Lua运算符重载

Question

我在网上找到一些地方说Lua的操作员是多余的,但我似乎找不到任何例子.

有人可以提供一个例子,比如重载+运算符,就像..运算符用于字符串连接一样吗？

编辑1: 对Alexander Gladysh和RBerteig:
如果操作符重载只在两个操作数相同的类型下工作并且改变这种行为并不容易,那么下面的代码怎么工作呢？(我不是指任何冒犯,我刚开始学习这种语言):

printf = function(fmt, ...)
    io.write(string.format(fmt, ...))
end

Set = {}
Set.mt = {}    -- metatable for sets

function Set.new (t)
    local set = {}
    setmetatable(set, Set.mt)
    for _, l in ipairs(t) do set[l] = true end
    return set
end


function Set.union (a,b)
    -- THIS IS THE PART THAT MANAGES OPERATOR OVERLOADING WITH OPERANDS OF DIFFERENT TYPES
    -- if user built new set using: new_set = some_set + some_number
    if type(a) == "table" and type(b) == "number" then
        print("building set...")
        local mixedset = Set.new{}
        for k in pairs(a) do mixedset[k] = true end
        mixedset[b] = true
        return mixedset
    -- elseif user built new set using: new_set = some_number + some_set
    elseif type(b) == "table" and type(a) == "number" then
        print("building set...")
        local mixedset = Set.new{}
        for k in pairs(b) do mixedset[k] = true end
        mixedset[a] = true
        return mixedset
    end

    if getmetatable(a) ~= Set.mt or
        getmetatable(b) ~= Set.mt then
        error("attempt to 'add' a set with a non-set value that is also not a number", 2)
    end

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


function Set.tostring (set)
    local s = "{"
    local sep = ""
    for e in pairs(set) do
        s = s .. sep .. e
        sep = ", "
    end
    return s .. "}"
end

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

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

Set.mt.__add = Set.union

-- now try to make a new set by unioning a set plus a number:
s3 = s1 + 8
Set.print(s3)  --> {1, 10, 20, 30, 50}

Answer 1

该metatable函数仅适用于表,但您可以使用debug.metatable设置字符串metatable ...

> mt = {}
> debug.setmetatable("",mt)
> mt.__add = function (op1, op2) return op1 .. op2 end
> ="foo"+"bar"
foobar
> 

另一种方法是使用debug.getmetatable扩充内置字符串metatable(回答下面注释中的问题):

~ e$ lua
Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> debug.getmetatable("").__add = function (op1, op2) return op1 .. op2 end
> ="foo"+"bar"
foobar
> 

Answer 2

参见Lua编程手册的Metatables部分和Lua第2版编程中的Metatables和Metamethods章节.

请注意,对于比较运算符,运算符重载仅在两个操作数类型相同时才起作用.