Version 3 of static data in command procedures

Updated 2004-10-04 17:46:03

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.

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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_Objs, 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.