This comes up every so often on '''comp.lang.tcl''':

 Chris Nelson wrote:
 > For example, whereas I might write C like:
 > 
 >     struct s {
 >        int foo
 >        char bar
 >        float grill
 >     } 
 > 
 >     s.foo = 1;
 >     s.bar = 'c';
 >     s.grill = 3.14159;

======
 proc struct { name thingys } {
     foreach thing $thingys {
        uplevel set $name.$thing \[ list \]
     }
 }

 struct s {
   foo
   bar
   grill
 }

 set s.foo 1
 set s.bar c
 set s.grill 3.14159
======

Of course, you probably dont just want to init the "struct"
to nulls, but to reasonable values:

======
 proc struct { name thingys } {
     foreach { key value } $thingys {
        uplevel set $name.$key $value 
     } 
 }

 struct s {
   foo 1
   bar c
   grill 3.14159
 }
======

Tcl has no types, but if you want to create a typing convention
and then use types in your struct it is only a matter of adding
''type'' to the foreach that now iterates ''key'' and ''value''
over ''$thingys''. '''-PSE'''

----

How about this--values are optional; and there's a variable that
lists all the fields in the structure:

======
 proc struct {name fields} {
     upvar $name srec
     foreach fld $fields {
         set fldn [lindex $fld 0]
         set fldv [lindex $fld 1]
         lappend srec $fldn
         uplevel set $name.$fldn [list $fldv]
     }
 }

 struct s {
    foo
    {bar 1}
    {pi 3.1415926}
 }

 foreach fld $s {upvar 0 s.$fld field; puts "s.$fld: '$field'"}
======
 

'''HJ'''

10-12-2002 [escargo]: I have my own clumsy implementation of
something like this; there are some properties that I found desirable
for such structures to have:

   1. Each structure can tell you its type.
   1. Each structure can tell you the names of its fields.
   1. It is possible to determine if the structure has a field with a specific name.
   1. There are ways to set and get each field from the structure by name.
   1. The structure is a value that does not depend on being assigned to a variable with a particular name.

Is there a C struct analog that has these properties?

----



So how do I create a linked list, as I would in C?

[Ro]: Well if you just want a list of objects you could just make a regular tcl list - they grow dynamically.  Each element in the list could be a reference to an object... what are you trying to represent?

[RS]: [List]s we have. The purpose of linked lists is often to allow insertion or deletion of elements, which our flat lists allow with [linsert], [lreplace] and (probably most often wanted), [lappend]. You could simulate a linked list with an array, e.g.,
======
 set a(1,content) "hello world"
 set a(1,prev) 0 ;# "pointer" to previous
 set a(1,next) 2 ;# "pointer" to next element
======
... but I doubt whether that's more efficient than linsert/lreplace...

[escargo]: This does beg the question of how you would do a ''circular'' linked list
using a tcl list, which
is easy enough to do in C (or other language with pointers or references).

[RS]: Indeed, that's the strongest point against(?) flat lists, which is occasionally also raised in favor of Lisp. But iterating over a circular list may get you nowhere... However, to ''cycle'' over a list is easy in Tcl, and that is a frequent use for circular lists:
======
 proc cycle listName {
    upvar 1 $listName list
    set first [lindex $list 0]
    set list [concat [lrange $list 1 end] [list $first]
    set first
 }
======
'''Lars H''': Even better would be
======
 proc cycle listName {
    upvar 1 $listName list
    set first [lindex $list 0]
    set list [lreplace $list [set list 0] 0] 
    lappend list $first
    set first
 }
======
since it (usually) allows the original object of the list to be modified in place and thus avoids allocating new storage for the list.

'''yajiang''': in side tcllib '''struct::record''' 
usage above are all covered in this package struct::record. 
maybe you guys wanner take a look

http://tcllib.sourceforge.net/doc/record.html

<<categories>> Arts and crafts of Tcl-Tk programming | Category Data Structure