Make the QMultiHash(const QHash &) constructor explicit

And add a QMultiHash::unite(const QHash &) method to avoid
a copy of the data when inserting a QHash into a multi hash.

Change-Id: I864aa9d2b9b7b2c367c3c4d140a2ce2f5408ae09
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qhash.cpp

@@ -2538,6 +2538,16 @@ size_t qHash(long double key, size_t seed) noexcept
     \sa insert()
 */
 
+
+/*! \fn template <class Key, class T> QMultiHash &QMultiHash<Key, T>::unite(const QHash<Key, T> &other)
+    \since 6.0
+
+    Inserts all the items in the \a other hash into this hash
+    and returns a reference to this hash.
+
+    \sa insert()
+*/
+
 /*! \fn template <class Key, class T> QList<Key> QMultiHash<Key, T>::uniqueKeys() const
     \since 5.13
 

src/corelib/tools/qhash.h

@@ -1219,7 +1219,7 @@ public:
         return *this;
     }
 
-    QMultiHash(const QHash<Key, T> &other)
+    explicit QMultiHash(const QHash<Key, T> &other)
         : QMultiHash(other.begin(), other.end())
     {}
     void swap(QMultiHash &other) noexcept { qSwap(d, other.d); qSwap(m_size, other.m_size); }
@@ -1808,6 +1808,13 @@ public:
         return *this;
     }
 
+    QMultiHash &unite(const QHash<Key, T> &other)
+    {
+        for (auto cit = other.cbegin(); cit != other.cend(); ++cit)
+            insert(cit.key(), *cit);
+        return *this;
+    }
+
     QPair<iterator, iterator> equal_range(const Key &key)
     {
         detach();

src/tools/moc/generator.cpp

@@ -465,7 +465,7 @@ void Generator::generateCode()
 // Build extra array
 //
     QList<QByteArray> extraList;
-    QMultiHash<QByteArray, QByteArray> knownExtraMetaObject = knownGadgets;
+    QMultiHash<QByteArray, QByteArray> knownExtraMetaObject(knownGadgets);
     knownExtraMetaObject.unite(knownQObjectClasses);
 
     for (int i = 0; i < cdef->propertyList.count(); ++i) {

tests/auto/corelib/tools/qhash/tst_qhash.cpp

@@ -1320,6 +1320,12 @@ void tst_QHash::qmultihash_specific()
     QVERIFY(map1.remove(42,5));
     QVERIFY(map2.remove(42,5));
     QVERIFY(map1 == map2);
+
+    QHash<int, int> hash;
+    hash.insert(-1, -1);
+    map2.unite(hash);
+    QCOMPARE(map2.count(), 6);
+    QCOMPARE(map2[-1], -1);
     }
 }