在 Lua 表中实现后备/默认 getter

发布于 2024-12-21 00:01:25 字数 198 浏览 0 评论 0原文

有没有办法实现类似python的__getitem__的机制？

例如，具有以下内容：

local t1 = {a=1, b=2, c=3, d=4}

如果在代码中，将调用 t1.e，那么我希望返回其他内容而不是 nil

原文

Is there a way to implement a mechanism similar to python's __getitem__?

for instance, having the following:

local t1 = {a=1, b=2, c=3, d=4}

if in code, t1.e will be called, then I wish to have something else returned rather than nil

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暖风昔人 2024-12-28 00:01:25

您可以使用 setmetatable 和 __index 元方法：

local t1 = {a = 1, b = 2, c = 3, d = 4}

setmetatable(t1, {
    __index = function(table, key)
        return "something"
    end
})

print(t1.hi) -- prints "something"

请注意，当您执行 t.nonexistant = Something 时，不会调用此方法。为此，您需要 __newindex 元方法：

local t1 = {a = 1, b = 2, c = 3, d = 4}

setmetatable(t1, {
    __index = function(table, key)
        return "something"
    end,

    __newindex = function(table, key, value)
        rawset(table, tostring(key) .. '_nope', value)
    end
})

print(t1.hi) -- prints "something"
t1.hi = 'asdf'
print(t1.hi) -- prints "something"
print(t1.hi_nope) -- prints "asdf"

You can use setmetatable and the __index metamethod:

local t1 = {a = 1, b = 2, c = 3, d = 4}

setmetatable(t1, {
    __index = function(table, key)
        return "something"
    end
})

print(t1.hi) -- prints "something"

Note that this will not be called when you do t.nonexistant = something. For that, you need the __newindex metamethod:

local t1 = {a = 1, b = 2, c = 3, d = 4}

setmetatable(t1, {
    __index = function(table, key)
        return "something"
    end,

    __newindex = function(table, key, value)
        rawset(table, tostring(key) .. '_nope', value)
    end
})

print(t1.hi) -- prints "something"
t1.hi = 'asdf'
print(t1.hi) -- prints "something"
print(t1.hi_nope) -- prints "asdf"

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