QVarLengthArray: add support for emplacement

Take the rvalue insert() function and turn it into the emplace()
function. Reformulate rvalue-insert using emplace(). Lvalue insert()
is using a different code path, so leave that alone. This way, we
don't need to go overboard with testing.

[ChangeLog][QtCore][QVarLengthArray] Added emplace(), emplace_back().

Change-Id: I3e1400820ae0dd1fe87fd4b4c518f7f40be39f8b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/tools/qvarlengtharray.h

@@ -288,7 +288,7 @@ public:
     const_reverse_iterator crbegin() const { return const_reverse_iterator(end()); }
     const_reverse_iterator crend() const { return const_reverse_iterator(begin()); }
     iterator insert(const_iterator before, qsizetype n, const T &x);
-    iterator insert(const_iterator before, T &&x);
+    iterator insert(const_iterator before, T &&x) { return emplace(before, std::move(x)); }
     inline iterator insert(const_iterator before, const T &x) { return insert(before, 1, x); }
     iterator erase(const_iterator begin, const_iterator end);
     inline iterator erase(const_iterator pos) { return erase(pos, pos + 1); }
@@ -303,6 +303,11 @@ public:
     inline T &back() { return last(); }
     inline const T &back() const { return last(); }
     void shrink_to_fit() { squeeze(); }
+    template <typename...Args>
+    iterator emplace(const_iterator pos, Args &&...args);
+    template <typename...Args>
+    T &emplace_back(Args &&...args) { return *emplace(cend(), std::forward<Args>(args)...); }
+
 
 #ifdef Q_QDOC
     template <typename T, qsizetype Prealloc1, qsizetype Prealloc2>
@@ -612,7 +617,8 @@ inline void QVarLengthArray<T, Prealloc>::replace(qsizetype i, const T &t)
 }
 
 template <class T, qsizetype Prealloc>
-Q_OUTOFLINE_TEMPLATE typename QVarLengthArray<T, Prealloc>::iterator QVarLengthArray<T, Prealloc>::insert(const_iterator before, T &&t)
+template <typename...Args>
+Q_OUTOFLINE_TEMPLATE auto QVarLengthArray<T, Prealloc>::emplace(const_iterator before, Args &&...args) -> iterator
 {
     Q_ASSERT_X(isValidIterator(before), "QVarLengthArray::insert", "The specified const_iterator argument 'before' is invalid");
 
@@ -627,14 +633,14 @@ Q_OUTOFLINE_TEMPLATE typename QVarLengthArray<T, Prealloc>::iterator QVarLengthA
             new (--j) T(std::move(*--i));
             while (i != b)
                 *--j = std::move(*--i);
-            *b = std::move(t);
+            *b = T(std::forward<Args>(args)...);
         } else {
-            new (b) T(std::move(t));
+            new (b) T(std::forward<Args>(args)...);
         }
     } else {
         T *b = ptr + offset;
         memmove(static_cast<void *>(b + 1), static_cast<const void *>(b), (s - offset) * sizeof(T));
-        new (b) T(std::move(t));
+        new (b) T(std::forward<Args>(args)...);
     }
     s += 1;
     return ptr + offset;

src/corelib/tools/qvarlengtharray.qdoc

@@ -753,6 +753,28 @@
     \a before. Returns an iterator pointing at the inserted item.
 */
 
+/*!
+    \fn template <class T, qsizetype Prealloc> QVarLengthArray<T, Prealloc>::emplace(const_iterator before, Args &&...args);
+
+    \since 6.3
+
+    Inserts an item in front of the item pointed to by the iterator
+    \a before, passing \a args to its constructor.
+
+    Returns an iterator pointing at the emplaced item.
+*/
+
+/*!
+    \fn template <class T, qsizetype Prealloc> QVarLengthArray<T, Prealloc>::emplace_back(Args &&...args);
+
+    \since 6.3
+
+    Inserts an item at the back of this QVarLengthArray, passing
+    \a args to its constructor.
+
+    Returns a reference to the emplaced item.
+*/
+
 /*! \fn template<class T, qsizetype Prealloc> QVarLengthArray<T, Prealloc>::iterator QVarLengthArray<T, Prealloc>::insert(const_iterator before, qsizetype count, const T &value)
 
     \since 4.8

tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp

@@ -70,6 +70,7 @@ class tst_QVarLengthArray : public QObject
 private slots:
     void append();
     void prepend();
+    void emplace();
     void move_int_1() { move_int<1>(); }
     void move_int_2() { move_int<2>(); }
     void move_int_3() { move_int<3>(); }
@@ -164,6 +165,24 @@ void tst_QVarLengthArray::prepend()
     QCOMPARE(v.front(), v.back());
 }
 
+void tst_QVarLengthArray::emplace()
+{
+    {
+        QVarLengthArray<QString, 2> strings;
+        strings.emplace_back();
+        QCOMPARE(strings.size(), 1);
+        QCOMPARE(strings.front().isNull(), true);
+        strings.emplace(strings.begin(), 42, u'x');
+        QCOMPARE(strings.size(), 2);
+        QCOMPARE(strings.back().isNull(), true);
+        QCOMPARE(strings.front(), QString(42, u'x'));
+        auto &r = strings.emplace_back(42, u'y');
+        QCOMPARE(&r, &strings.back());
+        QCOMPARE(strings.size(), 3);
+        QCOMPARE(strings.back(), QString(42, u'y'));
+    }
+}
+
 template <qsizetype N>
 void tst_QVarLengthArray::move_Tracker()
 {