Question

Say I have the following structure:

package require Itcl


itcl::class AAA {

private variable m_list {}

constructor {} {
    fill m_list list
}

}

How to get a reference on the m_list in order to write

foreach elem $reference {.......} 

Consider that list is really big and I don’t want to copy it!

Answer 1

Tcl variables use copy-on-write semantics. You can safely pass a value around, assigning multiple variables to it, without worrying about it taking up more space in memory.

For example

set x {some list} ;# there is one copy of the list, one variable pointing at it
set y $x          ;# there is one copy of the list, two variables pointing at it
set z $y          ;# there is one copy of the list, three variables pointing at it
lappend z 123     ;# there are two copies of the list
                  ;# x and y pointing at one
                  ;# z pointing at the other 
                  ;#     which is different from the first via an extra 123 at the end

The above code will result in two giant lists, one with the original data that both x any y point at, and one with the extra element of 123 that only z points to. Prior to the lappend statement, there was only one copy of the list and all three variables pointed at it.