R中的S4類的一元加號

Question

我在R中試驗S4類，我試圖爲我的對象定義一個加號（+）運算符，即超載加運算符。我設法超載二進制+，但我不知道如何超載一元加。下面是一個最小的工作（元運算符不工作）例如什麼，我想實現：

setClass("HWtest", 
     representation(expr = "character"), 
     prototype = list(expr = NA_character_) 
) 

H <- new("HWtest", expr="Hello") 
W <- new("HWtest", expr="World") 

setMethod("+", signature(e1="HWtest", e2="HWtest"), 
      function(e1,e2){ 
      new("HWtest", 
       expr = paste(e1@expr," + ",e2@expr)) 
      } 
) 

現在我可以使用+子及工程進展順利：

H+W 
An object of class "HWtest" 
Slot "expr": 
[1] "Hello + World" 

現在當然，一元加不工作，從而使已超載

+H 
Error in +H : invalid argument to unary operator 

於是，我就超載它以下列方式：

setMethod("+", signature(e="HWtest"), 
      function(e){ 
      new("HWtest", 
       expr = paste("+ ",e@expr)) 
      } 
) 

但這生成錯誤：

Error in match.call(fun, fcall) : 
    argument 1 matches multiple formal arguments 

是否有可能超載一元加號？如果是這樣，我將如何做這個最小的例子？

Answer 1

嘗試加入這個（除了你的二進制過載）：

setMethod("+", signature(e1 = "HWtest", e2 = "missing"), 
      function(e1){ 
      new("HWtest", 
       expr = paste("+ ", e1@expr)) 
      } 
) 

---

R> +H 
An object of class "HWtest" 
Slot "expr": 
[1] "+ Hello" 

R> H + W 
An object of class "HWtest" 
Slot "expr": 
[1] "Hello + World" 

---

引用的幫助文件?groupGeneric，

[...] when a unary operator is encountered the Ops method is called with one argument and e2 is missing.

正如弗蘭克指出，?setMethod幫助文件包含一些有關使用missing作爲方法簽名（強調增加）的一些有用信息：

Two additional special class names can appear: "ANY", meaning that this argument can have any class at all; and "missing", meaning that this argument must not appear in the call in order to match this signature.