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>

7 files changed, 10 insertions, 16 deletions

diff --git a/src/gui/text/qfont.h b/src/gui/text/qfont.h
index 4079e48dda..1ef0497d39 100644
--- a/src/gui/text/qfont.h
+++ b/src/gui/text/qfont.h
@@ -252,8 +252,7 @@ public:
     bool operator<(const QFont &) const;
     operator QVariant() const;
     bool isCopyOf(const QFont &) const;
-    inline QFont &operator=(QFont &&other) noexcept
-    { qSwap(d, other.d); qSwap(resolve_mask, other.resolve_mask);  return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QFont)
 
     QString key() const;
 
diff --git a/src/gui/text/qfontengine_p.h b/src/gui/text/qfontengine_p.h
index a81e877677..5d73e2cf8e 100644
--- a/src/gui/text/qfontengine_p.h
+++ b/src/gui/text/qfontengine_p.h
@@ -304,14 +304,11 @@ public:
         explicit Holder(void *p, qt_destroy_func_t d) : ptr(p), destroy_func(d) {}
         ~Holder() { if (ptr && destroy_func) destroy_func(ptr); }
         Holder(Holder &&other) noexcept
-            : ptr(other.ptr),
-              destroy_func(other.destroy_func)
+            : ptr(qExchange(other.ptr, nullptr)),
+              destroy_func(qExchange(other.destroy_func, nullptr))
         {
-            other.ptr = nullptr;
-            other.destroy_func = nullptr;
         }
-        Holder &operator=(Holder &&other) noexcept
-        { swap(other); return *this; }
+        QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(Holder)
 
         void swap(Holder &other) noexcept
         {
diff --git a/src/gui/text/qfontmetrics.h b/src/gui/text/qfontmetrics.h
index 097d40bf8d..a199e20373 100644
--- a/src/gui/text/qfontmetrics.h
+++ b/src/gui/text/qfontmetrics.h
@@ -60,8 +60,7 @@ public:
     ~QFontMetrics();
 
     QFontMetrics &operator=(const QFontMetrics &);
-    inline QFontMetrics &operator=(QFontMetrics &&other) noexcept
-    { qSwap(d, other.d); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QFontMetrics)
 
     void swap(QFontMetrics &other) noexcept
     { qSwap(d, other.d); }
@@ -131,8 +130,7 @@ public:
 
     QFontMetricsF &operator=(const QFontMetricsF &);
     QFontMetricsF &operator=(const QFontMetrics &);
-    inline QFontMetricsF &operator=(QFontMetricsF &&other) noexcept
-    { qSwap(d, other.d); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QFontMetricsF)
 
     void swap(QFontMetricsF &other) noexcept { qSwap(d, other.d); }
 
diff --git a/src/gui/text/qglyphrun.h b/src/gui/text/qglyphrun.h
index 0da8eddda8..0e9f0ce468 100644
--- a/src/gui/text/qglyphrun.h
+++ b/src/gui/text/qglyphrun.h
@@ -66,7 +66,7 @@ public:
 
     QGlyphRun();
     QGlyphRun(const QGlyphRun &other);
-    QGlyphRun &operator=(QGlyphRun &&other) noexcept { swap(other); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QGlyphRun)
     QGlyphRun &operator=(const QGlyphRun &other);
     ~QGlyphRun();
 
diff --git a/src/gui/text/qrawfont.h b/src/gui/text/qrawfont.h
index e7e12df040..51f7ba8044 100644
--- a/src/gui/text/qrawfont.h
+++ b/src/gui/text/qrawfont.h
@@ -79,7 +79,7 @@ public:
              qreal pixelSize,
              QFont::HintingPreference hintingPreference = QFont::PreferDefaultHinting);
     QRawFont(const QRawFont &other);
-    QRawFont &operator=(QRawFont &&other) noexcept { swap(other); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QRawFont)
     QRawFont &operator=(const QRawFont &other);
     ~QRawFont();
 
diff --git a/src/gui/text/qstatictext.h b/src/gui/text/qstatictext.h
index e8c94a6add..778861ed79 100644
--- a/src/gui/text/qstatictext.h
+++ b/src/gui/text/qstatictext.h
@@ -64,7 +64,7 @@ public:
     QStaticText();
     explicit QStaticText(const QString &text);
     QStaticText(const QStaticText &other);
-    QStaticText &operator=(QStaticText &&other) noexcept { swap(other); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QStaticText)
     QStaticText &operator=(const QStaticText &);
     ~QStaticText();
 
diff --git a/src/gui/text/qtextcursor.h b/src/gui/text/qtextcursor.h
index 7cad3cc5e8..59b32e0874 100644
--- a/src/gui/text/qtextcursor.h
+++ b/src/gui/text/qtextcursor.h
@@ -73,7 +73,7 @@ public:
     explicit QTextCursor(QTextFrame *frame);
     explicit QTextCursor(const QTextBlock &block);
     QTextCursor(const QTextCursor &cursor);
-    QTextCursor &operator=(QTextCursor &&other) noexcept { swap(other); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QTextCursor)
     QTextCursor &operator=(const QTextCursor &other);
     ~QTextCursor();