Tcl offers several techniques that can be
used to implement static data in command
procedures.  This page is intended to
present the pros and cons.

----
Example:

Here is a command procedure for a simple
Tcl command that makes use of a "static"
value.  Roughly, it's a constant value
that each invocation of the command will
use without changing, but a value that
need not be created at all if the command
is never evaluated.

  int
  OneObjCommand(ClientData cd, Tcl_Interp *interp,
    int objc, Tcl_Obj *const objv[])
  {
    Tcl_SetObjResult(interp, Tcl_NewIntObj(1));
    return TCL_OK;
  }

  Tcl_CreateObjCommand(interp, "one", OneObjCommand, NULL, NULL);

Note that each time [[one]] is evaluated, a new
'''Tcl_Obj''' is created to hold the value '''1'''.
Since Tcl is capable of managing shared '''Tcl_Obj'''s,
other alternatives are possible that would set
the result to be an additional reference to one shared
'''Tcl_Obj''' rather than a new one each time.
The alternatives differ mostly on where that shared
'''Tcl_Obj''' is stored between calls.

Why might we want an alternative?  Most compelling
reason is memory efficiency.  Consider the script:

  for {set i 0} {$i < 1000000} {incr i} {
    set a($i) [one]
  }

With the implementation of [[one]] above, 1 million
'''Tcl_Obj''' structs have to be allocated.
----
'''Alternative 1: ClientData'''

  int
  OneObjCommand(ClientData cd, Tcl_Interp *interp,
    int objc, Tcl_Obj *const objv[])
  {
    Tcl_SetObjResult(interp, (Tcl_Obj *)cd);
    return TCL_OK;
  }
  void
  OneDelete(ClientData cd)
  {
    Tcl_Obj *objPtr = (Tcl_Obj *)cd;
    Tcl_DecrRefCount(objPtr);
  }

  objPtr = Tcl_NewIntObj(1);
  Tcl_IncrObjCount(objPtr);
  Tcl_CreateObjCommand(interp, "one", OneObjCommand,
    (ClientData)objPtr, OneDelete);

Here the shared '''Tcl_Obj''' with the value '''1'''
is stuffed in the '''ClientData''' of the [[one]] command.
One disadvantage of this alternative is that the
shared '''Tcl_Obj''' is created whether or not [[one]]
is ever called.  For larger amounts of static data, that
might be a waste worth avoiding.  Another possible
disadvantage might be conflict with other uses of the
'''ClientData''' word that a command might have.
----
'''Alternative 2: One field in ClientData'''

  typedef struct OneData {
    Tcl_Obj *one;
  } OneData;
  int
  OneObjCommand(ClientData cd, Tcl_Interp *interp,
    int objc, Tcl_Obj *const objv[])
  {
    OneData *dataPtr = (OneData *)cd;
    if (dataPtr->one == NULL) {
      dataPtr->one = Tcl_NewIntObj(1);
      Tcl_IncrRefCount(dataPtr->one);
    }
    Tcl_SetObjResult(interp, dataPtr->one);
    return TCL_OK;
  }
  void
  OneDelete(ClientData cd)
  {
    OneData *dataPtr = (OneData *)cd;
    if (dataPtr->one != NULL) {
      Tcl_DecRefCount(dataPtr->one);
    }
    Tcl_Free(dataPtr);
  }

  dataPtr = Tcl_Alloc((int)sizeof(OneData));
  Tcl_CreateObjCommand(interp, "one", OneObjCommand,
    (ClientData)dataPtr, OneDelete);

----
'''[DGP]'''