[PD] equality and void * pointers

[Matt Barber]

As I understand it, you can compare void pointers because they just store
addresses.

In g_editor.c:


static int glist_dofinderror(t_glist *gl, void *error_object)
{
    t_gobj *g;
    for (g = gl->gl_list; g; g = g->g_next)
    {
        if ((void *)g == error_object)
        {
            /* got it... now show it. */
            glist_noselect(gl);
            canvas_vis(glist_getcanvas(gl), 1);
            canvas_editmode(glist_getcanvas(gl), 1.);
            glist_select(gl, g);
            return (1);
        }
        else if (g->g_pd == canvas_class)
        {
            if (glist_dofinderror((t_canvas *)g, error_object))
                return (1);
        }
    }
    return (0);
}


this function takes a pointer to void (storing the address of an object) as
its second argument void *error_object. It can't know what the type of that
object is, because it's being called from somewhere else, and it could be
any kind of Pd object. That somewhere else knows what kind of object it is
and (more importantly) where that object's address is, and just passes that
address in. Then t_gobj *g; traverses the canvas, and since the address of
each object is known, each of those can be compared to the object address
passed in until there's a match. This is kind of a way of getting around
the usual strong typing in c; as long as we know from both ends of the
transaction that we can get valid addresses of what we're interested in,
there's no problem just comparing those addresses to see if we've found the
same object.



On Thu, Oct 1, 2015 at 4:45 PM, Jonathan Wilkes via Pd-list <
pd-list at lists.iem.at> wrote:

> Hi list,
>
> int compare_pointers(t_pd *foo)
> {
>      long bar = generate_random_long();
>      return (((void *)foo) == ((void *)bar));
> }
>
> (I probably have unnecessary parens there...)
>
> Is the check for equality a case of undefined behavior?
>
> If so, doesn't glob_findinstance of s_print.c also lead to the same
> undefined behavior?
>
> -Jonathan
>
>
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