QVarLengthArray: add missing move special member functions

A QVLA is copyable, so it should be movable, too.

Added a helper function a la P1144's uninitialized_relocate_n to deal
with the QTypeInfoQuery stuff. This way, the code is re-usable
everywhere it's needed. The same cannot be said for QArrayDataOps,
which only a parent can love...

[ChangeLog][QtCore][QVarLengthArray] Added missing move constructor
and move-assignment operator.

Task-number: QTBUG-39111
Change-Id: If0dc2aa78eb29062d73dcd3dc4647ba345ae39e6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

4 files changed, 150 insertions, 15 deletions

diff --git a/src/corelib/tools/qcontainertools_impl.h b/src/corelib/tools/qcontainertools_impl.h
index 3a0c4381f1..b43c3bb1c4 100644
--- a/src/corelib/tools/qcontainertools_impl.h
+++ b/src/corelib/tools/qcontainertools_impl.h
@@ -47,12 +47,33 @@
 #define QCONTAINERTOOLS_IMPL_H
 
 #include <QtCore/qglobal.h>
+#include <QtCore/qtypeinfo.h>
+
 #include <iterator>
+#include <memory>
 
 QT_BEGIN_NAMESPACE
 
 namespace QtPrivate
 {
+
+template <typename T, typename N>
+void q_uninitialized_relocate_n(T* first, N n, T* out)
+{
+    if constexpr (QTypeInfoQuery<T>::isRelocatable) {
+        if (n != N(0)) { // even if N == 0, out == nullptr or first == nullptr are UB for memmove()
+            memmove(static_cast<void*>(out),
+                    static_cast<const void*>(first),
+                    n * sizeof(T));
+        }
+    } else {
+        std::uninitialized_move_n(first, n, out);
+        if constexpr (QTypeInfoQuery<T>::isComplex)
+            std::destroy_n(first, n);
+    }
+}
+
+
 template <typename Iterator>
 using IfIsInputIterator = typename std::enable_if<
     std::is_convertible<typename std::iterator_traits<Iterator>::iterator_category, std::input_iterator_tag>::value,
diff --git a/src/corelib/tools/qvarlengtharray.h b/src/corelib/tools/qvarlengtharray.h
index f90e455fe4..cea0e14d41 100644
--- a/src/corelib/tools/qvarlengtharray.h
+++ b/src/corelib/tools/qvarlengtharray.h
@@ -71,6 +71,26 @@ public:
         append(other.constData(), other.size());
     }
 
+    QVarLengthArray(QVarLengthArray &&other)
+            noexcept(std::is_nothrow_move_constructible_v<T>)
+        : a{other.a},
+          s{other.s},
+          ptr{other.ptr}
+    {
+        const auto otherInlineStorage = reinterpret_cast<T*>(other.array);
+        if (ptr == otherInlineStorage) {
+            // inline buffer - move into our inline buffer:
+            ptr = reinterpret_cast<T*>(array);
+            QtPrivate::q_uninitialized_relocate_n(otherInlineStorage, s, ptr);
+        } else {
+            // heap buffer - we just stole the memory
+        }
+        // reset other to internal storage:
+        other.a = Prealloc;
+        other.s = 0;
+        other.ptr = otherInlineStorage;
+    }
+
     QVarLengthArray(std::initializer_list<T> args)
         : QVarLengthArray(args.begin(), args.end())
     {
@@ -102,6 +122,27 @@ public:
         return *this;
     }
 
+    QVarLengthArray &operator=(QVarLengthArray &&other)
+        noexcept(std::is_nothrow_move_constructible_v<T>)
+    {
+        // we're only required to be self-move-assignment-safe
+        // when we're in the moved-from state (Hinnant criterion)
+        // the moved-from state is the empty state, so we're good with the clear() here:
+        clear();
+        Q_ASSERT(capacity() >= Prealloc);
+        const auto otherInlineStorage = reinterpret_cast<T*>(other.array);
+        if (other.ptr != otherInlineStorage) {
+            // heap storage: steal the external buffer, reset other to otherInlineStorage
+            a = std::exchange(other.a, Prealloc);
+            ptr = std::exchange(other.ptr, otherInlineStorage);
+        } else {
+            // inline storage: move into our storage (doesn't matter whether inline or external)
+            QtPrivate::q_uninitialized_relocate_n(other.ptr, other.s, ptr);
+        }
+        s = std::exchange(other.s, 0);
+        return *this;
+    }
+
     QVarLengthArray<T, Prealloc> &operator=(std::initializer_list<T> list)
     {
         resize(qsizetype(list.size()));
diff --git a/src/corelib/tools/qvarlengtharray.qdoc b/src/corelib/tools/qvarlengtharray.qdoc
index e43d6f152a..6371419ae5 100644
--- a/src/corelib/tools/qvarlengtharray.qdoc
+++ b/src/corelib/tools/qvarlengtharray.qdoc
@@ -404,6 +404,12 @@
     Assigns \a other to this array and returns a reference to this array.
  */
 
