QJniArray: implement operator-> for the iterator

The QJniArray doesn't store values, so at() always returns a temporary.
As we cannot hand out a pointer to a temporary, use a wrapper reference
struct that stores the value and implements operator->. It's all inline,
so we can move it out in the future to return mutable references, if we
ever want to enable write access to QJniArray elements.

Change-Id: I3962df6160db8c5b573d47ebb7975864f8ea7a8b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 3d5af4b912e60beb791f874c2dbfef5597b9aad7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qjniarray.h

@@ -25,6 +25,15 @@ template <typename T> class QJniArray;
 template <typename T>
 struct QJniArrayIterator
 {
+private:
+    // Since QJniArray doesn't hold values, we need a wrapper to be able to hand
+    // out a pointer to a value.
+    struct QJniArrayValueRef {
+        T ref;
+        const T *operator->() const { return &ref; }
+    };
+
+public:
     QJniArrayIterator() = default;
 
     constexpr QJniArrayIterator(const QJniArrayIterator &other) noexcept = default;
@@ -34,7 +43,7 @@ struct QJniArrayIterator
 
     using difference_type = jsize;
     using value_type = T;
-    using pointer = T *;
+    using pointer = QJniArrayValueRef;
     using reference = T; // difference to container requirements
     using const_reference = reference;
     using iterator_category = std::random_access_iterator_tag;
@@ -44,6 +53,11 @@ struct QJniArrayIterator
         return m_array->at(m_index);
     }
 
+    QJniArrayValueRef operator->() const
+    {
+        return {m_array->at(m_index)};
+    }
+
     const_reference operator[](difference_type n) const
     {
         return m_array->at(m_index + n);

tests/auto/corelib/kernel/qjniarray/tst_qjniarray.cpp

@@ -22,6 +22,7 @@ private slots:
     void operators();
     void ordering();
     void toContainer();
+    void pointerToValue();
 };
 
 using namespace QtJniTypes;
@@ -308,6 +309,15 @@ void tst_QJniArray::toContainer()
     QCOMPARE(charArray.toContainer<std::vector<jchar>>(), charVector);
 }
 
+void tst_QJniArray::pointerToValue()
+{
+    const QJniArray stringArray{u"one"_s, u"two"_s, u"three"_s};
+    auto it = std::find(stringArray.begin(), stringArray.end(), u"two"_s);
+    QCOMPARE_NE(it, stringArray.end());
+    QCOMPARE(it->size(), 3);
+    QCOMPARE((++it)->size(), 5);
+}
+
 QTEST_MAIN(tst_QJniArray)
 
 #include "tst_qjniarray.moc"