QVector: add an rvalue overload of push_back/append

This is low-hanging fruit, for two reasons:
1. The implementation is dead-simple (unlike, say, in QList).
2. It's completely transparent to the QVector user (unlike,
   say, emplace_back, which can only be used inside an ifdef).

Change-Id: Iaf750100cf61ced77aa452f0e4e3c4ec36b29639
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/doc/snippets/code/src_corelib_tools_qvector.cpp

@@ -84,14 +84,27 @@ for (int i = 0; i < 10; ++i)
 
 
 //! [7]
-QVector<QString> vector(0);
+QVector<QString> vector;
 vector.append("one");
 vector.append("two");
-vector.append("three");
+QString three = "three";
+vector.append(three);
 // vector: ["one", "two", "three"]
+// three: "three"
 //! [7]
 
 
+//! [move-append]
+QVector<QString> vector;
+vector.append("one");
+vector.append("two");
+QString three = "three";
+vector.append(std::move(three));
+// vector: ["one", "two", "three"]
+// three: ""
+//! [move-append]
+
+
 //! [8]
 QVector<QString> vector;
 vector.prepend("one");

src/corelib/tools/qvector.cpp

@@ -522,6 +522,16 @@
     \sa operator<<(), prepend(), insert()
 */
 
+/*!
+    \fn void QVector::append(T &&value)
+    \since 5.6
+
+    \overload
+
+    Example:
+    \snippet code/src_corelib_tools_qvector.cpp move-append
+*/
+
 /*! \fn void QVector::append(const QVector<T> &value)
 
     \overload
@@ -1026,6 +1036,11 @@
     to append(\a value).
 */
 
+/*! \fn void QVector::push_back(T &&value)
+    \since 5.6
+    \overload
+*/
+
 /*! \fn void QVector::push_front(const T &value)
 
     This function is provided for STL compatibility. It is equivalent

src/corelib/tools/qvector.h

@@ -129,6 +129,9 @@ public:
     T &operator[](int i);
     const T &operator[](int i) const;
     void append(const T &t);
+#ifdef Q_COMPILER_RVALUE_REFS
+    void append(T &&t);
+#endif
     inline void append(const QVector<T> &l) { *this += l; }
     void prepend(const T &t);
     void insert(int i, const T &t);
@@ -247,6 +250,9 @@ public:
     typedef const_iterator ConstIterator;
     typedef int size_type;
     inline void push_back(const T &t) { append(t); }
+#ifdef Q_COMPILER_RVALUE_REFS
+    void push_back(T &&t) { append(std::move(t)); }
+#endif
     inline void push_front(const T &t) { prepend(t); }
     void pop_back() { removeLast(); }
     void pop_front() { removeFirst(); }
@@ -643,6 +649,22 @@ void QVector<T>::append(const T &t)
     ++d->size;
 }
 
+#ifdef Q_COMPILER_RVALUE_REFS
+template <typename T>
+void QVector<T>::append(T &&t)
+{
+    const bool isTooSmall = uint(d->size + 1) > d->alloc;
+    if (!isDetached() || isTooSmall) {
+        QArrayData::AllocationOptions opt(isTooSmall ? QArrayData::Grow : QArrayData::Default);
+        reallocData(d->size, isTooSmall ? d->size + 1 : d->alloc, opt);
+    }
+
+    new (d->end()) T(std::move(t));
+
+    ++d->size;
+}
+#endif
+
 template <typename T>
 void QVector<T>::removeLast()
 {

tests/auto/corelib/tools/qvector/qvector.pro

@@ -1,4 +1,5 @@
 CONFIG += testcase parallel_test
+contains(QT_CONFIG, c++11):CONFIG += c++11
 TARGET = tst_qvector
 QT = core testlib
 SOURCES = $$PWD/tst_qvector.cpp

tests/auto/corelib/tools/qvector/tst_qvector.cpp

@@ -190,6 +190,7 @@ private slots:
     void appendInt() const;
     void appendMovable() const;
     void appendCustom() const;
+    void appendRvalue() const;
     void at() const;
     void capacityInt() const;
     void capacityMovable() const;
@@ -638,6 +639,21 @@ void tst_QVector::appendCustom() const
     QCOMPARE(instancesCount, Custom::counter.loadAcquire());
 }
 
+void tst_QVector::appendRvalue() const
+{
+#ifdef Q_COMPILER_RVALUE_REFS
+    QVector<QString> v;
+    v.append("hello");
+    QString world = "world";
+    v.append(std::move(world));
+    QVERIFY(world.isEmpty());
+    QCOMPARE(v.front(), QString("hello"));
+    QCOMPARE(v.back(),  QString("world"));
+#else
+    QSKIP("This test requires that C++11 move semantics support is enabled in the compiler");
+#endif
+}
+
 void tst_QVector::at() const
 {
     QVector<QString> myvec;