Question

I’ve seen mentioned in a few texts that Tcl uses reference counting for garbage collection. I was wondering what this means, since as far as I can tell Tcl doesn’t have pointers or references, so every variable is only referenced once. What exactly is it counting, then?

Answer 1

The idea is that Tcl has reference counting on values, not variables.

Let’s cite the wiki page on copy-on-write semantics:

Each value (Tcl_Obj) has a reference count. Whenever the value is passed to a command or assigned to a variable the reference count is incremented and no copy is made. When a value is to be changed the implementation first checks the reference count. If the count is 1 then there is no other reference to the value and it can be changed in place. If the count is greater than 1 then there are other references to this value. If the value were to be changed in place then those references would also be changed. To prevent this a copy of the value is made (with new reference count of 1) and the copy is changed in place.

The only thing missing in this explanation is that when the refcount of a value drops belov 1, the value is destroyed.

For hard-core information, read the “Storage Management of Objects” section of the Tcl_Obj manual page and these discussions.