From 721116de309f1933fad788fe4272c0d5c12b9909 Mon Sep 17 00:00:00 2001
From: Ulf Hermann <ulf.hermann@qt.io>
Date: Thu, 10 Sep 2020 18:36:27 +0200
Subject: Add QVariantRef, QVariantPointer, QVariantConstPointer

Change-Id: Ia329265826ab2f76e77ac14bb8d8e415a1b41dd6
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
---
 src/corelib/kernel/qvariant.cpp | 109 ++++++++++++++++++++++++++++++++++++++++
 src/corelib/kernel/qvariant.h   |  49 ++++++++++++++++++
 2 files changed, 158 insertions(+)

diff --git a/src/corelib/kernel/qvariant.cpp b/src/corelib/kernel/qvariant.cpp
index bfeee901da..d0a548bc74 100644
--- a/src/corelib/kernel/qvariant.cpp
+++ b/src/corelib/kernel/qvariant.cpp
@@ -2958,4 +2958,113 @@ QAssociativeIterable::const_iterator QAssociativeIterable::const_iterator::opera
     return const_iterator(impl, new QAtomicInt(0));
 }
 
+/*!
+    \class QVariantRef
+    \since 6.0
+    \inmodule QtCore
+    \brief The QVariantRef acts as a non-const reference to a QVariant.
+
+    As the generic iterators don't actually instantiate a QVariant on each
+    step, they cannot return a reference to one from operator*(). QVariantRef
+    provides the same functionality as an actual reference to a QVariant would,
+    but is backed by a pointer given as template parameter. The template is
+    implemented for pointers of type QSequentialIterator and
+    QAssociativeIterator.
+*/
+
+/*!
+    \fn QVariantRef::QVariantRef(const Pointer *pointer)
+
+    Creates a QVariantRef from an \a pointer.
+ */
+
+/*!
+    \fn QVariantRef &QVariantRef::operator=(const QVariantRef &value)
+
+    Assigns a new \a value to the value pointed to by the pointer this
+    QVariantRef refers to.
+ */
+
+/*!
+    \fn QVariantRef &QVariantRef::operator=(QVariantRef &&value)
+
+    Assigns a new \a value to the value pointed to by the pointer this
+    QVariantRef refers to.
+*/
+
+/*!
+    \fn void swap(QVariantRef a, QVariantRef b)
+
+    Swaps the values pointed to by the pointers the QVariantRefs
+    \a a and \a b refer to.
+*/
+
+/*!
+    \class QVariantConstPointer
+    \since 6.0
+    \inmodule QtCore
+    \brief Emulated const pointer to QVariant based on a pointer
+
+    QVariantConstPointer wraps a QVariant and returns it from its operator*().
+    This makes it suitable as replacement for an actual const pointer. We cannot
+    return an actual const pointer from generic iterators as the iterators don't
+    hold an actual QVariant.
+*/
+
+/*!
+    Constructs a QVariantConstPointer from a \a variant.
+ */
+QVariantConstPointer::QVariantConstPointer(QVariant variant)
+    : m_variant(std::move(variant))
+{
+}
+
+/*!
+    Dereferences the QVariantConstPointer to retrieve its internal QVariant.
+ */
+QVariant QVariantConstPointer::operator*() const
+{
+    return m_variant;
+}
+
+/*!
+    Returns a const pointer to the QVariant, conforming to the
+    conventions for operator->().
+ */
+const QVariant *QVariantConstPointer::operator->() const
+{
+    return &m_variant;
+}
+
+/*!
+    \class QVariantPointer
+    \since 6.0
+    \inmodule QtCore
+    \brief Emulated pointer to QVariant based on a pointer
+
+    QVariantConstPointer wraps a pointer and returns QVariantRef to it from its
+    operator*(). This makes it suitable as replacement for an actual pointer. We
+    cannot return an actual pointer from generic iterators as the iterators don't
+    hold an actual QVariant.
+*/
+
+/*!
+    \fn QVariantPointer::QVariantPointer(const Pointer *pointer)
+
+    Constructs a QVariantPointer from the given \a pointer.
+ */
+
+/*!
+    \fn QVariantRef QVariantPointer::operator*() const
+
+    Dereferences the QVariantPointer to a QVariantRef.
+ */
+
+/*!
+    \fn Pointer QVariantPointer::operator->() const
+
+    Dereferences and returns the pointer. The pointer is expected to also
+    implement operator->().
+ */
+
 QT_END_NAMESPACE
diff --git a/src/corelib/kernel/qvariant.h b/src/corelib/kernel/qvariant.h
index ad766c9a8e..796dd80fd7 100644
--- a/src/corelib/kernel/qvariant.h
+++ b/src/corelib/kernel/qvariant.h
@@ -662,6 +662,55 @@ Q_CORE_EXPORT QDebug operator<<(QDebug, const QVariant &);
 Q_CORE_EXPORT QDebug operator<<(QDebug, const QVariant::Type);
 #endif
 
+template<typename Pointer>
+class Q_CORE_EXPORT QVariantRef
+{
+private:
+    const Pointer *m_pointer = nullptr;
+
+public:
+    explicit QVariantRef(const Pointer *reference) : m_pointer(reference) {}
+    QVariantRef(const QVariantRef &) = default;
+    QVariantRef(QVariantRef &&) = default;
+    ~QVariantRef() = default;
+
+    operator QVariant() const;
+    QVariantRef &operator=(const QVariant &value);
+    QVariantRef &operator=(const QVariantRef &value) { return operator=(QVariant(value)); }
+    QVariantRef &operator=(QVariantRef &&value) { return operator=(QVariant(value)); }
+
+    friend void swap(QVariantRef a,  QVariantRef b)
+    {
+        QVariant tmp = a;
+        a = b;
+        b = std::move(tmp);
+    }
+};
+
+class Q_CORE_EXPORT QVariantConstPointer
+{
+private:
+    QVariant m_variant;
+
+public:
+    explicit QVariantConstPointer(QVariant variant);
+
+    QVariant operator*() const;
+    const QVariant *operator->() const;
+};
+
+template<typename Pointer>
+class Q_CORE_EXPORT QVariantPointer
+{
+private:
+    const Pointer *m_pointer = nullptr;
+
+public:
+    explicit QVariantPointer(const Pointer *pointer) : m_pointer(pointer) {}
+    QVariantRef<Pointer> operator*() const { return QVariantRef<Pointer>(m_pointer); }
+    Pointer operator->() const { return *m_pointer; }
+};
+
 QT_END_NAMESPACE
 
 #endif // QVARIANT_H
-- 
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