Properly implement qobject_cast for const pointers.

Instead of using ugly const_cast in public headers and Q_UNUSED
(which expands to nothing in release builds), use a properly-const
method.

Reviewed-By: Bradley T. Hughes
(cherry picked from commit 0b0aa7603b27447a5abe55c55d0514bc56b35f9e)

3 files changed, 21 insertions, 6 deletions

diff --git a/src/corelib/kernel/qmetaobject.cpp b/src/corelib/kernel/qmetaobject.cpp
index a70772f..ceb2a9c 100644
--- a/src/corelib/kernel/qmetaobject.cpp
+++ b/src/corelib/kernel/qmetaobject.cpp
@@ -266,7 +266,25 @@ QObject *QMetaObject::cast(QObject *obj) const
         const QMetaObject *m = obj->metaObject();
         do {
             if (m == this)
-                return const_cast<QObject*>(obj);
+                return obj;
+        } while ((m = m->d.superdata));
+    }
+    return 0;
+}
+
+/*!
+    \internal
+
+    Returns \a obj if object \a obj inherits from this
+    meta-object; otherwise returns 0.
+*/
+const QObject *QMetaObject::cast(const QObject *obj) const
+{
+    if (obj) {
+        const QMetaObject *m = obj->metaObject();
+        do {
+            if (m == this)
+                return obj;
         } while ((m = m->d.superdata));
     }
     return 0;
diff --git a/src/corelib/kernel/qobject.h b/src/corelib/kernel/qobject.h
index b5db936..6621efb 100644
--- a/src/corelib/kernel/qobject.h
+++ b/src/corelib/kernel/qobject.h
@@ -378,14 +378,10 @@ inline T qobject_cast(QObject *object)
 template <class T>
 inline T qobject_cast(const QObject *object)
 {
-    // this will cause a compilation error if T is not const
-    register T ptr = static_cast<T>(object);
-    Q_UNUSED(ptr);
-
 #if !defined(QT_NO_QOBJECT_CHECK)
     reinterpret_cast<T>(0)->qt_check_for_QOBJECT_macro(*reinterpret_cast<T>(const_cast<QObject *>(object)));
 #endif
-    return static_cast<T>(const_cast<QObject *>(reinterpret_cast<T>(0)->staticMetaObject.cast(const_cast<QObject *>(object))));
+    return static_cast<T>(reinterpret_cast<T>(0)->staticMetaObject.cast(object));
 }
 
 
diff --git a/src/corelib/kernel/qobjectdefs.h b/src/corelib/kernel/qobjectdefs.h
index 996fce2..db46ba5 100644
--- a/src/corelib/kernel/qobjectdefs.h
+++ b/src/corelib/kernel/qobjectdefs.h
@@ -298,6 +298,7 @@ struct Q_CORE_EXPORT QMetaObject
     const QMetaObject *superClass() const;
 
     QObject *cast(QObject *obj) const;
+    const QObject *cast(const QObject *obj) const;
 
 #ifndef QT_NO_TRANSLATION
     // ### Qt 4: Merge overloads