问题

I find that Lua's object-oriented system and metatables are very confusing, as they make simple class and instance grammar more complex than that of C++. I was looking at this code (for this example https://www.runoob.com/lua/lua-metatables.html)

mytable = setmetatable({key1 = "value1"}, {
  __index = function(mytable, key)
    if key == "key2" then
      return "metatablevalue"
    else
      return nil
    end
  end
})

print(mytable.key1, mytable.key2)

In line 2, the __index metamethod points to a function with two parameters. The first is mytable (itself), and the second is key. So when the last line of print tries to find key2 in mytable, mytable starts looking at the nameless metatable. However, I don't know why the second parameter is bound to the input-passing key. It can't be because the parameter's name is key, so Lua is smart enough to understand that the parameter is for key because it spells "k-e-y." I don't think key is a keyword in Lua. Are there any hidden conventions about this naming or this __index = function(arg1, arg2) stuff?

mytable = setmetatable({key1 = "value1"}, {
  __index = function(mytable, key)
    if key == "key2" then
      return "metatablevalue"
    else
      return nil
    end
  end
})

print(mytable.key1,mytable.key2)

答案1

得分: 2

mytable 和 key 只是参数名称（就像在常规 Lua 函数中一样），如果您希望，您可以重命名它们。

  __index = function(t, k)
    if k == "key2" then
      return "metatablevalue"
    else
      return nil
    end

为什么第二个参数绑定了输入的键。

这是元方法的定义。
当调用 __index 元方法时，Lua 按以下顺序传递两个参数：
参数 #1 = 要索引的对象，
参数 #2 = 索引。

  __index = function(t, k)
    if k == "key2" then
      return "metatablevalue"
    else
      return nil
    end

答案2

得分: 1

检查是否不确定，使用 print(...) 

lua_debug> test = setmetatable({}, {__index = function(...) print(...) return({...}) end})
lua_debug> print(test.hello) -- 触发 __index
table: 0x02a0a6549db0 hello -- print(...) 输出表对象和键
table: 0x02a0a65a99b0 -- __index 元方法函数的返回值

<details>
<summary>英文:</summary>

Check out if unsure with print(...)

lua_debug> test = setmetatable({}, {__index = function(...) print(...) return({...}) end})
lua_debug> print(test.hello) -- Triggering __index
table: 0x02a0a6549db0 hello -- print(...) gives out the table object and key
table: 0x02a0a65a99b0 -- The return of the __index Metatmethod Function

</details>