Add qobject_cast operators for std::shared_ptr

Mimicking what we currently have for QSharedPointer, but also adding
* snake_case version (matching the ones in std)
* rvalue-overloaded versions (matching the C++2a overloads).

[ChangeLog][QtCore][QSharedPointer] Overloads of
qSharedPointerObjectCast have been added to work on std::shared_ptr.

Change-Id: I26ddffd82b000bf876e7c141fdce86a7b8c1d75a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qsharedpointer.cpp

@@ -1299,6 +1299,57 @@
     \sa QSharedPointer::objectCast(), qSharedPointerCast(), qSharedPointerConstCast()
 */
 
+/*!
+    \fn template <class X, class T> std::shared_ptr<X> qSharedPointerObjectCast(const std::shared_ptr<T> &src)
+    \relates QSharedPointer
+    \since 5.14
+
+    Returns a shared pointer to the pointer held by \a src, using a
+    \l qobject_cast() to type \tt X to obtain an internal pointer of the
+    appropriate type. If the \tt qobject_cast fails, the object
+    returned will be null.
+
+    Note that \tt X must have the same cv-qualifiers (\tt const and
+    \tt volatile) that \tt T has, or the code will fail to
+    compile. Use const_pointer_cast to cast away the constness.
+*/
+
+/*!
+    \fn template <class X, class T> std::shared_ptr<X> qobject_pointer_cast(const std::shared_ptr<T> &src)
+    \relates QSharedPointer
+    \since 5.14
+
+    Same as qSharedPointerObjectCast(). This function is provided for STL
+    compatibility.
+*/
+
+/*!
+    \fn template <class X, class T> std::shared_ptr<X> qSharedPointerObjectCast(std::shared_ptr<T> &&src)
+    \relates QSharedPointer
+    \since 5.14
+
+    Returns a shared pointer to the pointer held by \a src, using a
+    \l qobject_cast() to type \tt X to obtain an internal pointer of the
+    appropriate type.
+
+    If the \tt qobject_cast succeeds, the function will return a valid shared
+    pointer, and \a src is reset to null. If the \tt qobject_cast fails, the
+    object returned will be null, and \a src will not be modified.
+
+    Note that \tt X must have the same cv-qualifiers (\tt const and
+    \tt volatile) that \tt T has, or the code will fail to
+    compile. Use const_pointer_cast to cast away the constness.
+*/
+
+/*!
+    \fn template <class X, class T> std::shared_ptr<X> qobject_pointer_cast(std::shared_ptr<T> &&src)
+    \relates QSharedPointer
+    \since 5.14
+
+    Same as qSharedPointerObjectCast(). This function is provided for STL
+    compatibility.
+*/
+
 /*!
     \fn template <class X> template <class T> QSharedPointer<X> qSharedPointerObjectCast(const QWeakPointer<T> &src)
     \relates QSharedPointer

src/corelib/tools/qsharedpointer_impl.h

@@ -67,6 +67,8 @@ QT_END_NAMESPACE
 #endif
 #include <QtCore/qhashfunctions.h>
 
+#include <memory>
+
 QT_BEGIN_NAMESPACE
 
 
@@ -996,6 +998,46 @@ qSharedPointerFromVariant(const QVariant &variant)
     return qSharedPointerObjectCast<T>(QtSharedPointer::sharedPointerFromVariant_internal(variant));
 }
 
+// std::shared_ptr helpers
+
+template <typename X, class T>
+std::shared_ptr<X> qobject_pointer_cast(const std::shared_ptr<T> &src)
+{
+    using element_type = typename std::shared_ptr<X>::element_type;
+    return std::shared_ptr<X>(src, qobject_cast<element_type *>(src.get()));
+}
+
+template <typename X, class T>
+std::shared_ptr<X> qobject_pointer_cast(std::shared_ptr<T> &&src)
+{
+    using element_type = typename std::shared_ptr<X>::element_type;
+    auto castResult = qobject_cast<element_type *>(src.get());
+    if (castResult) {
+        auto result = std::shared_ptr<X>(std::move(src), castResult);
+#if __cplusplus <= 201703L
+        // C++2a's move aliasing constructor will leave src empty.
+        // Before C++2a we don't really know if the compiler has support for it.
+        // The move aliasing constructor is the resolution for LWG2996,
+        // which does not impose a feature-testing macro. So: clear src.
+        src.reset();
+#endif
+        return result;
+    }
+    return std::shared_ptr<X>();
+}
+
+template <typename X, class T>
+std::shared_ptr<X> qSharedPointerObjectCast(const std::shared_ptr<T> &src)
+{
+    return qobject_pointer_cast<X>(src);
+}
+
+template <typename X, class T>
+std::shared_ptr<X> qSharedPointerObjectCast(std::shared_ptr<T> &&src)
+{
+    return qobject_pointer_cast<X>(std::move(src));
+}
+
 #endif
 
 template<typename T> Q_DECLARE_TYPEINFO_BODY(QWeakPointer<T>, Q_MOVABLE_TYPE);

tests/auto/corelib/tools/qsharedpointer/tst_qsharedpointer.cpp

@@ -78,6 +78,7 @@ private slots:
     void sharedPointerFromQObjectWithWeak();
     void weakQObjectFromSharedPointer();
     void objectCast();
+    void objectCastStdSharedPtr();
     void differentPointers();
     void virtualBaseDifferentPointers();
 #ifndef QTEST_NO_RTTI
@@ -1113,6 +1114,60 @@ void tst_QSharedPointer::objectCast()
     safetyCheck();
 }
 
+
+void tst_QSharedPointer::objectCastStdSharedPtr()
+{
+    {
+        OtherObject *data = new OtherObject;
+        std::shared_ptr<QObject> baseptr = std::shared_ptr<QObject>(data);
+        QVERIFY(baseptr.get() == data);
+
+        // perform successful object cast
+        std::shared_ptr<OtherObject> ptr = qobject_pointer_cast<OtherObject>(baseptr);
+        QVERIFY(ptr.get());
+        QVERIFY(ptr.get() == data);
+
+        QVERIFY(baseptr.get() == data);
+    }
+
+    {
+        OtherObject *data = new OtherObject;
+        std::shared_ptr<QObject> baseptr = std::shared_ptr<QObject>(data);
+        QVERIFY(baseptr.get() == data);
+
+        // perform successful object cast
+        std::shared_ptr<OtherObject> ptr = qobject_pointer_cast<OtherObject>(std::move(baseptr));
+        QVERIFY(ptr.get());
+        QVERIFY(ptr.get() == data);
+
+        QVERIFY(!baseptr.get());
+    }
+
+    {
+        QObject *data = new QObject;
+        std::shared_ptr<QObject> baseptr = std::shared_ptr<QObject>(data);
+        QVERIFY(baseptr.get() == data);
+
+        // perform unsuccessful object cast
+        std::shared_ptr<OtherObject> ptr = qobject_pointer_cast<OtherObject>(baseptr);
+        QVERIFY(!ptr.get());
+
+        QVERIFY(baseptr.get() == data);
+    }
+
+    {
+        QObject *data = new QObject;
+        std::shared_ptr<QObject> baseptr = std::shared_ptr<QObject>(data);
+        QVERIFY(baseptr.get() == data);
+
+        // perform unsuccessful object cast
+        std::shared_ptr<OtherObject> ptr = qobject_pointer_cast<OtherObject>(std::move(baseptr));
+        QVERIFY(!ptr.get());
+
+        QVERIFY(baseptr.get() == data);
+    }
+}
+
 void tst_QSharedPointer::differentPointers()
 {
     {