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TIP #470: Reliable Access to OO Definition Context Object

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TIP #470: Reliable Access to OO Definition Context Object

Donal K. Fellows-3

 TIP #470: RELIABLE ACCESS TO OO DEFINITION CONTEXT OBJECT
===========================================================
 Version:      $Revision: 1.1 $
 Author:       Donal Fellows <dkf_at_users.sf.net>
 State:        Draft
 Type:         Project
 Tcl-Version:  8.7
 Vote:         Pending
 Created:      Sunday, 23 April 2017
 URL:          http://purl.org/tcl/tip/470.html
 WebEdit:      http://purl.org/tcl/tip/edit/470
 Post-History:

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 ABSTRACT
==========

 This TIP makes it easier for people to write procedures to extend
 TclOO's definition sublanguage.

 RATIONALE
===========

 One of the fundamental features of Tcl is that you can extend it with
 more capabilities by writing your own procedures (and other commands,
 if you prefer the C API). However, it is somewhat awkward to do so when
 using TclOO, as the *oo::define* and *oo::objdefine* commands don't
 make it easy to find out what the context class or object is.

 For example, in the /oo::util/ package of Tcllib, the code for
 discovering what the context class is includes this
 [<URL:http://core.tcl-lang.org/tcllib/artifact/51d71f560ceb7d63?ln=77>]:

       # Get the name of the current class or class delegate
       set cls [namespace which [lindex [info level -1] 1]]

 That is ugly, and won't even work reliably for getting the context
 object in *oo::objdefine* as that can be entered into by multiple paths
 (i.e., there's a shortcut from *oo::define*).

 PROPOSED CHANGE
=================

 I propose to make the existing *self* command in *oo::define*, when
 invoked without arguments, return the context class (provided it is
 evaluated in the correct stack frame, as usual with definition
 commands). Similarly, I also propose to add a *self* command to the
 *oo::objdefine* system that takes no arguments and returns the context
 object.

 This will enable to code listed above in the /Rationale/ to become:

       # Get the name of the current class or class delegate
       set cls [uplevel 1 self]

 In the C API, I propose adding a function:

       Tcl_Object *Tcl_GetDefineContextObject*(Tcl_Interp */interp/)

 which will get the context object, or return NULL and put an error in
 the interpreter if there is no context object in the frame or the
 context object has been deleted. The functionality is that of
 *TclOOGetDefineCmdContext* in /tclOODefineCmds.c/
 [<URL:http://core.tcl-lang.org/tcl/artifact/7d58f1a701168168?ln=682>],
 but the text of the error messages might be changed.

 COPYRIGHT
===========

 This document has been placed in the public domain.

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 TIP AutoGenerator - written by Donal K. Fellows

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