+/*! \fn template<class T, qsizetype Prealloc> QVarLengthArray<T, Prealloc> &QVarLengthArray<T, Prealloc>::operator=(QVarLengthArray<T, Prealloc> &&other)
+    Move-assigns \a other to this array and returns a reference to this array.
+    After the move, \a other is empty.
+    \since 6.0
+ */
+
 /*! \fn template<class T, qsizetype Prealloc> QVarLengthArray<T, Prealloc> &QVarLengthArray<T, Prealloc>::operator=(std::initializer_list<T> list)
     \since 5.5
 
@@ -417,6 +423,11 @@
     Constructs a copy of \a other.
  */
 
+/*! \fn template<class T, qsizetype Prealloc> QVarLengthArray<T, Prealloc>::QVarLengthArray(QVarLengthArray<T, Prealloc> &&other)
+    Move-constructs this variable-length array from \a other. After the move, \a other is empty.
+    \since 6.0
+ */
+
 /*! \fn template<class T, qsizetype Prealloc> const T &QVarLengthArray<T, Prealloc>::at(qsizetype i) const
 
     Returns a reference to the item at index position \a i.
diff --git a/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp b/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
index 74654a3862..1ce5f1e0e6 100644
--- a/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
+++ b/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
@@ -29,14 +29,55 @@
 #include <QtTest/QtTest>
 #include <qvarlengtharray.h>
 #include <qvariant.h>
+#include <qscopeguard.h>
 
 #include <memory>
 
+struct Tracker
+{
+    static int count;
+    Tracker() { ++count; }
+    Tracker(const Tracker &) { ++count; }
+    Tracker(Tracker &&) { ++count; }
+
+    Tracker &operator=(const Tracker &) = default;
+    Tracker &operator=(Tracker &&) = default;
+
+    ~Tracker() { --count; }
+
+};
+
+int Tracker::count = 0;
+
+template <typename T>
+class ValueTracker
+{
+    Tracker m_tracker;
+public:
+    ValueTracker() = default;
+    ValueTracker(T value) : value{std::move(value)} {}
+    T value;
+
+    friend bool operator==(const ValueTracker &lhs, const ValueTracker &rhs) noexcept
+    { return lhs.value == rhs.value; }
+    friend bool operator!=(const ValueTracker &lhs, const ValueTracker &rhs) noexcept
+    { return !operator==(lhs, rhs); }
+};
+
 class tst_QVarLengthArray : public QObject
 {
     Q_OBJECT
 private slots:
     void append();
+    void move_int_1() { move_int<1>(); }
+    void move_int_2() { move_int<2>(); }
+    void move_int_3() { move_int<3>(); }
+    void move_QString_1() { move_QString<1>(); }
+    void move_QString_2() { move_QString<2>(); }
+    void move_QString_3() { move_QString<3>(); }
+    void move_Tracker_1() { move_Tracker<1>(); }
+    void move_Tracker_2() { move_Tracker<2>(); }
+    void move_Tracker_3() { move_Tracker<3>(); }
     void removeLast();
     void oldTests();
     void appendCausingRealloc();
@@ -61,25 +102,18 @@ private slots:
     void implicitDefaultCtor();
 
 private:
+    template <qsizetype N, typename T>
+    void move(T t1, T t2);
+    template <qsizetype N>
+    void move_int() { move<N, int>(42, 24); }
+    template <qsizetype N>
+    void move_QString() { move<N, QString>("Hello", "World"); }
+    template <qsizetype N>
+    void move_Tracker();
     template<typename T>
     void initializeList();
 };
 
-struct Tracker
-{
-    static int count;
-    Tracker() { ++count; }
-    Tracker(const Tracker &) { ++count; }
-    Tracker(Tracker &&) { ++count; }
-
-    Tracker &operator=(const Tracker &) = default;
-    Tracker &operator=(Tracker &&) = default;
-
-    ~Tracker() { --count; }
-};
-
-int Tracker::count = 0;
-
 void tst_QVarLengthArray::append()
 {
     QVarLengthArray<QString, 2> v;
@@ -102,6 +136,34 @@ void tst_QVarLengthArray::append()
     v2.append(5);
 }
 
+template <qsizetype N>
+void tst_QVarLengthArray::move_Tracker()
+{
+    const auto reset = qScopeGuard([] { Tracker::count = 0; });
+    move<N, ValueTracker<int>>({24}, {24});
+    QCOMPARE(Tracker::count, 0);
+}
+
+template <qsizetype N, typename T>
+void tst_QVarLengthArray::move(T t1, T t2)
+{
+    {
+        QVarLengthArray<T, N> v;
+        v.append(t1);
+        v.append(t2);
+
+        auto moved = std::move(v);
+        QCOMPARE(moved.size(), 2);
+        QCOMPARE(moved[0], t1);
+        QCOMPARE(moved[1], t2);
+
+        v = std::move(moved);
+        QCOMPARE(v.size(), 2);
+        QCOMPARE(v[0], t1);
+        QCOMPARE(v[1], t2);
+    }
+}
+
 void tst_QVarLengthArray::removeLast()
 {
     {