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我不知道如何命名这个问题，因为我对正在发生的事情了解得不够多。（随意编辑）

考虑下面的代码。

函数对象：new()
    o = o 或 {
        x = 0
    }
    setmetatable(o, self)
    self.__index = 自我
    自我.y = 0

返回
结尾

表 = 对象：新（）

后面的变量（ox 和 self.y）有什么区别？

如果我print_r the variable table，则只返回 x 。但是，两者table.x都table.y可以访问。这让我意识到两者之间是有区别的。

有人可以解释有什么区别以及将变量放在不同位置的原因是什么？

4

1 回答 1

4

后面的变量（ox 和 self.y）有什么区别？

您在这里有两张桌子，object并且o.

里面的object:new，self指的是桌子object。self.y表中的字段也是如此object。

o是您在每次调用时创建的新表object:new。o.x是表中的一个字段o。

该o表只有一个条目: o["x"]，因此当您迭代表中的条目时（就像print_r这样），这就是您将看到的全部内容。

那么为什么o.y会给你一个价值呢？因为您将表设置object为o'元表并且该元表设置了它的__index字段，所以当索引尝试失败时o，Lua 将通过o' 元表再次尝试（如果它设置了 __index）。

一点代码可能会使这一点更清楚：

o = { x = 33 }

print(o.x, o.y) --> 33 nil

-- let's give o a metatable
mt = { y = 20 }
setmetatable(o, mt)

-- we've given o a metatable, but that metatable doesn't have an __index member set
print(o.y) --> nil

-- metatable `handlers` are normally functions that handle events
-- here we create a handler for indexing a table if the key doesn't exist in the table:
mt.__index = function(t,k)
  print("An attempt was made to index table", t, "with the key", k)
  return 5150
end

-- we've given o's metatable a handler for indexing which always returns 5150
print(o.x) --> 33
print(o.y) --> 5150
print(o.z) --> 5150
print(o.donut) --> 5150

-- note that Lua has a "rawget" function, which bypasses the metatable mechanics
print(rawget(o,'x')) --> 33, we get a value, because o actually contains "x"
print(rawget(o,'y')) --> nil, back to nil, because the metatable is being ignored

-- the __index metatable handler is special. Instead of providing a function
-- to handle missing key events, you can give it a table. If an index attempt fails,
-- it will try again in the __index table
mt.__index = mt -- we could have used a third table here, but mt has the y entry we want

-- we've changed the metatable handler, so now we'll get 777
print(o.y) --> 777

-- changes to the metatable are reflected in any object using it as metatable
mt.__index.y = 999
print(o.y) --> 999

-- now we give `o` it's OWN "y" entry, so the metatable handler will no longer be invoked
o.y = 2112

print(o.y) --> 2112
print(rawget(o, 'y')) --> o really owns this "y" entry
print(mt.__index.y) --> 999, the metatable was untouched by our write to o.y
于 2012-05-19T22:45:35.793 回答