在 webcrawler/webscraper 设置中，我想动态扩展我的基本参考类URL，以便能够为各个主机/域编写特定的方法。为了清楚起见，动态我的意思是“在遇到新域时自动生成类定义（例如URL_something.com，将从 class 继承的类URL）”。
一种享受，唯一的问题是我的班级WebPage期望 field 的值url是 class URL。它将接受 class 的对象，URL_something.com因为 this 从 class 继承URL，但实际上将对象转换为 class 的实例URL。所以我失去了它实际上是 class 的信息URL_something.com。
您知道如何防止丢失这些重要信息吗？
代码示例
setRefClass(Class="URL", fields=list(x="character"))
setRefClass(Class="WebPage", fields=list(url="URL"))
obj <- new("WebPage", url=new("URL", x="http://www.something.com/home/index.html"))
obj$url
# Method would recognize that there is no class 'URL_something.com'
# yet and thus create it:
setRefClass(Class="URL_something.com", contains="URL")
# Another method would take care of mapping field values to
# an instance of the new class:
> url.obj <- new("URL_something.com", x="http://www.something.com/home/index.html")
> inherits(url.obj, "URL")
[1] TRUE
> obj$url <- url.obj
> class(obj$url)
[1] "URL"
# So I lose the information that it was actually of class "URL_something.com"