Lua-メタテーブル

メタテーブルは、キーセットと関連するメタメソッドを使用して、アタッチされているテーブルの動作を変更するのに役立つテーブルです。これらのメタメソッドは、次のような機能を可能にする強力なLua機能です。

• テーブルの演算子への機能の変更/追加。

• メタテーブルの__indexを使用して、キーがテーブルで使用できない場合にメタテーブルを検索します。

メタテーブルの処理に使用される2つの重要な方法があります。

• setmetatable(table,metatable) −このメソッドは、テーブルのメタテーブルを設定するために使用されます。

• getmetatable(table) −このメソッドは、テーブルのメタテーブルを取得するために使用されます。

まず、あるテーブルを別のテーブルのメタテーブルとして設定する方法を見てみましょう。以下に示します。

mytable = {}
mymetatable = {}
setmetatable(mytable,mymetatable)

上記のコードは、以下に示すように1行で表すことができます。

mytable = setmetatable({},{})

_index

テーブルで使用できない場合にメタテーブルを検索するためのメタテーブルの簡単な例を以下に示します。

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

print(mytable.key1,mytable.key2)

上記のプログラムを実行すると、次の出力が得られます。

value1 metatablevalue

上記の例で何が起こったのかを段階的に説明しましょう。

• ここのテーブルmytableは {key1 = "value1"}.

• Metatable is set for mytable that contains a function for __index, which we call as a metamethod.

• The metamethod does a simple job of looking up for an index "key2", if it's found, it returns "metatablevalue", otherwise returns mytable's value for corresponding index.

We can have a simplified version of the above program as shown below.

mytable = setmetatable({key1 = "value1"}, 
   { __index = { key2 = "metatablevalue" } })
print(mytable.key1,mytable.key2)

__newindex

When we add __newindex to metatable, if keys are not available in the table, the behavior of new keys will be defined by meta methods. A simple example where metatable's index is set when index is not available in the main table is given below.

mymetatable = {}
mytable = setmetatable({key1 = "value1"}, { __newindex = mymetatable })

print(mytable.key1)

mytable.newkey = "new value 2"
print(mytable.newkey,mymetatable.newkey)

mytable.key1 = "new  value 1"
print(mytable.key1,mymetatable.newkey1)

When you run the above program, you get the following output.

value1
nil	new value 2
new  value 1	nil

You can see in the above program, if a key exists in the main table, it just updates it. When a key is not available in the maintable, it adds that key to the metatable.

Another example that updates the same table using rawset function is shown below.

mytable = setmetatable({key1 = "value1"}, {

   __newindex = function(mytable, key, value)
      rawset(mytable, key, "\""..value.."\"")
   end
})

mytable.key1 = "new value"
mytable.key2 = 4

print(mytable.key1,mytable.key2)

When we run the above program we will get the following output.

new value	"4"

rawset sets value without using __newindex of metatable. Similarly there is rawget that gets value without using __index.

Adding Operator Behavior to Tables

A simple example to combine two tables using + operator is shown below −

mytable = setmetatable({ 1, 2, 3 }, {
   __add = function(mytable, newtable)
	
      for i = 1, table.maxn(newtable) do
         table.insert(mytable, table.maxn(mytable)+1,newtable[i])
      end
      return mytable
   end
})

secondtable = {4,5,6}

mytable = mytable + secondtable

for k,v in ipairs(mytable) do
   print(k,v)
end

When we run the above program, we will get the following output.

1	1
2	2
3	3
4	4
5	5
6	6

The __add key is included in the metatable to add behavior of operator +. The table of keys and corresponding operator is shown below.

Sr.No.	Mode & Description
1	__add Changes the behavior of operator '+'.
2	__sub Changes the behavior of operator '-'.
3	__mul Changes the behavior of operator '*'.
4	__div Changes the behavior of operator '/'.
5	__mod Changes the behavior of operator '%'.
6	__unm Changes the behavior of operator '-'.
7	__concat Changes the behavior of operator '..'.
8	__eq Changes the behavior of operator '=='.
9	__lt Changes the behavior of operator '<'.
10	__le Changes the behavior of operator '<='.

__call

Adding behavior of method call is done using __call statement. A simple example that returns the sum of values in main table with the passed table.

mytable = setmetatable({10}, {
   __call = function(mytable, newtable)
   sum = 0
	
      for i = 1, table.maxn(mytable) do
         sum = sum + mytable[i]
      end
	
      for i = 1, table.maxn(newtable) do
         sum = sum + newtable[i]
      end
	
      return sum
   end
})

newtable = {10,20,30}
print(mytable(newtable))

When we run the above program, we will get the following output.

70

__tostring

To change the behavior of the print statement, we can use the __tostring metamethod. A simple example is shown below.

mytable = setmetatable({ 10, 20, 30 }, {
   __tostring = function(mytable)
   sum = 0
	
      for k, v in pairs(mytable) do
         sum = sum + v
      end
		
      return "The sum of values in the table is " .. sum
   end
})
print(mytable)

When we run the above program, we will get the following output.

The sum of values in the table is 60

If you know the capabilities of meta table fully, you can really perform a lot of operations that would be very complex without using it. So, try to work more on using metatables with different options available in meta tables as explained in the samples and also create your own samples.