Re: Implementing a customtree with Gtk2::TreeModel

[muppet <scott asofyet org>]

On Jan 13, 2005, at 12:18 AM, ofey aikon wrote:

> If any one has code snippets related to this task, please throw it in. 
> My (non-working) code is
> attached, in case some one has the time to review it and let me know 
> how disturbing it is :)

You're already running up against one of the basic difficulties, 
imposed by the stack-use-based design of C GtkTreeIter.

What you're seeing ("There is a disparity...") is an internal 
GtkTreeView sanity check failing, because one of the items you've 
placed into your iter ($iter->[2]) is a string, and by the time the 
iter gets back to a place where it can be used, the scalar has already 
been destroyed.


The problem is the complete lack of lifetime management on GtkTreeIter 
in the C api, which is a side effect of its design for speed and 
efficiency.  There is no destructor called when the GtkTreeIter is 
destroyed.  This is alright for binding the client-side use of the 
iter, but is deadly for implementing a TreeIter in a language like 
perl, in which even primitive values (strings, numbers, references) are 
lifetime-managed complex objects (SVs).  Because the TreeModel 
interface is made up of virtual functions from which we return new 
objects to calling C code, and because the Iter is necessarily bound in 
a way that doesn't know about the internals of the structure, we can 
neither use mortal SVs (they will be destroyed at the next perl 
statement, which, because the bindings can't look inside Iters from the 
client side, won't know to keep the object alive) nor typical SVs with 
a single reference (because without a destruction notification, we'll 
simply leak them).  Our only viable solution is to place a "weak 
reference" to a scalar in the iter --- that is, just the address of a 
scalar that is lifetime-managed by the model implementation.

In other words, you have to cache any non-integer value you place into 
an iter.


(sorry to leave it at that, i'm out of time for the morning)

--
To me, "hajime" means "the man standing opposite you is about to hit 
you with a stick".
  -- Ian Malpass, speaking of the Japanese word for "the beginning"