Add a function to QMetaObject to check for inheritance

This is analogous to QObject::inherits() but only requires the
metaobjects rather than pointers to a QObject instances. This is needed
for type checking on the backend of Qt 3D where we do not have access
to QObject pointers.

Change-Id: I14d26c4cbb5cc3fbecb57725f2c14ee0ffda4a11
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

src/corelib/kernel/qmetaobject.cpp

@@ -325,6 +325,24 @@ const char *QMetaObject::className() const
     \sa className()
 */
 
+/*
+    Returns \c true if the class described by this QMetaObject inherits
+    the type described by \a metaObject; otherwise returns false.
+
+    A type is considered to inherit itself.
+
+    \since 5.7
+*/
+bool QMetaObject::inherits(const QMetaObject *metaObject) const Q_DECL_NOEXCEPT
+{
+    const QMetaObject *m = this;
+    do {
+        if (metaObject == m)
+            return true;
+    } while ((m = m->d.superdata));
+    return false;
+}
+
 /*!
     \internal
 

src/corelib/kernel/qobjectdefs.h

@@ -327,6 +327,7 @@ struct Q_CORE_EXPORT QMetaObject
     const char *className() const;
     const QMetaObject *superClass() const;
 
+    bool inherits(const QMetaObject *metaObject) const Q_DECL_NOEXCEPT;
     QObject *cast(QObject *obj) const;
     const QObject *cast(const QObject *obj) const;
 

tests/auto/corelib/kernel/qmetaobject/tst_qmetaobject.cpp

@@ -131,6 +131,22 @@ namespace MyNamespace {
         MyEnum m_enum;
         MyFlags m_flags;
     };
+
+    // Test inherits
+    class MyClassSubclass : public MyClass
+    {
+        Q_OBJECT
+    };
+
+    class MyClassSubclass2 : public MyClass2
+    {
+        Q_OBJECT
+    };
+
+    class MyClass2Subclass : public MyClass
+    {
+        Q_OBJECT
+    };
 }
 
 
@@ -222,6 +238,9 @@ private slots:
     void signalIndex();
     void enumDebugStream();
 
+    void inherits_data();
+    void inherits();
+
 signals:
     void value6Changed();
     void value7Changed(const QString &);
@@ -1425,5 +1444,34 @@ void tst_QMetaObject::enumDebugStream()
     qDebug() << f1 << f2;
 }
 
+void tst_QMetaObject::inherits_data()
+{
+    QTest::addColumn<const QMetaObject *>("derivedMetaObject");
+    QTest::addColumn<const QMetaObject *>("baseMetaObject");
+    QTest::addColumn<bool>("inheritsResult");
+
+    QTest::newRow("MyClass inherits QObject")
+        << &MyNamespace::MyClass::staticMetaObject << &QObject::staticMetaObject << true;
+    QTest::newRow("QObject inherits MyClass")
+        << &QObject::staticMetaObject << &MyNamespace::MyClass::staticMetaObject << false;
+    QTest::newRow("MyClass inherits MyClass")
+        << &MyNamespace::MyClass::staticMetaObject << &MyNamespace::MyClass::staticMetaObject << true;
+    QTest::newRow("MyClassSubclass inherits QObject")
+        << &MyNamespace::MyClassSubclass::staticMetaObject << &QObject::staticMetaObject << true;
+    QTest::newRow("MyClassSubclass2 inherits QObject")
+        << &MyNamespace::MyClassSubclass2::staticMetaObject << &QObject::staticMetaObject << true;
+    QTest::newRow("MyClassSubclass2 inherits MyClass2")
+        << &MyNamespace::MyClassSubclass2::staticMetaObject << &MyNamespace::MyClass2Subclass::staticMetaObject << false;
+}
+
+void tst_QMetaObject::inherits()
+{
+    QFETCH(const QMetaObject *, derivedMetaObject);
+    QFETCH(const QMetaObject *, baseMetaObject);
+    QFETCH(bool, inheritsResult);
+
+    QCOMPARE(derivedMetaObject->inherits(baseMetaObject), inheritsResult);
+}
+
 QTEST_MAIN(tst_QMetaObject)
 #include "tst_qmetaobject.moc"