For instance,

I have extern (C) int sortList1(GtkListBoxRow* _row1, GtkListBoxRow* _row2, void* userData)

I pass it as listbox.setSortFunc(&sortList1, cast(void*)this, &_destroy2);

In the sortList1 function I want to access derived members of my ListBoxRowWithData such as:

auto row1 = cast(ListBoxRowWithData)getMyDObject!ListBoxRow(_row1);
auto row2 = cast(ListBoxRowWithData)getMyDObject!ListBoxRow(_row2);
if(row1.id < row2.id) return 1;

A regular type cast returns null. T opCast(T)() in ObjectG.d returns null in this case. I dug into the sources a little, and my working solution is like:

template getCType(T)
{
    static if ( is(T == class) )
        alias getCType = typeof(T.tupleof[0]);
    else
        alias getCType = void*;
}

T getMyDObject(T, TC = getCType!T)(void* data){

    T ret = ObjectG.getDObject!(T)(cast(TC)data);

    return ret;
}

Is there an existing implementation for T getMyDObject(T, TC = getCType!T)(void* data)?