NSString未指定“分配"，“保留"或“复制"属性 [英] NSString no 'assign', 'retain', or 'copy' attribute is specified

问题描述

我在类中声明了NSString属性，而Objective-c则在抱怨:

I'm declaring an NSString property in a class and objective-c is complaining that:

NSString未指定分配"，保留"或复制"属性

NSString no 'assign', 'retain', or 'copy' attribute is specified

然后随便让我知道改为使用分配".

It then casually lets me know that "assign is used instead".

有人可以向我解释分配，保留和复制在正常C内存管理方面的区别 >功能?

Can someone explain to me the difference between assign, retain and copy in terms of normal C memory management functions?

推荐答案

我想引起您注意使用assign而不是retain或copy的事实.由于NSString是对象，因此在引用计数的环境(即，没有垃圾回收)中，这可能是危险的"(除非是有意设计的).

I think it is drawing your attention to the fact that a assign is being used, as opposed to retain or copy. Since an NSString is an object, in a reference-counted environment (ie without Garbage Collection) this can be potentially "dangerous" (unless it is intentional by design).

但是，assign，retain和copy之间的区别如下:

However, the difference between assign, retain and copy are as follows:

• 分配:在属性的设置方法中，可以简单地将实例变量分配给新值，例如:

• assign: In your setter method for the property, there is a simple assignment of your instance variable to the new value, eg:

- (void)setString:(NSString*)newString
{
    string = newString;
}

这可能会导致问题，因为Objective-C对象使用引用计数，因此如果不保留对象，则有可能在您仍在使用字符串时将其释放.

This can cause problems since Objective-C objects use reference counting, and therefore by not retaining the object, there is a chance that the string could be deallocated whilst you are still using it.

保留:此保留 setter方法中的新值.例如:

retain: this retains the new value in your setter method. For example:

- (void)setString:(NSString*)newString
{
    [newString retain];
    [string release];
    string = newString;
}

这是更安全的方法，因为您明确声明要维护该对象的引用，并且必须在释放该对象之前释放它.

This is safer, since you explicitly state that you want to maintain a reference of the object, and you must release it before it will be deallocated.

复制:这会在您的setter方法中复制字符串:

copy: this makes a copy of the string in your setter method:

- (void)setString:(NSString*)newString
{
    if(string!=newString)
    {
        [string release];
        string = [newString copy];
    }
}

这通常与字符串一起使用，因为制作原始对象的副本可确保在您使用它时不会对其进行更改.

This is often used with strings, since making a copy of the original object ensures that it is not changed whilst you are using it.

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