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, 8 insertions, 0 deletions

diff --git a/src/corelib/kernel/qmetaobject.cpp b/src/corelib/kernel/qmetaobject.cpp
index c537293347..3bb3e33465 100644
--- a/src/corelib/kernel/qmetaobject.cpp
+++ b/src/corelib/kernel/qmetaobject.cpp
@@ -1598,6 +1598,14 @@ bool QMetaObject::invokeMethodImpl(QObject *object, QtPrivate::QSlotObjectBase *
 */
 
 /*!
+    \fn QMetaObject::Connection::swap(Connection &other)
+    \since 5.15
+
+    Swaps this Connection instance with \a other. This operation is very fast
+    and never fails.
+*/
+
+/*!
     \class QMetaMethod
     \inmodule QtCore