have a class TDBeam which inherits from superclass TDWeapon.

超类 TDWeapon 在TDWeapon.h文件中声明@property：

The superclass TDWeapon declares a @property in the TDWeapon.h file:

@interface TDWeapon : UIView

@property (nonatomic) int damage;

@end

我没有明确地@synthesize属性，因为我让Xcode自动这样做。

I do not explicitly @synthesize the property, as I'm letting Xcode automatically do so.

在子类 TDBeam 中，我覆盖了TDBeam.m文件中的getter：

In the subclass TDBeam I override the getter in the TDBeam.m file:

#import "TDBeam.h"

@implementation TDBeam

- (int)damage {
    return _damage;
}

@end

Xcode自动完成getter方法名称，如预期的那样。但是当我尝试引用_damage实例变量（继承自超类）时，我得到一个编译器错误：

Xcode auto-completes the getter method name, as expected. But when I attempt to reference the _damage instance variable (inherited from the superclass), I get a compiler error:

Use of undeclared identifier '_damage'

我在这里做错了什么？我已经尝试显式添加@synthesize，并更改_damage ivar的名称，但编译器不会看到它或来自超类的任何其他ivars。我认为ivars是可见的并且可以从子类访问？

What am I doing wrong here? I've tried explicitly adding @synthesize, and changing the name of the _damage ivar, but the compiler doesn't "see" it or any other ivars from the superclass. I thought ivars were visible and accessible from subclasses?

推荐答案

合成的ivars 不对子类可见，是明确还是自动创建： @的可见性是什么？合成实例变量？由于它们在实现文件中有效声明，因此它们的声明不包含在包含子类的翻译单元中。

Synthesized ivars are not visible to subclasses, whether they are explicitly or automatically created: What is the visibility of @synthesized instance variables? Since they are effectively declared in the implementation file, their declaration isn't included in the "translation unit" that includes the subclass.

如果你真的想直接访问那个ivar，你必须在子类可以看到它的地方显式地声明它（在它的默认受保护形式），例如私有头中超类的类扩展。

If you really want to access that ivar directly, you'll have to explicitly declare it (in its default "protected" form) somewhere that the subclass can see it, such as a class extension of the superclass in a private header.

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