Objective-C动态广播? [英] Objective-C dynamic_cast?

问题描述

Objective-C是否等效于C ++的dynamic_cast?

Is there an Objective-C equivalent of C++'s dynamic_cast?

可以使用以下方法伪造它:

It can be faked using this:

MyClass *safeObject = [object isKindOfClass: [MyClass class]]
                      ? (MyClass *)originalObject : nil;

但这需要输入很多代码，即使我不需要经常输入.

But this is a lot of code to type, even if I don't need to type it often.

我有点生锈，所以这可能不太正确，但是我相信C ++中的等效条件是:

I am a bit rusty so this might not be quite right, but I believe the equivalent in C++ would be:

MyClass safeObject = dynamic_cast<MyClass>(orginalObject);

这里的上下文是一个块，其中参数定义为更通用的类的类型，但是在此块中，我知道"它是特定的子类.尽管如此，我不想只是用(MyClass *)originalObject盲目地将其强制转换而忽略了理论上可能的错误.

The context here is a block where the parameter is defined as a type of a more generic class, but in this block I "know" it's a specific subclass. Nevertheless, I don't want to just cast it blindly with (MyClass *)originalObject and ignore a theoretically possible error.

要清楚一点，虽然我很喜欢dynamic_cast，但我也对能够安全处理此案的另一种方法感到满意.

To be clear, while I'd love a dynamic_cast, I'd be happy with an alternate approach to safely handle this case as well.

推荐答案

如果您愿意使用Objective-C ++，则可以轻松编写以下代码:

If you're willing to use Objective-C++, you can write this pretty easily:

template<typename T> inline T* objc_cast(id from) {
    if ([from isKindOfClass:[T class]]) {
        return static_cast<T*>(from);
    }
    return nil;
}

除了obj-c对象外，它的行为应与dynamic_cast<>完全相同.

This should behave exactly as dynamic_cast<> except for obj-c objects.

如果您要坚持使用香草Obj-C，则可以在NSObject上使用类方法获得类似的行为:

If you want to stick with vanilla Obj-C you can get similar behavior with a class method on NSObject:

@interface NSObject (Cast)
+ (instancetype)cast:(id)from;
@end

@implementation NSObject (Cast)
+ (instancetype)cast:(id)from {
    if ([from isKindOfClass:self]) {
        return from;
    }
    return nil;
}
@end

此版本使用起来不太好，因为您必须说类似

This version just isn't as nice to use since you have to say something like

UIButton *button = [UIButton cast:someView];

在两个版本中，如果强制转换失败，则结果值为nil.

In both versions the resulting value is nil if the cast fails.

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