[Solved] Accessing Object from child

bareil76

Hi all,

I have a basic problem which should be easy to solve, but It won't work.

I have a mainwindow class with two other classes. "first" and "second"

@namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

private:
Ui::MainWindow *ui;
first *fristClass;
second *secondClass;

public slots:

signals:

private slots:

protected:

protected slots:

};@

In my mainwindow class, I have the following constructor

@MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
setupActions();

firstClass= new first();
secondClass=new second();--

}@

In fact, secondClass NEEDS first class to function... so I would need to include the firstClass pointer to its constructor, but it won't work!!!!

How do I do that in Qt?

raven-worx

can you please be more specific.
whats your exact problem?

when you create a instance of "first" before "second" you can easily pass it by the constructor...

bareil76

Yes... I know.. but can you explain HOW to pass the pointer in the constructor.

I tried adding parent &p in the argument and it did not worked.

NicuPopescu

second's constructor's parameters list is empty, so how would you think to pass the first's pointer into constructor ?

NicuPopescu

bq. I tried adding parent &p in the argument and it did not worked

as first and seconds are encapsulated in the same class their type object's can be directly passed from one to another ... so in arg list must be (first &f) ... it would work through parent's type, as (MainWindow &p) but you need a public geter for first ... this is awkward anyway

bareil76

what do you mean it is awkward? How would you proceed?

raven-worx

[quote author="bareil76" date="1381322725"]Yes... I know.. but can you explain HOW to pass the pointer in the constructor.

I tried adding parent &p in the argument and it did not worked.[/quote]

depends on your class definition of "second":

@
#include "first.h"

class Second
{
public:
Second(First* first) : m_First(first)
{
}

void doSomethingWithFirst()
{
     if( m_First )
         m_First->callMethod();
}

protected:
First* m_First;
}
@

Then you can do this in your MainWindow constructor:
@
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
setupActions();

    firstClass= new first();
    secondClass=new second(firstClass);
}

@

NicuPopescu

bq. what do you mean it is awkward? How would you proceed?

in .h

@namespace Ui {
class MainWindow;
}

class MainWindow; //<-- forward declaration

class first
{

};

class second
{
first *_f;

public:
second(MainWindow *p);
};

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
first *getFirst(){ return f;}

private:
Ui::MainWindow *ui;
first *f;
second *s;

protected:
bool eventFilter(QObject *obj, QEvent *event);
};@

in .cpp you must define the second's constructor:

@...
second::second(MainWindow *p)
{
_f = p->getFirst();
}@

bareil76

Thanks a lot! Both of your method work great.

However, what is the most correct approach?

SGaist

Hi,

raven-worx method is cleaner, your second class doesn't need to know two different classes, especially since the only purpose of MainWindow would be to provide the pointer to the "first" object.

NicuPopescu

no doubt! ... my point was to unchain a little bit the imagination and in programming you need this a lot :)

SGaist

Sure you need imagination, but clean code should be a top priority, without that you get quickly a lot of useless tight couplings and messy code that become nightmares to maintain.