Centralize the implementation of move assignment operators

At the moment we have two main strategies for dealing with move
assignment in Qt:

1) move-and-swap, used by "containers" (in the broad sense): containers,
but also smart pointers and similar classes that can hold user-defined
types;

2) pure swap, used by containers that hold only memory (e.g. QString,
QByteArray, ...) as well as most implicitly shared datatypes.

Given the fact that a move assignment operator's code is just
boilerplate (whether it's move-and-swap or pure swap), provide two
_strictly internal_ macros to help write them, and apply the macros
across corelib and gui, porting away from the hand-rolled
implementations.

The rule of thumb when porting to the new macros is:

* Try to stick to the existing code behavior, unless broken

* if changing, then follow this checklist:

  * if the class does not have a move constructor => pure swap
    (but consider ADDING a move constructor, if possible!)

  * if the class does have a move constructor, try to follow the
     criteria above, namely:

    * if the class holds only memory, pure swap;

    * if the class may hold anything else but memory (file handles,
      etc.), then move and swap.

Noteworthy details:

* some operators planned to be removed in Qt 6 were not ported;

* as drive-by, some move constructors were simplified to be using
qExchange(); others were outright broken and got fixed;

* some contained some more interesting code and were not touched.

Change-Id: Idaab3489247dcbabb6df3fa1e5286b69e1d372e9
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 2 insertions, 3 deletions

diff --git a/src/gui/painting/qregion.h b/src/gui/painting/qregion.h
index 8b991f2fb2..8f52185ed9 100644
--- a/src/gui/painting/qregion.h
+++ b/src/gui/painting/qregion.h
@@ -70,12 +70,11 @@ public:
     QRegion(const QPolygon &pa, Qt::FillRule fillRule = Qt::OddEvenFill);
     QRegion(const QRegion &region);
     QRegion(QRegion &&other) noexcept
-        : d(other.d) { other.d = const_cast<QRegionData*>(&shared_empty); }
+        : d(qExchange(other.d, const_cast<QRegionData*>(&shared_empty))) {}
     QRegion(const QBitmap &bitmap);
     ~QRegion();
     QRegion &operator=(const QRegion &);
-    inline QRegion &operator=(QRegion &&other) noexcept
-    { qSwap(d, other.d); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QRegion);
     inline void swap(QRegion &other) noexcept { qSwap(d, other.d); }
     bool isEmpty() const;
     bool isNull() const;