QModelIndex/QAIM: improve const correctness

QAIM::createIndex() takes a non-const void*. It's
typically called from inside QAIM::index(), which is a
const member function. If the data storage wrapped
by the model is const correct, it means that we have
to drop constness somewhere before calling createIndex().

To support this: change createIndex() to take a
const void * instead. This is painless.

Accessing the pointer is a bit more troubling, because
the accessor (QModelIndex::internalPointer()) returns
void *. (Effectively, now it does a const_cast...).
To avoid a massive source break, I've left it alone,
and instead added another function to retrieve a
const void *.

Read-only models can now be fully (deep) const correct.

[ChangeLog][QtCore][QAbstractItemModel] The
createIndex() function now takes a const void *, rather
than a void *.

[ChangeLog][QtCore][QModelIndex] Added the
constInternalPointer() function, to retrieve the
internal pointer as a pointer-to-const.

Change-Id: I108912b6814fcd5fe0c5cb7db6c721ba51e83de0
Reviewed-by: Samuel Gaist <samuel.gaist@idiap.ch>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/itemmodels/qabstractitemmodel.cpp

@@ -351,6 +351,22 @@ void *QPersistentModelIndex::internalPointer() const
     return nullptr;
 }
 
+/*!
+    \fn const void *QPersistentModelIndex::constInternalPointer() const
+    \since 6.0
+    \internal
+
+    Returns a \c{const void} \c{*} pointer used by the model to
+    associate the index with the internal data structure.
+*/
+
+const void *QPersistentModelIndex::constInternalPointer() const
+{
+    if (d)
+        return d->index.constInternalPointer();
+    return nullptr;
+}
+
 /*!
     \fn quintptr QPersistentModelIndex::internalId() const
 
@@ -1064,6 +1080,15 @@ void QAbstractItemModel::resetInternalData()
     \sa QAbstractItemModel::createIndex()
 */
 
+/*!
+    \fn const void *QModelIndex::constInternalPointer() const
+
+    Returns a \c{const void} \c{*} pointer used by the model to associate
+    the index with the internal data structure.
+
+    \sa QAbstractItemModel::createIndex()
+*/
+
 /*!
     \fn quintptr QModelIndex::internalId() const
 
@@ -2589,7 +2614,7 @@ bool QAbstractItemModel::setHeaderData(int section, Qt::Orientation orientation,
 }
 
 /*!
-    \fn QModelIndex QAbstractItemModel::createIndex(int row, int column, void *ptr) const
+    \fn QModelIndex QAbstractItemModel::createIndex(int row, int column, const void *ptr) const
 
     Creates a model index for the given \a row and \a column with the internal
     pointer \a ptr.

src/corelib/itemmodels/qabstractitemmodel.h

@@ -63,6 +63,7 @@ public:
     Q_DECL_CONSTEXPR inline int column() const noexcept { return c; }
     Q_DECL_CONSTEXPR inline quintptr internalId() const noexcept { return i; }
     inline void *internalPointer() const noexcept { return reinterpret_cast<void*>(i); }
+    inline const void *constInternalPointer() const noexcept { return reinterpret_cast<const void *>(i); }
     inline QModelIndex parent() const;
     inline QModelIndex sibling(int row, int column) const;
     inline QModelIndex siblingAtColumn(int column) const;
@@ -86,7 +87,7 @@ public:
                                                   || (i == other.i && std::less<const QAbstractItemModel *>()(m, other.m))))));
         }
 private:
-    inline QModelIndex(int arow, int acolumn, void *ptr, const QAbstractItemModel *amodel) noexcept
+    inline QModelIndex(int arow, int acolumn, const void *ptr, const QAbstractItemModel *amodel) noexcept
         : r(arow), c(acolumn), i(reinterpret_cast<quintptr>(ptr)), m(amodel) {}
     Q_DECL_CONSTEXPR inline QModelIndex(int arow, int acolumn, quintptr id, const QAbstractItemModel *amodel) noexcept
         : r(arow), c(acolumn), i(id), m(amodel) {}
@@ -129,6 +130,7 @@ public:
     int row() const;
     int column() const;
     void *internalPointer() const;
+    const void *constInternalPointer() const;
     quintptr internalId() const;
     QModelIndex parent() const;
     QModelIndex sibling(int row, int column) const;
@@ -307,7 +309,7 @@ protected Q_SLOTS:
 protected:
     QAbstractItemModel(QAbstractItemModelPrivate &dd, QObject *parent = nullptr);
 
-    inline QModelIndex createIndex(int row, int column, void *data = nullptr) const;
+    inline QModelIndex createIndex(int row, int column, const void *data = nullptr) const;
     inline QModelIndex createIndex(int row, int column, quintptr id) const;
 
     void encodeData(const QModelIndexList &indexes, QDataStream &stream) const;
@@ -378,7 +380,7 @@ inline bool QAbstractItemModel::moveRow(const QModelIndex &sourceParent, int sou
 inline bool QAbstractItemModel::moveColumn(const QModelIndex &sourceParent, int sourceColumn,
                                            const QModelIndex &destinationParent, int destinationChild)
 { return moveColumns(sourceParent, sourceColumn, 1, destinationParent, destinationChild); }
-inline QModelIndex QAbstractItemModel::createIndex(int arow, int acolumn, void *adata) const
+inline QModelIndex QAbstractItemModel::createIndex(int arow, int acolumn, const void *adata) const
 { return QModelIndex(arow, acolumn, adata, this); }
 inline QModelIndex QAbstractItemModel::createIndex(int arow, int acolumn, quintptr aid) const
 { return QModelIndex(arow, acolumn, aid, this); }