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Smart-pointers and memory management in Qt4

Hi everybody.

Smart points are very important for managing the lifetime of objects. Qt has a model for managing the lifetime of objects, when objects are inherited from the base class QObject and the ”relationship” parent/child is set. It deletes all children of the project. This model of object lifetime management is successfully combined with the technology of interaction between “Signals & Slots” objects, and when using smart pointers, hard-to-debug bugs may occur.

This article will review the QScopedPointer, a “lightweight” version of QSharedPointer. QScopedPointer is introduced in version 4.6.

Boost has analogues to Qt-based smart pointers: QSharedPointer — boost::shared_ptr QWeakPointer — boost::weak_ptr QScopedPointer — boost::scoped_ptr

Example 1

A simple program that does not delete the m_socket object and, accordingly, when the application is terminated, the socket is not disconnected:

class test_socket : public QTcpSocket
{
public:
   test_socket( QObject * parent )
       : QTcpSocket( parent )
   {
   }
   virtual ~test_socket( )
   {
       qDebug( ) << "~test_socket";
   }
};

class test_scop_ptr_obj : public QObject
{
   Q_OBJECT
   test_socket * m_socket;
public:
   test_scop_ptr_obj( )
       : m_socket( new test_socket( 0 ) )
   {
       m_socket->connectToHost( "www.ru", 80 );
       connect( m_socket, SIGNAL( stateChanged ( QAbstractSocket::SocketState ) ),
               SLOT( on_state_changed( QAbstractSocket::SocketState ) ) );
   }
   virtual ~test_scop_ptr_obj( )
   {
       qDebug( ) << "~test_scop_ptr_obj";
   }
private Q_SLOTS:
   void on_state_changed( QAbstractSocket::SocketState socket_state )
   {
       qDebug( ) << socket_state;
   }
};

int main( int argc, char** argv )
{
   QCoreApplication a( argc, argv );
   test_scop_ptr_obj obj;
   QTimer::singleShot( 3000, &a, SLOT( quit( ) ) );
   return a.exec( );
}

After starting the program, we get the following output:

QAbstractSocket::ConnectingState
QAbstractSocket::ConnectedState
~test_scop_ptr_obj

Example 2

Now let us change the line:

    : m_socket( new test_socket( 0 ) )

per line:

    : m_socket( new test_socket( this ) )

and we will get the output:

QAbstractSocket::ConnectingState
QAbstractSocket::ConnectedState
~test_scop_ptr_obj
~test_socket

Example 3

Let us add a smart pointer and now the test_scop_ptr_obj class will look like this:

class test_scop_ptr_obj : public QObject
{
   Q_OBJECT
   QScopedPointer< test_socket > m_socket;
public:
   test_scop_ptr_obj( )
       : m_socket( new test_socket( 0 ) )
   {
       m_socket->connectToHost( "www.ru", 80 );
       connect( m_socket.data( ), SIGNAL( stateChanged ( QAbstractSocket::SocketState ) ),
                SLOT( on_state_changed( QAbstractSocket::SocketState ) ) );
   }
   virtual ~test_scop_ptr_obj( )
   {
       qDebug( ) << "~test_scop_ptr_obj";
   }
private Q_SLOTS:
   void on_state_changed( QAbstractSocket::SocketState socket_state )
   {
       qDebug( ) << socket_state;
   }
};

We get the output:

QAbstractSocket::ConnectingState
QAbstractSocket::ConnectedState
~test_scop_ptr_obj
~test_socket
QAbstractSocket::UnconnectedState

Analysis

Example 1 is not interesting to us because memory leaks occur in this example. Examples 2 and 3 differ in the technology of freeing previously allocated memory. In both examples, the object of the test_socket class is deleted, but in the 3rd example, the on_state_changed function is called after calling the ~test_scop_ptr_obj destructor, which is an error. Before deleting children, the parent object disconnects its signals and slots, which is why in Example 2 the stateChanged signal does not arrive to the test_scop_ptr_obj object.

Results

To correct the error in Example 3, it is necessary to add disabling signals of the deleted object to the test_scop_ptr_obj destructor:

virtual ~test_scop_ptr_obj( )
{
    m_socket->disconnect( );
    qDebug( ) << "~test_scop_ptr_obj";
}

In this case, the output will be the same as in Example 2.