NSString属性：copy或retain？ [英] NSString property: copy or retain?

问题描述

假设我有一个类 SomeClass 和 string 属性名称：

  @interface SomeClass：NSObject 
 {
 NSString * name; 
} 
 
 @property（nonatomic，retain）NSString * name; 
 
 @end 
  

我知道名称可能会分配一个 NSMutableString 在这种情况下，这可能会导致错误的行为。

• 对于一般的字符串，总是是一个好主意，使用 属性而不是 retain ？


• 是否是复制的属性的效率低于这种保留属性？

NSCopying 协议的不可变值类的属性，你几乎总是应该指定 @property 声明中的code> copy 。指定 retain 是在这种情况下几乎不想要的东西。

这里是为什么要这样做： / p>

  NSMutableString * someName = [NSMutableString stringWithString：@Chris]; 
 
 Person * p = [[[Person alloc] init] autorelease]; 
 p.name = someName; 
 
 [someName setString：@Debajit]; 
  

Person.name 属性将根据属性是否声明 retain 或 copy 而不同 - 将会是 @Debajit如果属性标记为 retain ，但 @Chris if该属性标记为 copy 。

由于几乎在所有情况下您都希望突变背后的对象属性，您应该标记表示它们的属性 copy 。 （如果你自己编写安装器而不是使用 @synthesize ，你应该记住使用 copy 而不是保留。）

Let's say I have a class called SomeClass with a string property name:

@interface SomeClass : NSObject
{
    NSString* name;
}

@property (nonatomic, retain) NSString* name;

@end

I understand that name may be assigned a NSMutableString in which case this may lead to errant behavior.

• For strings in general, is it always a good idea to use the copy attribute instead of retain?
• Is a "copied" property in any way less efficient than such a "retain-ed" property?

解决方案

For attributes whose type is an immutable value class that conforms to the NSCopying protocol, you almost always should specify copy in your @property declaration. Specifying retain is something you almost never want in such a situation.

Here's why you want to do that:

NSMutableString *someName = [NSMutableString stringWithString:@"Chris"];

Person *p = [[[Person alloc] init] autorelease];
p.name = someName;

[someName setString:@"Debajit"];

The current value of the Person.name property will be different depending on whether the property is declared retain or copy — it will be @"Debajit" if the property is marked retain, but @"Chris" if the property is marked copy.

Since in almost all cases you want to prevent mutating an object's attributes behind its back, you should mark the properties representing them copy. (And if you write the setter yourself instead of using @synthesize you should remember to actually use copy instead of retain in it.)

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