Pointers and Q_PROPERTY

[andersin]

Hi,

I want to configure objects that are encapsulated in other objects using a configuration file and a plugin architecture, so I do not know in advance which objects are contained within the object. Is it possible to do this using the QObject:roperty() function?

Example:

class BInterface {
        public:
               virtual ~BInterface() {}
               virtual int foo() = 0;
};
 
class B : public QObject, public BInterface
{
       Q_OBJECT
       Q_PROPERTY(int b READ b WRITE setB );
       public:
               virtual ~B() {}
               virtual int foo() { return mv_b; }
               const int b() const { return mv_b; }
               void setB( const int b ) { mv_b = b; }
      private:
              int mv_b;
};
 
class A : public QObject{
   Q_OBJECT
   Q_PROPERTY(BInterface* B READ B WRITE setB);
   public:
         A() { mv_B = NULL; }
         virtual ~A() { if(mv_B) delete mv_B; }
         const BInterface * B() { return mv_B; }
         void setB( const BInterface * B ) { if (mv_B) delete mv_B; mv_B = B; }
   private:
         BInterface * mv_B;
};

class BInterface {
        public:
               virtual ~BInterface() {}
               virtual int foo() = 0;
};

class B : public QObject, public BInterface
{
       Q_OBJECT
       Q_PROPERTY(int b READ b WRITE setB );
       public:
               virtual ~B() {}
               virtual int foo() { return mv_b; }
               const int b() const { return mv_b; }
               void setB( const int b ) { mv_b = b; }
      private:
              int mv_b;
};

class A : public QObject{
   Q_OBJECT
   Q_PROPERTY(BInterface* B READ B WRITE setB);
   public:
         A() { mv_B = NULL; }
         virtual ~A() { if(mv_B) delete mv_B; }
         const BInterface * B() { return mv_B; }
         void setB( const BInterface * B ) { if (mv_B) delete mv_B; mv_B = B; }
   private:
         BInterface * mv_B;
};

To copy to clipboard, switch view to plain text mode 

what I want to do is something like:

QObject * objA = new A();
BInterface* b = a.property("B");
b.setProperty("b", someValue);

QObject * objA = new A();
BInterface* b = a.property("B");
b.setProperty("b", someValue);

To copy to clipboard, switch view to plain text mode 

can this be done at all?

[jacek]

can this be done at all?

To some extent:

#include <QtDebug>
 
#include <QCoreApplication>
#include <QObject>
#include <QVariant>
 
class BInterface
{
   public:
      virtual int foo() = 0;
};
 
class B : public QObject, public BInterface
{
   Q_OBJECT
   Q_PROPERTY( int b READ b WRITE setB )
   public:
 
      virtual int foo() { return mv_b; }
 
      const int b() const { return mv_b; }
      void setB( int b ) { mv_b = b; }
 
   private:
      int mv_b;
};
 
class A : public QObject
{
   Q_OBJECT
   Q_PROPERTY( QObject* B READ B WRITE setB )
   public:
      A( QObject *b = 0 ) : mv_B( b ) {}
      virtual ~A() { if(mv_B) delete mv_B; }
     QObject * B() { return mv_B; }
      void setB( QObject* B ) { if (mv_B) delete mv_B; mv_B = B; }
 
   private:
      QObject* mv_B;
};
 
int main( int argc, char **argv )
{
  QCoreApplication app( argc, argv );
 
  QObject *a = new A( new B() );
 
  // QObject *a = new A();
  // a->setProperty( "B", 0 );   // works
 
  // QObject *a = new A();
  // a->setProperty( "B", new B() ); // crashes
 
  QObject* b1 = a->property( "B" ).value< QObject *>();
  if( b1 != 0 ) {
     b1->setProperty( "b", 1234 );
  }
  else {
     qDebug() << "b1 is null";
  }
 
  QObject* b2 = a->property( "B" ).value< QObject *>();
  if( b2 != 0 ) {
     qDebug() << b2->property( "b" );
  }
  else {
     qDebug() << "b2 is null";
  }
 
  return 0;
}
 
#include "main.moc"

