[Richard Suchenwirth] 2002-11-14 - A ''reference'' is something that refers, or points, to another something (if you pardon the scientific expression). In C, references are done with *pointers* (memory addresses); in Tcl, references are strings ([everything is a string]), namely names of variables, which via a hash table can be resolved (dereferenced) to the "other something" they point to:
 puts foo       ;# just the string foo
 puts $foo      ;# dereference variable with name of foo
 puts [set foo] ;# the same
This can be done more than one time with nested [set] commands. Compare the following C and Tcl programs, that do the same (trivial) job, and exhibit remarkable similarity:
 #include <stdio.h>
 int main(void) {
   int    i =      42;
   int *  ip =     &i;
   int ** ipp =   &ip;
   int ***ippp = &ipp;
   printf("hello, %d\n", ***ippp);
   return 0;
 }
...and Tcl:
 set i    42
 set ip   i
 set ipp  ip
 set ippp ipp
 puts "hello, [set [set [set [set ippp]]]]"
The C asterisks correlate to [set] calls in derefencing, while in Tcl similar markup is not needed in declaring. But why four [set]s for three asterisks? Because also the first mention of ''i'' in c is a dereference, to pass its value into ''printf''; Tcl makes all four of them explicit (otherwise, you'd see ''hello, i''). One dereference is so frequent that it is typically abbreviated with ''$varname'', e.g.
 puts "hello, [set [set [set $ippp]]]"
has [set] where C uses asterisks, and $ for the last (default) dereference.

The hashtable for variable names is either global, for code evaluated in that scope, or local to a [proc]. You can still "import" references to variables in scopes that are "higher" in the call stack, with the [upvar] and [global] commands. (The latter being automatic in C, given the names are unique; else, the innermost scope wins).

(This little piece was written to defuse the superstition that Tcl has no references, or needs more of the same. See also [Pass by reference], which is also called "call by name", pretty exactly describes how Tcl implements it...)
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