[Qt5-feedback] Add internalVariant to QModelIndex

[Charley Bay]

I completely support Matthias' proposal for an internal "variant" type thing
for QModelIndex.  My use case is different, but my solution was the same as
what Matthias implemented:  Generate a "permanant tree" of utility instances
(handles) that reference the "real" stuff in the model, because QModelIndex
cannot resolve the type of object it references.  (This intermediate tree is
difficult to maintain/synchronize with model changes, and should be
completely unnecessary.)

I understand the issues regarding performance and the current trivial
constructor/copy-constructor for QModelIndex, so I can "settle" for an
*additional* int-or-enum value (in addition to the current void* inside the
QModelIndex).

Our use case, very painful:  Applications commonly use "native" models of
heterogeneous types, and we need to create QModelIndex references to those
model "nodes".  With the current "void*", we do not know the *type* to which
it points.  So, we create an "adapter utility" instance that *does* know the
type.  Unfortunately, these "adapter utility" instances come-and-go like the
QModelIndex comes-and-goes, and the current Qt MVC design provides no
possible mechanism to delete these "adapter utility" instances.  We need
value semantics inside the QModelIndex itself, or the ability to inspect the
"void*" from the QModelIndex to know *what type* it references.

(Because there is no possible mechanism for value semantics for these
"adapter utility" instances, we have a permanently maintained "utility
instance tree" that mirrors the *real* model, just as did Matthias.)

This is my *only* serious problem with the Qt model/view infrastructure, and
it is a (very) big problem.

Previous discussion at: <
http://lists.qt.nokia.com/pipermail/qt5-feedback/2011-May/000047.html>

--charley
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