Give QPixmap a move constructor

It already has a move-assignment operator, and implements swapping.

This requires a specialization of the QExplicitlySharedDataPointer
destructor to be forward declared, and an implementation that is
identical to the default version. Otherwise we would need
QPlatformPixmap to be fully defined in the public QPixmap header.

Change-Id: I2651bbc29a7083a93e3b3ad671d3aeea659b7d5a
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/gui/image/qpixmap.cpp

@@ -224,6 +224,15 @@ QPixmap::QPixmap(const QPixmap &pixmap)
     }
 }
 
+/*! \fn QPixmap::QPixmap(QPixmap &&other)
+    Move-constructs a QPixmap instance from \a other.
+
+    \sa swap() operator=(QPixmap&&)
+*/
+template<>
+QExplicitlySharedDataPointer<QPlatformPixmap>::~QExplicitlySharedDataPointer()
+{ if (d && !d->ref.deref()) delete d; }
+
 /*!
     Constructs a pixmap from the given \a xpm data, which must be a
     valid XPM image.

src/gui/image/qpixmap.h

@@ -57,6 +57,7 @@ class QImageReader;
 class QColor;
 class QVariant;
 class QPlatformPixmap;
+template<> Q_GUI_EXPORT QExplicitlySharedDataPointer<QPlatformPixmap>::~QExplicitlySharedDataPointer();
 
 class Q_GUI_EXPORT QPixmap : public QPaintDevice
 {
@@ -70,6 +71,7 @@ public:
     explicit QPixmap(const char * const xpm[]);
 #endif
     QPixmap(const QPixmap &);
+    QPixmap(QPixmap &&other) noexcept : QPaintDevice(), data(std::move(other.data)) {}
     ~QPixmap();
 
     QPixmap &operator=(const QPixmap &);

tests/auto/gui/image/qpixmap/tst_qpixmap.cpp

@@ -118,6 +118,7 @@ private slots:
     void refUnref();
 
     void copy();
+    void move();
     void deepCopyPreservesDpr();
     void dprPassthrough();
     void depthOfNullObjects();
@@ -1142,6 +1143,20 @@ void tst_QPixmap::copy()
     QCOMPARE(trans, transCopy);
 }
 
+void tst_QPixmap::move()
+{
+    QPixmap moveFrom(32, 32);
+
+    QPixmap moveAssigned;
+    moveAssigned = std::move(moveFrom);
+    QVERIFY(!moveAssigned.isNull());
+    QVERIFY(moveFrom.isNull());
+
+    QPixmap moveConstructed(std::move(moveAssigned));
+    QVERIFY(moveAssigned.isNull());
+    QVERIFY(!moveConstructed.isNull());
+}
+
 // QTBUG-58653: Force a deep copy of a pixmap by
 // having a QPainter and check whether DevicePixelRatio is preserved
 void tst_QPixmap::deepCopyPreservesDpr()