lua metatables中操做__index

reference:

http://www.lua.org/manual/5.3/manual.html

__index: The indexing access table[key]. This event happens when table is not a table or when key is not present in table. The metamethod is looked up in table.

Despite the name, the metamethod for this event can be either a function or a table. If it is a function, it is called with table and key as arguments, and the result of the call (adjusted to one value) is the result of the operation. If it is a table, the final result is the result of indexing this table with key. (This indexing is regular, not raw, and therefore can trigger another metamethod.)

 

__index这个重载，主要是重载了find key的操做，这操做可让Lua变得有点面向对象的感受。 所谓__index，说得明确一点，若是咱们有两个对象a和b，咱们想让b做为a的prototype只须要：

setmetatable(a, {__index = b})

 

metamethod 为function

示例：

Window_Prototype = {x=0, y=0, width=100, height=100}
MyWin = {title="Hello"}
Window = {}
Window.__index = function(table, key)
  return Window_Prototype[key]
end
setmetatable(MyWin,Window)
print(MyWin.title, MyWin.width)

执行结果：

Hello	100

metamethod 为table

Window_Prototype = {x=0, y=0, width=100, height=100}
MyWin = {title="Hello"}
Window = {}
Window.__index = Window_Prototype
setmetatable(MyWin,Window)

print(MyWin.title, MyWin.width)

执行结果：

Hello	100

metamethod 为table简洁的书写方式：

Window_Prototype = {x=0, y=0, width=100, height=100}
MyWin = {title="Hello"}
setmetatable(MyWin, {__index = Window_Prototype})

print(MyWin.title, MyWin.width)