Lua More metamethods


Example

There are many more metamethods, some of them are arithmetic (e.g. addition, subtraction, multiplication), there are bitwise operations (and, or, xor, shift), comparison (<, >) and also basic type operations like == and # (equality and length). Lets build a class which supports many of these operations: a call for rational arithmetic. While this is very basic, it shows the idea.

local meta = {
    -- string representation
    __tostring = function(self)
        return string.format("%s/%s", self.num, self.den)
    end,
    -- addition of two rationals
    __add = function(self, rhs)
        local num = self.num * rhs.den + rhs.num * self.den
        local den = self.den * rhs.den
        return new_rational(num, den)
    end,
    -- equality
    __eq = function(self, rhs)
        return self.num == rhs.num and self.den == rhs.den
    end
}

-- a function for the creation of new rationals
function new_rational(num, den)
    local self = { num = num, den = den }
    setmetatable(self, meta)

    return self
end

local r1 = new_rational(1, 2)
print(r1) -- 1/2

local r2 = new_rational(1, 3)
print(r1 + r2) -- 5/6

local r3 = new_rational(1, 2)
print(r1 == r3) -- true
-- this would be the behaviour if we hadn't implemented the __eq metamethod.
-- this compares the actual tables, which are different
print(rawequal(r1, r3)) -- false