Lua默认值代理：__index读缺省的配置对象

-- 错误：每个实例都拷贝一整套默认值
function newCfg()
    return {size = 20, color = "white",
        bold = false}
end

local DEFAULTS = {size = 20, color = "white"}
local function newCfg(over)
    return setmetatable(over or {},
        {__index = DEFAULTS})
end
local cfg = newCfg({color = "red"})
local label = panel:getChildByName("cfgText")
label:setString(cfg.size .. "/" .. cfg.color
    .. "/" .. tostring(rawget(cfg, "size") == nil))