[Gambas-devel] Store GB_VARIANT in memory

[Tobias Boege]

On Tue, 14 Aug 2012, Beno?t Minisini wrote:
> > BEGIN_METHOD(Deque_PushFront, GB_VARIANT value)
> >
> > 	void *newval;
> >
> > 	GB.Alloc(&newval, sizeof(GB_VARIANT_VALUE));
> > 	GB.StoreVariant(ARG(value), newval);
> > 	CDEQUE_push_front(THIS, newval); // Just links the copy to the list
> >
> > END_METHOD
> >
> > BEGIN_METHOD_VOID(Deque_PopFront)
> >
> > 	void *value;
> >
> > 	if (CDEQUE_is_empty(THIS)) {
> > 		GB.ReturnNull();
> > 		return;
> > 	}
> > 	value = CDEQUE_pop_front(THIS); // Retrieves newval from above and
> > 					// deletes the list node
> > 	GB.ReturnVariant(value);
> > 	GB.Free(&value);
> >
> > END_METHOD
> >
> > (Don't mind the names ;-))
> >
> 
> Too much english for me in one sole mail. :-)
> 

You haven't had to write it! ;-)

> Did you look at how the Collection class has been implemented?
> 
> 	--> /main/gbx/gbx_c_collection.c
> 
> Or simpler the Variant[] array?
> 
> 	--> /main/gbx/gbx_c_array.c
> 

The latter, yes. Until this minute, I looked again and saw that all Arrays
simply hold the memory they once occupied - kind of optimisation, I think,
and necessary as you said below...

> They both store and retrieve variants. Or you can look at how the 
> Control.Tag property in gb.qt4, it's a variant stored inside a 
> dynamically allocated object.
> 
> 	--> /gb.qt4/src/CWidget.cpp
> 
> If the Variant is not global, but stored inside a dynamically allocated 
> C structure, nothing changes:

I'll have a look at the other two then.

> 
> --8<-------------------------------------------------------
> 
> // My object
> 
> typedef
>    struct {
>      GB_BASE ob;
>      GB_VARIANT_VALUE tag;
>      }
>    MYOBJECT;
> 
> #define THIS ((MYOBJECT *)_object)
> 
> BEGIN_METHOD(Store, GB_VARIANT value)
> 
>    GB.StoreVariant(ARG(value), &THIS->tag);
> 
> END_METHOD
> 
> BEGIN_METHOD_VOID(Restore)
> 
>    // The variant is copied in a temporary place.
>    GB.ReturnVariant(&THIS->tag);
> 
>    // Free the stored variant.
>    // But you can't do it now, because the Variant will
>    // be referenced after the function has returned only.
>    // GB.StoreVariant(NULL, &THIS->tag);
> 
> END_METHOD
> 
> --8<-------------------------------------------------------
> 
> Regards,
> 
> -- 
> Beno?t Minisini
> 

_This_ is confusing and may be the error in my code. I thought, the Variant
is copied to a temporary place by GB.Return*() (AFAICS this "TEMP" variable.
Haven't I once heard that Gambas is not thread-safe? :-))
Why can't I free up my storage of the Variant?
Maybe my approach is just too different from what you proposed... What comes
to my mind now is this: GB.StoreVariant() refs the Object it takes but it
gets never unref'd by my code. If this is true, no wonder why I had memory
errors...

I can (and actually wanted to) rewrite the code a bit so I can test your
suggestion - which is not to try to free the buffer but instead keep it and
re-use like the Array classes do. We'll see... Thanks.

Regards,
Tobi