Long live QHVContainer!

Various places in Qt use arrays indexed by some function of
Qt::Orientation input. None document their dependence on the numerical
values of the Qt::Orientation enum, some waste space, none is
type-safe.

QHVContainer is a private container of two values, one for
Qt::Horizontal and one for Qt::Vertical. Its salient API is the
indexing operator, taking Qt::Orientation, thus making the class
type-safe.

Use it to port QGridLayoutItem and QAbstractScrollAreaPrivate.

Change-Id: I0d9f17431a5eb141bfb0763c83155710bb82a537
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/gui/util/qgridlayoutengine.cpp

@@ -507,24 +507,21 @@ void QGridLayoutRowData::dump(int indent) const
 
 QGridLayoutItem::QGridLayoutItem(int row, int column, int rowSpan, int columnSpan,
                                  Qt::Alignment alignment)
-    : q_alignment(alignment)
+    : q_firstRows{column, row},
+      q_rowSpans{columnSpan, rowSpan},
+      q_stretches{-1, -1},
+      q_alignment(alignment)
 {
-    q_firstRows[Hor] = column;
-    q_firstRows[Ver] = row;
-    q_rowSpans[Hor] = columnSpan;
-    q_rowSpans[Ver] = rowSpan;
-    q_stretches[Hor] = -1;
-    q_stretches[Ver] = -1;
 }
 
 int QGridLayoutItem::firstRow(Qt::Orientation orientation) const
 {
-    return q_firstRows[orientation == Qt::Vertical];
+    return q_firstRows[orientation];
 }
 
 int QGridLayoutItem::firstColumn(Qt::Orientation orientation) const
 {
-    return q_firstRows[orientation == Qt::Horizontal];
+    return q_firstRows.transposed()[orientation];
 }
 
 int QGridLayoutItem::lastRow(Qt::Orientation orientation) const
@@ -539,27 +536,27 @@ int QGridLayoutItem::lastColumn(Qt::Orientation orientation) const
 
 int QGridLayoutItem::rowSpan(Qt::Orientation orientation) const
 {
-    return q_rowSpans[orientation == Qt::Vertical];
+    return q_rowSpans[orientation];
 }
 
 int QGridLayoutItem::columnSpan(Qt::Orientation orientation) const
 {
-    return q_rowSpans[orientation == Qt::Horizontal];
+    return q_rowSpans.transposed()[orientation];
 }
 
 void QGridLayoutItem::setFirstRow(int row, Qt::Orientation orientation)
 {
-    q_firstRows[orientation == Qt::Vertical] = row;
+    q_firstRows[orientation] = row;
 }
 
 void QGridLayoutItem::setRowSpan(int rowSpan, Qt::Orientation orientation)
 {
-    q_rowSpans[orientation == Qt::Vertical] = rowSpan;
+    q_rowSpans[orientation] = rowSpan;
 }
 
 int QGridLayoutItem::stretchFactor(Qt::Orientation orientation) const
 {
-    int stretch = q_stretches[orientation == Qt::Vertical];
+    int stretch = q_stretches[orientation];
     if (stretch >= 0)
         return stretch;
 
@@ -577,7 +574,7 @@ int QGridLayoutItem::stretchFactor(Qt::Orientation orientation) const
 void QGridLayoutItem::setStretchFactor(int stretch, Qt::Orientation orientation)
 {
     Q_ASSERT(stretch >= 0); // ### deal with too big stretches
-    q_stretches[orientation == Qt::Vertical] = stretch;
+    q_stretches[orientation] = stretch;
 }
 
 QLayoutPolicy::ControlTypes QGridLayoutItem::controlTypes(LayoutSide /*side*/) const
@@ -695,9 +692,9 @@ QRectF QGridLayoutItem::geometryWithin(qreal x, qreal y, qreal width, qreal heig
 
 void QGridLayoutItem::transpose()
 {
-    qSwap(q_firstRows[Hor], q_firstRows[Ver]);
-    qSwap(q_rowSpans[Hor], q_rowSpans[Ver]);
-    qSwap(q_stretches[Hor], q_stretches[Ver]);
+    q_firstRows.transpose();
+    q_rowSpans.transpose();
+    q_stretches.transpose();
 }
 
 void QGridLayoutItem::insertOrRemoveRows(int row, int delta, Qt::Orientation orientation)
@@ -746,10 +743,10 @@ void QGridLayoutItem::dump(int indent) const
     qDebug("%*s (%d, %d) %d x %d", indent, "", firstRow(), firstColumn(),   //###
            rowSpan(), columnSpan());
 
