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>

3 files changed, 73 insertions, 0 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.