Introduce Q{Json,Cbor}ValueConstRef

I screwed up when I wrote QCborValueRef by not having the ConstRef
type. The code worked, but it wasn't const-correct, allowing you to
write:

  const QCborArray &arr = something();
  *arr.begin() = QCborArray();

This mistake was brought over to QJsonValue in Qt 6.0, so it has to be
fixed for QJsonValue too. The actual fixes are in the next couple of
commits.

This change is believed to be binary-compatible: the Q{Json,Cbor}ValueRef
classes continue to have the exact same size, except that they're now
empty and have a new base class. They weren't trivial before this commit
doesn't change that.

[ChangeLog][Potentially Source-Incompatible Changes] The iterator
classes for Qt's JSON and CBOR containers (array and map/object) had a
const correctness issue which allowed a const_iterator to mutate the
container being iterated on, even if that container was itself const. Qt
6.4 has a fix for this, but will cause compilation issues where
QCborValueRef and QJsonValueRef were used where the correctness could be
violated. To keep code compiling with both 6.3 and 6.4, either change to
non-const iteration or replace the QxxxValueRef with a const QxxxValue
reference. This change is binary-compatible.

Change-Id: I5e52dc5b093c43a3b678fffd16b6063333765ae0
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

1 files changed, 14 insertions, 2 deletions

diff --git a/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp b/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp
index 4fd4255b7d..fcbe739a39 100644
--- a/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp
+++ b/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp
@@ -135,6 +135,8 @@ private slots:
 
     void cborValueRef_data();
     void cborValueRef();
+    void cborValueConstRef_data() { cborValueRef_data(); }
+    void cborValueConstRef();
     void cborValueRefMutatingArray_data() { cborValueRef_data(); }
     void cborValueRefMutatingArray();
     void cborValueRefMutatingMapIntKey_data() { cborValueRef_data(); }
@@ -2637,7 +2639,7 @@ void tst_QCborValue::cborValueRef_data()
     QTest::newRow("Tagged") << QCborValue::Tag << QCborValue(QCborKnownTags::Base64, QByteArray());
 }
 
-void tst_QCborValue::cborValueRef()
+template <typename ValueRef> static void cborValueRef_template()
 {
     const QCborArray otherArray = {2};
     const QCborMap otherMap = { { 2, 21 } };
@@ -2648,7 +2650,7 @@ void tst_QCborValue::cborValueRef()
 
     QFETCH(QCborValue, v);
     QCborArray a = { v };
-    const QCborValueRef ref = a[0];
+    const ValueRef ref = a[0];
 
     QCOMPARE(ref, v);
     QVERIFY(ref.compare(v) == 0);
@@ -2717,6 +2719,16 @@ void tst_QCborValue::cborValueRef()
     QCOMPARE(ref.toDiagnosticNotation(), v.toDiagnosticNotation());
 }
 
+void tst_QCborValue::cborValueRef()
+{
+    cborValueRef_template<QCborValueRef>();
+}
+
+void tst_QCborValue::cborValueConstRef()
+{
+    cborValueRef_template<QCborValueConstRef>();
+}
+
 void tst_QCborValue::cborValueRefMutatingArray()
 {
     // complements arrayMutation()