coppy ds_list item to variable in GameMaker

Question

function foo(_bar) constructor
{
    bar = _bar;
}

__foo = ds_list_create();

ds_list_add(__foo, new foo(5));

__bar = __foo[| 0];

__bar.bar++;


show_debug_message(string(__foo[| 0].bar));

show_debug_message(string(__bar.bar));

And I have in Output:

6
6

How I can __bar and __foo[| 0] as Difference, I want to __bar.bar = 6 and __foo[| 0].bar = 5 (default).