#include <QtDebug>

#include <QCoreApplication>
#include <QObject>
#include <QVariant>

class BInterface
{
   public:
      virtual int foo() = 0;
};

class B : public QObject, public BInterface
{
   Q_OBJECT
   Q_PROPERTY( int b READ b WRITE setB )
   public:

      virtual int foo() { return mv_b; }

      const int b() const { return mv_b; }
      void setB( int b ) { mv_b = b; }

   private:
      int mv_b;
};

class A : public QObject
{
   Q_OBJECT
   Q_PROPERTY( QObject* B READ B WRITE setB )
   public:
      A( QObject *b = 0 ) : mv_B( b ) {}
      virtual ~A() { if(mv_B) delete mv_B; }
     QObject * B() { return mv_B; }
      void setB( QObject* B ) { if (mv_B) delete mv_B; mv_B = B; }

   private:
      QObject* mv_B;
};

int main( int argc, char **argv )
{
  QCoreApplication app( argc, argv );

  QObject *a = new A( new B() );

  // QObject *a = new A();
  // a->setProperty( "B", 0 );   // works

  // QObject *a = new A();
  // a->setProperty( "B", new B() ); // crashes

  QObject* b1 = a->property( "B" ).value< QObject *>();
  if( b1 != 0 ) {
     b1->setProperty( "b", 1234 );
  }
  else {
     qDebug() << "b1 is null";
  }

  QObject* b2 = a->property( "B" ).value< QObject *>();
  if( b2 != 0 ) {
     qDebug() << b2->property( "b" );
  }
  else {
     qDebug() << "b2 is null";
  }

  return 0;
}

#include "main.moc"

To copy to clipboard, switch view to plain text mode 

Output:

$ ./properties
QVariant(int, 1234)

[andersin]

Ok, that creates a workaround. The problem there is that I lose the type information of the property that I want to set.

I want to write plugins, so I am forced to use QObject and the meta object. I hoped that I could use const char * QMetaProperty::typeName together with void QObject::inherits(const char*) to check if I can use a particular plugin for the property. Like so:

// initialize the plugin loaders and all that jazz
 
QObject * pluginA = pluginLoaderForA.instance();
QObject * pluginB = pluginLoaderForB.instance();
 
const QMetaObject * mobjA = pluginA->metaObject;
QMetaProperty propA = mobj->property(mobj->indexOfProperty("B"));
QString typeName = QString(propA.typeName());
typeName.chop(1); //remove the * at the end
if ( pluginB.inherits(typeName.toAscii().constData()) ) {
   // it inherits the object, so we can use it.
   pluginA.setProperty("B", QVariant::fromValue(pluginB));
}

// initialize the plugin loaders and all that jazz

QObject * pluginA = pluginLoaderForA.instance();
QObject * pluginB = pluginLoaderForB.instance();

const QMetaObject * mobjA = pluginA->metaObject;
QMetaProperty propA = mobj->property(mobj->indexOfProperty("B"));
QString typeName = QString(propA.typeName());
typeName.chop(1); //remove the * at the end
if ( pluginB.inherits(typeName.toAscii().constData()) ) {
   // it inherits the object, so we can use it.
   pluginA.setProperty("B", QVariant::fromValue(pluginB));
}

To copy to clipboard, switch view to plain text mode 

That is of course lost when I use QObject* as the type. On the other hand I am not sure if the setProperty still works. The object is still a QObject, but I doubt that the meta object stuff knows this.

Now one solution would probably be to inherit the interface from QObject, since then I know about the interface in the meta object system. It gets tricky with multiple inheritance of interfaces though, since the inheritance path to QObject is not unique any longer.

I guess I just need to provide the type of BInterface as a const char* property and force some naming scheme on the whole thing.

p.s.: I figured out that the crash in your code is due to some QVariant stuff. This is how I got that to work

// QObject *a = new A();
  // a->setProperty( "B", new B() ); // crashes
 
  QObject *a = new A();
  a->setProperty("B", QVariant::fromValue(new B())); //works

// QObject *a = new A();
  // a->setProperty( "B", new B() ); // crashes

  QObject *a = new A();
  a->setProperty("B", QVariant::fromValue(new B())); //works

To copy to clipboard, switch view to plain text mode 

[jacek]

OK, another try:

