当它们被初始化的对象改变时，引用类对象的列表不会改变

Question

我有一个引用类对象列表，我希望当我更改初始化列表的对象之一时，列表中的相应条目也应该更改。情况并非如此，如以下示例代码所示，如​​果我列出square 和triangle，然后将square 更改为octagon。该列表仍将分别具有 4 边和 3 边的多边形。反过来也是如此。如果我更改列表的元素，初始变量保持不变。有谁知道这是为什么，或者是否有解决方法？谢谢！

Polygon <- setRefClass("Polygon", fields = c("sides"))
square <- Polygon$new(sides = 4)
triangle <- Polygon$new(sides = 3)
octagon <- Polygon$new(sides = 8)

pList <-list(square,triangle)

print(pList)
# [[1]] Reference class object of class "Polygon"
# Field "sides": [1] 4
# [[2]]    # Reference class object of class "Polygon"
# Field "sides":  [1] 3

square<-octagon
# Reference class object of class "Polygon"
# Field "sides":  [1] 8

square #the variable square changes
# Reference class object of class "Polygon"
# Field "sides": [1] 8

pList # but the entry in pList does now.
# [[1]] Reference class object of class "Polygon"
# Field "sides": [1] 4
# [[2]] Reference class object of class "Polygon"
# Field "sides":  [1] 3

#the same is true the other way around, changing the item corresponding to triangle in the list, 
#does not change the object triangle
triangle$sides #3
pList[[2]]$sides #3
pList[2]<-octagon
pList[[2]]$sides #8
triangle$sides #3

有谁知道这是为什么，或者是否有解决办法？

如果我更改对象的各个字段，更改确实会传播，如下面的代码所示，但我正在使用的实际类比此处使用的简单多边形类复杂得多，因此更改每个对象内的所有字段对我来说并不是真正的解决方案。

square <- Polygon$new(sides = 4)
triangle <- Polygon$new(sides = 3)
octagon <- Polygon$new(sides = 8)
pList <-list(square,triangle)

#changing the list element changes the variable triangle
triangle$sides #3
pList[[2]]$sides#3
pList[[2]]$sides<-12
pList[[2]]$sides #12
triangle$sides #12

square <- Polygon$new(sides = 4)
triangle <- Polygon$new(sides = 3)
octagon <- Polygon$new(sides = 8)
pList <-list(square,triangle)

#changing the variable triangle changes the list element
triangle$sides #3
pList[[2]]$sides#3
triangle$sides<-12
pList[[2]]$sides #12
triangle$sides #12

Answer 1

列表条目是值而不是引用/指针。 ReferenceClass 对象引用（或指向）与它们关联的数据（即字段）。

如果您想在列表中编辑“类指针”而不直接接触列表对象，则新的“PolygonPointer”类可能是您的解决方法：

PolygonPointer <- setRefClass("PolygonPointer", fields = list(target = "Polygon"))

pointer1 <- PolygonPointer(target = square)
pointer2 <- PolygonPointer(target = triangle)

lst <- list(pointer1, pointer2)

pointer1$target <- octagon

输出是：

> lst
[[1]]
Reference class object of class "PolygonPointer"
Field "target":
Reference class object of class "Polygon"
Field "sides":
[1] 8

[[2]]
Reference class object of class "PolygonPointer"
Field "target":
Reference class object of class "Polygon"
Field "sides":
[1] 3

但请注意，这样的构造是危险的——例如，默认的复制构造函数会执行“深度复制”，同时也会复制指向的对象。例如：

pointer3 <- pointer1$copy()
pointer3 <- triangle
pointer1 # still points to the octagon

如果您只想复制指针本身而不是其引用的对象，则必须覆盖 PolygonPointer 的复制构造函数。