Null pattern with QObject

Question

(C++/Qt) I have a smart pointer to a QObject. Let's say a QWeakPointer. For some external reason (something that might happen in another object or due to an event), it is possible that the pointed object gets destroyed. Since I have a smart pointer there will be no dangling reference, so there's no problem. But I always have to check if the pointer is null or not.

I'm thinking of using the null pattern in order to avoid checking this all the time but I'm not sure if this is possible or convenient with a QObject. The idea would be that the pointer points to the object and in case it gets destroyed, the smart pointer changes its pointed object to a null object. Is this a good idea or should I forget it and just check if the pointer is NULL all the time?

---

Let's show you an example. We have a worker who uses a tool to do its work:

class Worker : public QObject
{
    Q_OBJECT

public:
    Worker(QObject *parent = 0);
    void work()
    {
        if(m_tool)
            m_tool->use();
        emit workCompleted();
    };

signals:
    workCompleted();

public slots:
    void setTool(QWeakPointer<Tool> tool);

private:
    QWeakPointer<Tool> m_tool;
};


class Tool : public QObject
{
    Q_OBJECT

public:
    Tool();

public slots:
    void use() =0;
};


class Screwdriver : public Tool
{
    Q_OBJECT

public:
    Screwdriver() : Tool();

public slots:
    void use()
    {
        // do something
    };
};


class Hammer : public Tool;
class Saw : public Tool;
...

In this case, the Tool is a public domain object of a library, which is used by the Worker. I'm developing such library. So the worker is using a screwdriver but it gets broken and gets destroyed. No problem:

if(m_tool)
    m_tool->use();
emit workCompleted();

m_tool is 0 so it simply does nothing. But we have to check that it's not null everytime.

Now let's say we had a NullTool object:

class NullTool : public Tool
{
    Q_OBJECT

public:
    NullTool() : Tool();

public slots:
    void use()
    {
        // does nothing
    };
};

When the tool was destroyed, our pointer would be smart and would know it should point to a NullTool instance. So Worker::work() could be implemented like this:

void Worker::work()
{
    m_tool->use();
    emit workCompleted();
};

m_tool->use() would then get called on the NullTool which does nothing, so there would be no need to check the pointer is not null.

Is this a good idea? Is it possible with the smart pointer classes Qt provides or should I subclass QWeakPointer?

Answer 1

I don't understand clearly your use case, but your program can be signaled when the object has been destroy by connecting the following signal from QObject:

void destroyed ( QObject * obj = 0 );

Answer 2

I think the null object pattern makes most sense for value-like classes. Examples are QString or QVariant, were you don't want to have code like if ( str && !str->isEmpty() ) but just do if ( !str.isEmpty() ). For QObjects, which are not values but have "an identity", I never found this useful.

Answer 3

I don't see any problem in your idea. You just have to compare the work that it takes to implement it compared to the work for checking the pointer every time. Let's your checking the pointer 10.000 times it's a good idea to use your approach. Side note: Your null object pattern rely on the fact that Tool::use() has no side effects whatsoever.

Take care that possible side affects in Tool::use() don't get in the way when you replace it polymorphically with NullTool::use(). In other words: Be sure you don't break the Liskov Substitution Principle.