Two variables pointing to same mem location?

[TimeLoop]

Hi.

Lets say we got a piece of code like the next one:

Code: Select all

foo = {salute = "Hello World!"} -- some LUA table

myTable = nil
original = nil

function loadTable(tableName)
    myTable = tableName
end

function restoreOriginal()
    loadTable(original)
end

function love.load()
    original = foo
    loadTable(foo)
    myTable.salute = myTable.salute .. " Goodbye World!"
    restoreOriginal()
    print(myTable.salute) --====>> OUTPUTS "Hello World! Goodbye World!"
end

Anyone knows the problem there?

I noticed also that when I do "print(foo.salute)", the foo table got modified!!! How can that be possible?

Thanks

[veethree]

You're setting mytable and original to nil which is nothing basically. To define an empty table you would do this:

Code: Select all

mytable = {}
original = {}

[Azhukar]

Tables are references, the only time a new table is created is when you use {}.

mytable = {}
thisIsAReference = mytable

The above makes both variables point to the same exact table.

[ixjf]

As aforementioned, foo variable is a reference to the table, it doesn't hold the actual table. It is possible, though, to do what you want by creating a copy of the original table, although you have to copy field by field and save metatables yourself (I suppose there are some implementions on the internet, perhaps search for table.deepcopy or deep_copy).

EDIT: Close enough! It was deepcopy

Code: Select all

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

Source: http://lua-users.org/wiki/CopyTable

[TimeLoop]

Copying tables still doesn't work.


The values of the original table get modified anyway.


Any other solution?

[Azhukar]

Any other solution?

Yes, learn basic lua.

http://lua-users.org/wiki/TablesTutorial

[ixjf]

Copying tables still doesn't work.
The values of the original table get modified anyway.
Any other solution?

What are you talking about? How do they get modified with a copy of the original table? Assigning a table variable to another variable doesn't copy it.