3 files changed, 103 insertions, 0 deletions

diff --git a/src/corelib/tools/qsharedpointer.cpp b/src/corelib/tools/qsharedpointer.cpp
index 58a9a021d0..180f45eb56 100644
--- a/src/corelib/tools/qsharedpointer.cpp
+++ b/src/corelib/tools/qsharedpointer.cpp
@@ -372,6 +372,43 @@
 */
 
 /*!
+    \class QEnableSharedFromThis
+    \inmodule QtCore
+    \brief A base class that allows to obtain a QSharedPointer for an object already managed by a shared pointer
+    \since 5.4
+
+    You can inherit this class when you need to create a QSharedPointer
+    from any instance of a class -- for instance, from within the
+    object itself. The key point is that the "obvious" technique of
+    just returning QSharedPointer<T>(this) can not be used, because
+    this winds up creating multiple distinct QSharedPointer objects
+    with separate reference counts. For this reason you must never
+    create more than one QSharedPointer from the same raw pointer.
+
+    QEnableSharedFromThis defines two member functions called
+    sharedFromThis() that return a QSharedPointer<T> and
+    QSharedPointer<const T>, depending on constness, to \c this:
+
+    \code
+    class Y: public QEnableSharedFromThis<Y>
+    {
+    public:
+        QSharedPointer<Y> f()
+        {
+            return sharedFromThis();
+        }
+    };
+
+    int main()
+    {
+        QSharedPointer<Y> p(new Y());
+        QSharedPointer<Y> y = p->f();
+        Q_ASSERT(p == y); // p and q must share ownership
+    }
+    \endcode
+*/
+
+/*!
     \fn QSharedPointer::QSharedPointer()
 
     Creates a QSharedPointer that points to null (0).
@@ -924,6 +961,23 @@
 */
 
 /*!
+    \fn QSharedPointer<T> QEnableSharedFromThis::sharedFromThis()
+    \since 5.4
+
+    If \c this (that is, the subclass instance invoking this method) is being
+    managed by a QSharedPointer, returns a shared pointer instance pointing to
+    \c this; otherwise returns a QSharedPointer holding a null pointer.
+*/
+
+/*!
+    \fn QSharedPointer<const T> QEnableSharedFromThis::sharedFromThis() const
+    \overload
+    \since 5.4
+
+    Const overload of sharedFromThis().
+*/
+
+/*!
     \fn bool operator==(const QSharedPointer<T> &ptr1, const QSharedPointer<X> &ptr2)
     \relates QSharedPointer
 
diff --git a/src/corelib/tools/qsharedpointer.h b/src/corelib/tools/qsharedpointer.h
index d9de48b7f4..431868ee08 100644
--- a/src/corelib/tools/qsharedpointer.h
+++ b/src/corelib/tools/qsharedpointer.h
@@ -131,6 +131,14 @@ public:
     QSharedPointer<T> lock() const;
 };
 
+template <class T>
+class QEnableSharedFromThis
+{
+public:
+    QSharedPointer<T> sharedFromThis();
+    QSharedPointer<const T> sharedFromThis() const;
+};
+
 template<class T, class X> bool operator==(const QSharedPointer<T> &ptr1, const QSharedPointer<X> &ptr2);
 template<class T, class X> bool operator!=(const QSharedPointer<T> &ptr1, const QSharedPointer<X> &ptr2);
 template<class T, class X> bool operator==(const QSharedPointer<T> &ptr1, const X *ptr2);
diff --git a/src/corelib/tools/qsharedpointer_impl.h b/src/corelib/tools/qsharedpointer_impl.h
index 7dafa4981c..fe79aa1db1 100644
--- a/src/corelib/tools/qsharedpointer_impl.h
+++ b/src/corelib/tools/qsharedpointer_impl.h
@@ -91,6 +91,7 @@ template<typename T> inline void qt_sharedpointer_cast_check(T *) { }
 //
 template <class T> class QWeakPointer;
 template <class T> class QSharedPointer;
+template <class T> class QEnableSharedFromThis;
 
 class QVariant;
 
@@ -479,6 +480,14 @@ private:
             delete d;
     }
 
+    template <class X>
+    inline void enableSharedFromThis(const QEnableSharedFromThis<X> *ptr)
+    {
+        ptr->initializeFromSharedPointer(*this);
+    }
+
+    inline void enableSharedFromThis(...) {}
+
     template <typename Deleter>
     inline void internalConstruct(T *ptr, Deleter deleter)
     {
@@ -499,6 +508,7 @@ private:
         internalSafetyCheckAdd(d, ptr);
 #endif
         d->setQObjectShared(ptr, true);
+        enableSharedFromThis(ptr);
     }
 
     template <class X>
@@ -707,6 +717,37 @@ public:
     T *value;
 };
 
+template <class T>
+class QEnableSharedFromThis
+{
+protected:
+#ifdef Q_COMPILER_DEFAULT_MEMBERS
+    QEnableSharedFromThis() = default;
+#else
+    Q_DECL_CONSTEXPR QEnableSharedFromThis() {}
+#endif
+    QEnableSharedFromThis(const QEnableSharedFromThis &) {}
+    QEnableSharedFromThis &operator=(const QEnableSharedFromThis &) { return *this; }
+
+public:
+    inline QSharedPointer<T> sharedFromThis() { return QSharedPointer<T>(weakPointer); }
+    inline QSharedPointer<const T> sharedFromThis() const { return QSharedPointer<const T>(weakPointer); }
+
+#ifndef Q_NO_TEMPLATE_FRIENDS
+private:
+    template <class X> friend class QSharedPointer;
+#else
+public:
+#endif
+    template <class X>
+    inline void initializeFromSharedPointer(const QSharedPointer<X> &ptr) const
+    {
+        weakPointer = ptr;
+    }
+
+    mutable QWeakPointer<T> weakPointer;
+};
+
 //
 // operator== and operator!=
 //