当属性存在时，ivar 的目的是什么? [英] What's the purpose of an ivar when a property exists?

问题描述

以下内容不会在编译或运行时抱怨没有 name ivar.那么为什么看到 ivar 和 @property/@synthesize 如此普遍.

The following doesn't complain at compilation nor runtime about no name ivar. So why is it so common to see an ivar and @property/@synthesize.

@interface PropTest : NSObject
{
}
@property (retain) NSString *name;
@end

@implementation PropTest
@synthesize name;
@end

int main (int argc, const char * argv[]) {
  NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
  PropTest *p = [[PropTest new] autorelease];
  p.name = @"Hello, World!";
  NSLog(@"%@",p.name);
  [pool drain];
  return 0;
}

此代码打印

Hello, World!

事实上，如果我访问p->name，我会收到警告:

In fact, if i access p->name, i get a warning:

warning: instance variable 'name' is @private; this will be a hard error in the future

这表明如果不存在 ivar，则会为我创建一个.

which indicates that an ivar is created for me if one doesn't exist.

如果这是真的，那么手动创建 ivar 有什么意义(忽略显而易见的，有时不使用 g/setter 访问器的正当理由)?

If that's true, what's the point of creating the ivar manually (ignoring the obvious, that there are sometimes valid reasons for not using the g/setter accessor)?

或者换个说法，当我需要绕过访问器时，我是否应该只为一个属性创建一个 ivar?

Or asked differently, should i only ever create an ivar for a property when i need to bypass the accessors?

推荐答案

合成 ivars(不手动声明 ivars 的能力)是新的 Objective-C 运行时的一个特性，它仍然没有在所有系统上使用.对于 32 位 Mac(以及直到最近的 iPhone 模拟器)，您必须手动声明 ivars.如果您只针对具有新运行时的系统，则没有理由手动声明 ivars.

Synthesized ivars (the ability to not manually declare ivars) are a feature of the new Objective-C runtime, which still isn't being used on all systems. For 32-bit Macs (and, until recently, the iPhone simulator), you have to manually declare ivars. If you're only targeting systems with the new runtime, there's no reason to manually declare ivars.

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