QMutexLocker: add move semantics

The class is similar to unique_lock in that it allows for unlocking
and relocking. Since the locked state is tracked by QMutexLocker itself,
it's trivial to make it movable.

[ChangeLog][QtCore][QMutexLocker] The class is now movable.

Change-Id: I534044f8024575e996c12efb2236761d493798a3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/thread/qmutex.cpp

@@ -485,6 +485,40 @@ void QRecursiveMutex::unlock() noexcept
     \sa QMutex::lock()
 */
 
+/*!
+    \fn template <typename Mutex> QMutexLocker<Mutex>::QMutexLocker(QMutexLocker &&other) noexcept
+    \since 6.4
+
+    Move-constructs a QMutexLocker from \a other. The mutex and the
+    state of \a other is transferred to the newly constructed instance.
+    After the move, \a other will no longer be managing its mutex.
+
+    \sa QMutex::lock()
+*/
+
+/*!
+    \fn template <typename Mutex> QMutexLocker<Mutex> &QMutexLocker<Mutex>::operator=(QMutexLocker &&other) noexcept
+    \since 6.4
+
+    Move-assigns \a other onto this QMutexLocker. If this QMutexLocker
+    was holding a locked mutex before the assignment, the mutex will be
+    unlocked. The mutex and the state of \a other is then transferred
+    to this QMutexLocker. After the move, \a other will no longer be
+    managing its mutex.
+
+    \sa QMutex::lock()
+*/
+
+/*!
+    \fn template <typename Mutex> void QMutexLocker<Mutex>::swap(QMutexLocker &other) noexcept
+    \since 6.4
+
+    Swaps the mutex and the state of this QMutexLocker with \a other.
+    This operation is very fast and never fails.
+
+    \sa QMutex::lock()
+*/
+
 /*!
     \fn template <typename Mutex> QMutexLocker<Mutex>::~QMutexLocker() noexcept
 

src/corelib/thread/qmutex.h

@@ -247,6 +247,13 @@ public:
         }
     }
 
+    inline QMutexLocker(QMutexLocker &&other) noexcept
+        : m_mutex(std::exchange(other.m_mutex, nullptr)),
+          m_isLocked(std::exchange(other.m_isLocked, false))
+    {}
+
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_MOVE_AND_SWAP(QMutexLocker)
+
     inline ~QMutexLocker()
     {
         unlock();
@@ -275,6 +282,12 @@ public:
         }
     }
 
+    inline void swap(QMutexLocker &other) noexcept
+    {
+        qt_ptr_swap(m_mutex, other.m_mutex);
+        std::swap(m_isLocked, other.m_isLocked);
+    }
+
     Mutex *mutex() const
     {
         return m_mutex;

tests/auto/corelib/thread/qmutexlocker/tst_qmutexlocker.cpp

@@ -67,6 +67,7 @@ private slots:
     void scopeTest();
     void unlockAndRelockTest();
     void lockerStateTest();
+    void moveSemantics();
 };
 
 void tst_QMutexLocker::scopeTest()
@@ -214,5 +215,71 @@ void tst_QMutexLocker::lockerStateTest()
     thread = nullptr;
 }
 
+void tst_QMutexLocker::moveSemantics()
+{
+    {
+        QMutexLocker<QMutex> locker(nullptr);
+        QVERIFY(!locker.isLocked());
+        QCOMPARE(locker.mutex(), nullptr);
+
+        QMutexLocker locker2(std::move(locker));
+        QVERIFY(!locker.isLocked());
+        QVERIFY(!locker2.isLocked());
+        QCOMPARE(locker.mutex(), nullptr);
+        QCOMPARE(locker2.mutex(), nullptr);
+    }
+
+    QMutex mutex;
+
+    {
+        QMutexLocker locker(&mutex);
+        QVERIFY(locker.isLocked());
+        QCOMPARE(locker.mutex(), &mutex);
+        QVERIFY(!mutex.tryLock());
+
+        QMutexLocker locker2(std::move(locker));
+        QVERIFY(!locker.isLocked());
+        QVERIFY(locker2.isLocked());
+        QCOMPARE(locker.mutex(), nullptr);
+        QCOMPARE(locker2.mutex(), &mutex);
+        QVERIFY(!mutex.tryLock());
+    }
+
+    QVERIFY(mutex.tryLock());
+    mutex.unlock();
+
+    {
+        QMutex mutex;
+        QMutexLocker locker(&mutex);
+        QVERIFY(locker.isLocked());
+        QCOMPARE(locker.mutex(), &mutex);
+        QVERIFY(!mutex.tryLock());
+
+        locker.unlock();
+        QVERIFY(!locker.isLocked());
+        QCOMPARE(locker.mutex(), &mutex);
+        QVERIFY(mutex.tryLock());
+        mutex.unlock();
+
+        QMutexLocker locker2(std::move(locker));
+        QVERIFY(!locker.isLocked());
+        QVERIFY(!locker2.isLocked());
+        QCOMPARE(locker.mutex(), nullptr);
+        QCOMPARE(locker2.mutex(), &mutex);
+        QVERIFY(mutex.tryLock());
+        mutex.unlock();
+
+        locker2.relock();
+        QVERIFY(!locker.isLocked());
+        QVERIFY(locker2.isLocked());
+        QCOMPARE(locker.mutex(), nullptr);
+        QCOMPARE(locker2.mutex(), &mutex);
+        QVERIFY(!mutex.tryLock());
+    }
+
+    QVERIFY(mutex.tryLock());
+    mutex.unlock();
+}
+
 QTEST_MAIN(tst_QMutexLocker)
 #include "tst_qmutexlocker.moc"