Copy Table

wiki

A generic table.copy function is not guaranteed to suit all use-cases, as there are many different aspects which must be selected for the specific situation. For example: should metatables be shared or copied? Should we check userdata for a __copy metamethod? These questions (as well as many others) must be asked and answered by the developer.

Some of the Lua "extended standard libraries", such as Penlight and stdlib provide ready-made copy functions for convenience. Check if they suit your use-case before implementing your own.

The following functions provide a base to work off of:

Shallow Copy

function shallowcopy(orig)

    local orig_type = type(orig)

    local copy

    if orig_type == 'table' then

        copy = {}

        for orig_key, orig_value in pairs(orig) do

            copy[orig_key] = orig_value

        end

    else -- number, string, boolean, etc

        copy = orig

    end

    return copy

end

Deep Copy

Here is a simple recursive implementation that additionally handles metatables and avoid the __pairs metamethod.

function deepcopy(orig)

    local orig_type = type(orig)

    local copy

    if orig_type == 'table' then

        copy = {}

        for orig_key, orig_value in next, orig, nil do

            copy[deepcopy(orig_key)] = deepcopy(orig_value)

        end

        setmetatable(copy, deepcopy(getmetatable(orig)))

    else -- number, string, boolean, etc

        copy = orig

    end

    return copy

end

Additionally, it is recursive which means it may overflow the stack for large tables.

Non-recursive Deep Copy