Qt4 parent pointer usage

Question

I'm having some problem with understanding usage of parent pointer in QT4.

class firstClass : public QWidget
{
    Q_OBJECT

public:
     firstClass(QWidget *parent = 0);
    ~firstClass();

    void doSomething();

private:
    secondClass * myClass;
};

class secondClass : public QWidget
{
    Q_OBJECT

public:
    secondClass(QWidget *parent = 0);
    void doSomethingElse();
};

I want to call doSomething() method while running doSomethingElse(). Is there any way to do it using parent pointer?

I tried parent->doSomething() but it doesn't work. It seems that Qt Creator is suggesting only methods from QObject class after parent->.

On the other hand I can't write it like secondClass(firstClass *parent = 0); - compilator returns error:

Thanks for any suggestions.

Answer 1

If you are positive that the parent of secondClass is always going to be firstClass then you can do this:

static_cast<firstClass *>(parent)->doSomething();

Alternatively you can use qobject_cast and check to make sure that parent is actually an instance of firstClass:

firstClass *myParent = qobject_cast<firstClass *>(parent);
if(myParent){
    myParent->doSomething();
}

Answer 2

The more Qt-ish way to do this would be to use signals and slots, instead of trying to directly call a different function.

class firstClass : public QWidget
{
    Q_OBJECT

public:
     firstClass(QWidget *parent = 0);
    ~firstClass();

public slot:
    void doSomething();

private:
    secondClass * myClass;
};

class secondClass : public QWidget
{
    Q_OBJECT

public:
    secondClass(QWidget *parent = 0);
    void doSomethingElse()
    {
        // ...
        emit ( triggerDoSomething() );
        // ...
    }

signal:
    void triggerDoSomething();
};

firstClass::firstClass(QWidget *parent) : 
    QWidget(parent), myClass(new secondClass(this))
{
    // ...
    bool connected = connect(myClass, SIGNAL(triggerDoSomething()),
        SLOT(doSomething()));
    assert( connected );
}