shared_ptr 的动态转换 [英] dynamic cast of a shared_ptr

问题描述

我有几个类，其中我制作了 std::shared_ptr 版本，如下所示:

I have a few classes of which I've made std::shared_ptr versions, as follows:

typedef std::shared_ptr<MediaItem> MediaItemPtr;
typedef std::shared_ptr<ImageMediaItem> ImageMediaItemPtr;

class MediaItem
{
   //stuff here
}

class ImageMediaItem : public MediaItem
{
   //more stuff here
}

在内部，我将所有内容作为 MediaItemPtr 对象传递，但是当我尝试转换为 ImageMediaItemPtr 时，我尝试的任何内容似乎都不起作用.例如:

Internally, I pass everything around as a MediaItemPtr object, but when I try to cast to a ImageMediaItemPtr, nothing I try seems to work. For example:

ImageMediaItemPtr item = std::dynamic_pointer_cast<ImageMediaItemPtr>(theItem);
//theItem is MediaItemPtr

失败

错误 C2440:正在初始化":无法从'std::tr1::shared_ptr<_Ty>' 到 'std::tr1::shared_ptr<_Ty>'

error C2440: 'initializing' : cannot convert from 'std::tr1::shared_ptr<_Ty>' to 'std::tr1::shared_ptr<_Ty>'

对这个演员表应该如何实际工作有任何想法吗?我对 shared_ptr 有点陌生

Any thoughts of how this cast should actually work? I'm a bit new to shared_ptr

推荐答案

dynamic_pointer_cast 的模板参数应该是指向类型.换句话说，它应该是 T 而不是 shared_ptr.

The template argument to dynamic_pointer_cast should be the pointed-to type. In other words, it should be T and not shared_ptr<T>.

在这种情况下，它应该是 dynamic_pointer_cast 而不是 dynamic_pointer_cast.

In this case, it should be dynamic_pointer_cast<ImageMediaItem> and not dynamic_pointer_cast<ImageMediaItemPtr>.

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