QMetaObject::newInstance failes if type is in namespace

Question

I want to create a new instance of class A that inherits a base class derived from QObject.

In case A isn't in a namespace this works fine, but in case A is in a namespace this call returns a null pointer:

QObject *o = metaObject->newInstance(arg1,arg2,arg3);

The metaObject itself returns the correct class name (including the namespace)

std::cout << "Class name from staticMetaObject: " << metaObject->className() << std::endl;

The constructor of A is marked with Q_INVOKABLE. How can I use QMetaObject::newInstance with namespaces?

Answer 1

It definitely works for me under Qt 5.5.1.

Perhaps your base class is missing the Q_OBJECT macro.

// https://github.com/KubaO/stackoverflown/tree/master/questions/ns-meta-35505644
#include <QtCore>

int A_a, B_a;

class A : public QObject {
   Q_OBJECT
public:
   Q_INVOKABLE A(int a, QObject * parent = 0) : QObject{parent} {
      A_a = a;
   }
};

namespace NS {
class B : public A {
   Q_OBJECT
public:
   Q_INVOKABLE B(int a, QObject * parent = 0) : A{a, parent} {
      B_a = a;
   }
};
}

int main() {
   Q_ASSERT(A_a == 0);
   Q_ASSERT(B_a == 0);
   QScopedPointer<QObject> a {A::staticMetaObject.newInstance(Q_ARG(int, 10))};
   Q_ASSERT(A_a == 10);
   QScopedPointer<QObject> b {NS::B::staticMetaObject.newInstance(Q_ARG(int, 20))};
   Q_ASSERT(A_a == 20);
   Q_ASSERT(B_a == 20);
   QScopedPointer<QObject> c {b->metaObject()->newInstance(Q_ARG(int, 30))};
   Q_ASSERT(A_a == 30);
   Q_ASSERT(B_a == 30);
}

#include "main.moc"