-    if (q_stretches[Hor] >= 0)
-        qDebug("%*s Horizontal stretch: %d", indent, "", q_stretches[Hor]);
-    if (q_stretches[Ver] >= 0)
-        qDebug("%*s Vertical stretch: %d", indent, "", q_stretches[Ver]);
+    if (q_stretches[Qt::Horizontal] >= 0)
+        qDebug("%*s Horizontal stretch: %d", indent, "", q_stretches[Qt::Horizontal]);
+    if (q_stretches[Qt::Vertical] >= 0)
+        qDebug("%*s Vertical stretch: %d", indent, "", q_stretches[Qt::Vertical]);
     if (q_alignment != 0)
         qDebug("%*s Alignment: %x", indent, "", uint(q_alignment));
     qDebug("%*s Horizontal size policy: %x Vertical size policy: %x",

src/gui/util/qgridlayoutengine_p.h

@@ -102,6 +102,37 @@ enum {
     UnfeasibleConstraint    // not feasible, it be has some items with Vertical and others with Horizontal constraints
 };
 
+/*
+    Minimal container to store Qt::Orientation-discriminated values.
+
+    The salient feature is the indexing operator, which takes
+    Qt::Orientation (and assumes it's passed only Qt::Horizonal or Qt::Vertical).
+*/
+template <typename T>
+class QHVContainer {
+    T m_data[2];
+
+    Q_STATIC_ASSERT(Qt::Horizontal == 0x1);
+    Q_STATIC_ASSERT(Qt::Vertical == 0x2);
+    static constexpr int map(Qt::Orientation o) noexcept
+    {
+        return int(o) - 1;
+    }
+public:
+    constexpr QHVContainer(const T &h, const T &v)
+            noexcept(std::is_nothrow_copy_constructible_v<T>)
+        : m_data{h, v} {}
+    QHVContainer() = default;
+
+    constexpr T &operator[](Qt::Orientation o) noexcept { return m_data[map(o)]; }
+    constexpr const T &operator[](Qt::Orientation o) const noexcept { return m_data[map(o)]; }
+
+    constexpr void transpose() noexcept { qSwap(m_data[0], m_data[1]); }
+    constexpr QHVContainer transposed() const
+        noexcept(std::is_nothrow_copy_constructible_v<T>)
+    { return {m_data[1], m_data[0]}; }
+};
+
 template <typename T>
 class QLayoutParameter
 {
@@ -279,10 +310,10 @@ public:
                     Qt::Alignment alignment = { });
     virtual ~QGridLayoutItem() {}
 
-    inline int firstRow() const { return q_firstRows[Ver]; }
-    inline int firstColumn() const { return q_firstRows[Hor]; }
-    inline int rowSpan() const { return q_rowSpans[Ver]; }
-    inline int columnSpan() const { return q_rowSpans[Hor]; }
+    inline int firstRow() const { return q_firstRows[Qt::Vertical]; }
+    inline int firstColumn() const { return q_firstRows[Qt::Horizontal]; }
+    inline int rowSpan() const { return q_rowSpans[Qt::Vertical]; }
+    inline int columnSpan() const { return q_rowSpans[Qt::Horizontal]; }
     inline int lastRow() const { return firstRow() + rowSpan() - 1; }
     inline int lastColumn() const { return firstColumn() + columnSpan() - 1; }
 
@@ -329,9 +360,9 @@ public:
 #endif
 
 private:
-    int q_firstRows[NOrientations];
-    int q_rowSpans[NOrientations];
-    int q_stretches[NOrientations];
+    QHVContainer<int> q_firstRows;
+    QHVContainer<int> q_rowSpans;
+    QHVContainer<int> q_stretches;
     Qt::Alignment q_alignment;
 
 };

src/widgets/widgets/qabstractscrollarea_p.h

@@ -54,6 +54,7 @@
 #include <QtWidgets/private/qtwidgetsglobal_p.h>
 #include "private/qframe_p.h"
 #include "qabstractscrollarea.h"
+#include <QtGui/private/qgridlayoutengine_p.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -73,7 +74,7 @@ public:
 
     void replaceScrollBar(QScrollBar *scrollBar, Qt::Orientation orientation);
 
-    QAbstractScrollAreaScrollBarContainer *scrollBarContainers[int(Qt::Vertical) + 1];
+    QHVContainer<QAbstractScrollAreaScrollBarContainer *> scrollBarContainers;
     QScrollBar *hbar, *vbar;
     Qt::ScrollBarPolicy vbarpolicy, hbarpolicy;