Add swapItemsAt() to QVector

This closes one compatibility gap with QList, to make
it easier to replace QList with QVector in Qt6.

Change-Id: I5655bc4cd2150a6f09a1ed68c0742f3b42ca47e4
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/corelib/tools/qvector.h

@@ -251,6 +251,13 @@ public:
     T value(int i) const;
     T value(int i, const T &defaultValue) const;
 
+    void swapItemsAt(int i, int j) {
+        Q_ASSERT_X(i >= 0 && i < size() && j >= 0 && j < size(),
+                    "QVector<T>::swap", "index out of range");
+        detach();
+        qSwap(d->begin()[i], d->begin()[j]);
+    }
+
     // STL compatibility
     typedef T value_type;
     typedef value_type* pointer;

src/corelib/tools/qvector.qdoc

@@ -288,6 +288,16 @@
     never fails.
 */
 
+/*! \fn template <typename T> void QVector<T>::swapItemsAt(int i, int j)
+    \since 5.14
+
+    Exchange the item at index position \a i with the item at index
+    position \a j. This function assumes that both \a i and \a j are
+    at least 0 but less than size(). To avoid failure, test that both
+    \a i and \a j are at least 0 and less than size().
+*/
+
+
 /*! \fn template <typename T> bool QVector<T>::operator==(const QVector<T> &other) const
 
     Returns \c true if \a other is equal to this vector; otherwise

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

@@ -331,6 +331,8 @@ private slots:
 
     void insertMove() const;
 
+    void swapItemsAt() const;
+
 private:
     template<typename T> void copyConstructor() const;
     template<typename T> void add() const;
@@ -2990,5 +2992,22 @@ void tst_QVector::insertMove() const
     QCOMPARE(Movable::counter.loadAcquire(), instancesCount);
 }
 
+void tst_QVector::swapItemsAt() const
+{
+    QVector<int> v;
+    v << 0 << 1 << 2 << 3;
+
+    v.swapItemsAt(0, 2);
+    QCOMPARE(v.at(0), 2);
+    QCOMPARE(v.at(2), 0);
+
+    auto copy = v;
+    copy.swapItemsAt(0, 2);
+    QCOMPARE(v.at(0), 2);
+    QCOMPARE(v.at(2), 0);
+    QCOMPARE(copy.at(0), 0);
+    QCOMPARE(copy.at(2), 2);
+}
+
 QTEST_MAIN(tst_QVector)
 #include "tst_qvector.moc"