[litwindow-users] Re: Using aggregate/accessors

[Hajo K. <mai...@ha...>]

Hi,

>>What do you pass to "Items"?
> 
> 
> I am passing a vector<class MyListData> to Items.
> 
> Specifically, 
> 
> RULE("xrcMainList.Items", make_expr<accessor>("m_ListData"))

ah, of course. Tiny detail, big effect. I didn't pay close enough 
attention. You are not passing a vector<>, you are passing an accessor 
to a vector. Thats the same difference between Type and Type*. The 
accessor is like Type*. You need to dereference the accessor to get to 
the vector.

On the wxListCtrl side:

Items is not a vector<MyListData>, its an accessor, which in your 
situation currently happens to point at a vector<MyListData> but could 
point to a totally different data type a few microseconds later.

So you have

wxListCtrl *pList = wxDynamicCast(event.GetEventObject(), wxListCtrl);
aggregate ag(make_aggregate(*pList));
accessor ac(ag["Items"]);

ag["Items"] returns an accessor to the property. If the property is of 
type long, it returns an accessor to a long object. If the property is 
of type string, it returns an accessor to a string object.
In your case, the property is an accessor, so ag["Items"] returns an 
accessor to an accessor object.

To get to the real accessor, the one you are interested in, you must 
dereference the accessor, much like *ag["Items"].

But accessors are untyped objects, so you need to cast it into a typed 
accessor first, then dereference it. Here is how:

     typed_accessor tac=dynamic_cast_accessor<accessor>(ac);
     accessor the_real_items_accessor=tac.get(); // this is like '*ac'
     the_real_items_accessor.is_container()==true;




I'll rewrite the scenario using pseudo-code and replace accessor with 
'void*' to make things more clear.

wxListCtrl *pList=wxDynamicCast...
aggregate ag(make_aggregate(*pList));

// ag["Items"] returns a void* to the "Items" member property
// which is itself of type void*
// so ag["Items"] return type is void **
void **ac=ag["Items"]; // ag.Items

// cast it so we can get to the data

void *tac=*(void*)ac;
vector<MyDataStruct> *the_real_items_accessor=(vector<...>*)tac;



    ***
Here is the full version again, that should work now:

wxListCtrl *pList = wxDynamicCast(event.GetEventObject(), wxListCtrl);
aggregate ag(make_aggregate(*pList));
accessor ac(ag["Items"]);

typed_accessor tac=dynamic_cast_accessor<accessor>(ac);
accessor the_real_items=tac.get();

if (the_real_items.is_valid() && the_real_items.is_container())


Looks more complicated than it is until you think of accessors as void* 
pointers with type information, which is exactly what they are by the way :)


Hajo