#include <QtDebug>
 
#include <QCoreApplication>
#include <QMetaProperty>
#include <QObject>
#include <QVariant>
 
class BInterface : public QObject
{
   Q_OBJECT
   public:
      virtual int foo() = 0;
};
 
class B : public BInterface
{
   Q_OBJECT
   Q_PROPERTY( int b READ b WRITE setB )
   public:
 
      virtual int foo() { return mv_b; }
 
      const int b() const { return mv_b; }
      void setB( int b ) { mv_b = b; }
 
   private:
      int mv_b;
};
 
Q_DECLARE_METATYPE( BInterface* );
 
lass A : public QObject
{
   Q_OBJECT
   Q_PROPERTY( BInterface* B READ B WRITE setB )
   public:
      A( BInterface *b = 0 ) : mv_B( b ) {}
      virtual ~A() { if(mv_B) delete mv_B; }
 
      BInterface * B() { return mv_B; }
      void setB( BInterface *B ) { if (mv_B) delete mv_B; mv_B = B; }
 
   private:
      BInterface* mv_B;
};
 
int main( int argc, char **argv )
{
  QCoreApplication app( argc, argv );
 
  QObject *a = new A();
  BInterface * b = new B();
  a->setProperty( "B", QVariant::fromValue( b ) );
 
  const QMetaObject * metaObj = a->metaObject();
  int index = metaObj->indexOfProperty( "B" );
  if( index != -1 ) {
     QMetaProperty prop = metaObj->property( index );
     qDebug() << prop.typeName();
  }
  else {
     qDebug() << "property not found";
  }
 
  BInterface *b1 = a->property( "B" ).value< BInterface *>();
  if( b1 != 0 ) {
     b1->setProperty( "b", 1234 );
  }
  else {
     qDebug() << "b1 is null";
  }
 
  BInterface *b2 = a->property( "B" ).value< BInterface *>();
  if( b2 != 0 ) {
     qDebug() << b2->property( "b" );
  }
  else {
     qDebug() << "b2 is null";
  }
 
  return 0;
}
 
#include "main.moc"

#include <QtDebug>

#include <QCoreApplication>
#include <QMetaProperty>
#include <QObject>
#include <QVariant>

class BInterface : public QObject
{
   Q_OBJECT
   public:
      virtual int foo() = 0;
};

class B : public BInterface
{
   Q_OBJECT
   Q_PROPERTY( int b READ b WRITE setB )
   public:

      virtual int foo() { return mv_b; }

      const int b() const { return mv_b; }
      void setB( int b ) { mv_b = b; }

   private:
      int mv_b;
};

Q_DECLARE_METATYPE( BInterface* );

lass A : public QObject
{
   Q_OBJECT
   Q_PROPERTY( BInterface* B READ B WRITE setB )
   public:
      A( BInterface *b = 0 ) : mv_B( b ) {}
      virtual ~A() { if(mv_B) delete mv_B; }

      BInterface * B() { return mv_B; }
      void setB( BInterface *B ) { if (mv_B) delete mv_B; mv_B = B; }

   private:
      BInterface* mv_B;
};

int main( int argc, char **argv )
{
  QCoreApplication app( argc, argv );

  QObject *a = new A();
  BInterface * b = new B();
  a->setProperty( "B", QVariant::fromValue( b ) );

  const QMetaObject * metaObj = a->metaObject();
  int index = metaObj->indexOfProperty( "B" );
  if( index != -1 ) {
     QMetaProperty prop = metaObj->property( index );
     qDebug() << prop.typeName();
  }
  else {
     qDebug() << "property not found";
  }

  BInterface *b1 = a->property( "B" ).value< BInterface *>();
  if( b1 != 0 ) {
     b1->setProperty( "b", 1234 );
  }
  else {
     qDebug() << "b1 is null";
  }

  BInterface *b2 = a->property( "B" ).value< BInterface *>();
  if( b2 != 0 ) {
     qDebug() << b2->property( "b" );
  }
  else {
     qDebug() << "b2 is null";
  }

  return 0;
}

#include "main.moc"

To copy to clipboard, switch view to plain text mode 

Output:

$ ./properties
BInterface*
QVariant(int, 1234)