是否可以从使用instancetype的类构造函数中返回子类？ [英] Is it okay to return a subclass from a class constructor that uses instancetype?

问题描述

我有一个类中的类方法构建一个Cocoa集合在某些方式，内置的初始化器不允许。由于初始化器功能有限，我必须使用可变版本的集合来实际构建它。下面是 NS {Mutable} IndexSet 的示例：

I have a class method in a category to construct a Cocoa collection in some way that the built-in initializers don't allow. Due to the limited initializer functionality, I have to use the mutable version of the collection to actually build it. Here's an example for NS{Mutable}IndexSet:

@implementation NSIndexSet (WSSNonContiguous)

+ (instancetype)WSSIndexSetFromMask:(NSUInteger)mask
{
    NSMutableIndexSet * set = [NSMutableIndexSet indexSet];

    for( NSUInteger i = 0; i < (sizeof(NSUInteger) * 8); i++ ){
        if( mask & (1l << i) ){
            [set addIndex:i];
        }
    }

    return set;
}

我的回报类型有时是谎言，

My return type is sometimes a lie here -- there's always a mutable collection being returned, whether the user is requesting an immutable version or not.

仍然适合使用 instancetype code>在这样的情况下，或者我应该与 id ？如果我使用 instancetype ，我也应该明确地重新创建集合：

Is it still appropriate to use instancetype in cases like this, or should I go with id? If I do use instancetype, should I also be explicitly re-creating the collection:

// Ick?
return [[self alloc] initWithIndexSet:set];

以确保在调用 + [NSIndexSet]时返回不可变拷贝WSSIndexSetFromMask：] ？

推荐答案

一切正常：

NSIndexSet *set = [[NSIndexSet WSSIndexSetFromMask:0] addIndex:0];

No visible @interface for 'NSIndexSet' declares the selector 'addIndex:'

instancetype 对发件人说，返回一个接收者类型的实例，即使它是一个子类型。对于发送者，它是一个NSIndexSet，因为它被发送到NSIndexSet的类对象。

instancetype says to the sender, that you return a instance of the receivers type even it is a subtype. For the sender it is a NSIndexSet, because it is send to the class object of NSIndexSet.

这种自省，有人期待返回类型并看到一个子类并利用这些信息的任何优势，格式不正确。合同是用返回类型构建的，在这种情况下是NSIndexSet。

An introspection that way, that someone looks to the return type and sees a subclass and takes any advantage out of this information, is malformed. The contract is build with the return type and this is in this case NSIndexSet.

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