lua - 运算符重载不起作用

Question

在阅读《Lua 编程》时，我尝试了书中给出的运算符重载示例

Set = {}

mt = {}
mt.__add = Set.union

--create a new set with the values of the given list
function Set.new (l) 
    local set = {}
    setmetatable (set, mt)
    for _, v in ipairs (l) do 
        set [v] = true
    end
    return set
end 

function Set.union (a, b)
    local result = Set.new {}
    for k in pairs (a) do result [k] = true end
    for k in pairs (b) do result [k] = true end
    return result
end

function Set.intersection (a, b)
    local result = Set.new {}
    for k in pairs (a) do result [k] = b[k] end
    return result
end

function Set.tostring (set)
    local l = {}
    for e in pairs (set) do
        l[#l + 1] = e
    end
    return "{" .. table.concat (l, ", ") .. "}"
end 

function Set.print (s)
    print (Set.tostring (s))
end 

s1 = Set.new {10, 20, 30, 50}
s2 = Set.new {30, 1}

Set.print (s1)
Set.print (s2)

s3 = s1 + s2
Set.print (s3)

但是最近lua for windows我收到以下错误

lua: C:\meta.lua:47: attempt to perform arithmetic on global 's1' (a table value)
stack traceback:
    C:\meta.lua:47: in main chunk
    [C]: ?
{30, 10, 20, 50}
{1, 30}

Answer 1

你做这个任务太早了：

mt.__add = Set.union

因为Set.union还没有初始化。

将其移至下方Set.union，它将起作用。

出于同样的原因，如果你分配mt.__mul，这应该在下面Set.intersection

Answer 2

您需要定义mt为合适的元表：

mt = { __add = Set.union, __mul = Set.intersection, __tostring = Set.tostring }