@interface或@implementation中的私有ivar [英] Private ivar in @interface or @implementation

问题描述

是否有任何理由在@interface而不是@implementation中声明私有ivar?

Is there any reason to declare a private ivar in @interface instead of @implementation?

我在互联网上看到这样的代码(包括苹果):

I see code like this all over the internet (including documentation provided by Apple):

Foo.h

@interface Foo : NSObject {
@private
    id _foo;
}
@end

Foo.m

@implementation Foo
// do something with _foo
@end

头文件定义了一个类的公共接口，而一个私有的ivar是...好...私有的.那么为什么不这样声明呢?

The header file defines the public interface of a class, whereas a private ivar is... well... private. So why not declare it like this?

Foo.h

@interface Foo : NSObject
@end

Foo.m

@implementation Foo {
@private
    id _foo;
}

// do something with _foo
@end

推荐答案

在@implementation中声明实例变量是Obj-C的最新功能，这就是为什么您在@interface中看到很多带有它们的代码的原因-别无选择.

Declaring instance variables in the @implementation is a recent feature of Obj-C, this is why you see a lot of code with them in the @interface - there was no other choice.

如果您使用的编译器支持在实现中声明实例变量，则声明它们可能是最好的默认值-仅在需要其他人访问它们时才将它们放在接口中.

If you are using a compiler which supports declaring instance variables in the implementation declaring them there is probably the best default - only put them in the interface if they need to be accessed by others.

修改:其他信息

实例变量是隐式隐藏的(实际上是私有的)，并且可见性 无法更改-@public，@protected和@private不产生编译器错误(至少使用当前的Clang)，但会被忽略.

Instance variables declared in the implementation are implicitly hidden (effectively private) and the visibility cannot be changed - @public, @protected and @private do not produce compiler errors (with the current Clang at least) but are ignored.

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