Problems accessing static member variable from static member function

[montylee]

I have a class with a static variable of type QComboBox *. I have created a static member function from which i want to return this static variable. Here is the sample code:

Header code

Class TestPage : public QWidget
{
---
---
---
public:
       static QComboBox *getTestCombo ();
 
private:
       static QComboBox *sm_pTestCombo;
};

Class TestPage : public QWidget
{
---
---
---
public:
       static QComboBox *getTestCombo ();

private:
       static QComboBox *sm_pTestCombo;
};

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CPP code

QComboBox * TestPage::getTestCombo ()
{
       return sm_pTestCombo;
}

QComboBox * TestPage::getTestCombo ()
{
       return sm_pTestCombo;
}

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I am getting the following linking error:

undefined reference to 'TestPage::sm_pTestCombo'

Any ideas?

[spirit]

you must init static variable

QComboBox *Test::m_comboBox = new QComboBox();

QComboBox *Test::m_comboBox = new QComboBox();

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why do you need this method of storing/getting of QComboBox?

[montylee]

got it working...
searched the internet and found that i have to define the static member variable again in the CPP file at file scope.

So i have added this on top of the CPP file:

QComboBox* TestPage::sm_pTestCombo;

QComboBox* TestPage::sm_pTestCombo;

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But that brings me to a questions that is it worth it?
Now the static variable can be accessed by anyone using the scope resolution operator. Same thing can be done by simply using a global variable in the CPP file, so just take a global variable in the CPP file and be happy. Why should i use the static variable then?

[montylee]

you must init static variable

QComboBox *Test::m_comboBox = new QComboBox();

QComboBox *Test::m_comboBox = new QComboBox();

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why do you need this method of storing/getting of QComboBox?

i already got my solution... i have posted it...

I need to get the combo box pointer as i need to add items to it from a different class.

If i use a normal member function to return the combo box pointer it won't work as as i have to first create an instance of this class in the other class to invoke the member function. As soon as i create an instance, the member combo box variable is allocated a new memory and i am not able to get the original combo box pointer.

So , i have to call it directly without using an object, so i have to use either statiic or a global variable...

pls read my earlier post and suggest if i should go for global or static member variable.

[spirit]

this variable is private, you can't get access from another class.

[montylee]

i have made it public and my code is working. But i am not sure whether to use static member variable or use a global variable...
both approaches r working...

pls suggest which one is better.

[spirit]

I suggest you to add method in you class which will add item to a combobox and refuse using of global pointer to the combobox or using static variable.

[montylee]

I suggest you to add method in you class which will add item to a combobox and refuse using of global pointer to the combobox or using static variable.

The problem is:

Suppose the combo box is part of class A:

class A {
private:
         QComboBox *m_pTestCombo;
 
public:
         void updateCombo ();
}

class A {
private:
         QComboBox *m_pTestCombo;

public:
         void updateCombo ();
}

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Now, suppose i want to update the combo box from a different class B. For that, i have to call updateCombo () function of class A from class B.
For calling the updateCombo () function of class A, i have to first create an object of class A in class B. For e.g.

void B::someFunction ()
{
       A obj;
       obj.updateCombo;
}

void B::someFunction ()
{
       A obj;
       obj.updateCombo;
}

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Now, as soon as i create a new object of class A in class B, calling updateCombo function will have no effect on the GUI because when i create a new object of class A, new memory is allocated to m_pTestCombo variable. So, even if the code to add items is executed this way, i am unable to see the updated combo box in the GUI as the GUI is still showing the original combo box.

So, basically i have to get the same instance of the combo box which is being displayed on the GUI. Getting the existing instance of the pointer is only possible if i use a static member variable and call the updateCombo () function without creating a new object.
I hope i have explained my problem well...

[spirit]

you must provide interface in a class which keep combobox to setting/getting information, but don't allow to use members of this class to another classes. this is bad idea.

[spirit]

in you case, I suggest you add method to a class, which updates combobox, which will return QStringList with new items, and in a class, which keeps combox, method which will set new items (QStringList).

[montylee]

ok, i got a bit confused with my own code. Here's what i am doing now:

Header code:

Class TestPage : public QWidget
{
---
---
---
public:
       static QComboBox *getTestCombo ();
 
private:
       static QComboBox *sm_pTestCombo;
};

Class TestPage : public QWidget
{
---
---
---
public:
       static QComboBox *getTestCombo ();
 
private:
       static QComboBox *sm_pTestCombo;
};

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CPP code

QComboBox* TestPage::sm_pTestCombo; 
QComboBox * TestPage::getTestCombo ()
{
       return sm_pTestCombo;
}

QComboBox* TestPage::sm_pTestCombo; 
QComboBox * TestPage::getTestCombo ()
{
       return sm_pTestCombo;
}

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Now, sm_pTestCombo is a private variable so it can't be accessed from the other class directly.

From the other class, i am calling getTestCombo() to get the combo box pointer.

QComboBox *testCombo = TestPage::getTestComboBox ();

QComboBox *testCombo = TestPage::getTestComboBox ();

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Does this approach look ok? The member variable is now private and other classes have to use the public member function to get the variable's address.

[spirit]

this looks ok, but when you define QComboBox like static class member you didn't set a parent. so when you start application you will see this combobox like separate widget. so, you must reparent combobox in this case. anyway, I suggest you refuse this approach.

[montylee]

this looks ok, but when you define QComboBox like static class member you didn't set a parent. so when you start application you will see this combobox like separate widget. so, you must reparent combobox in this case. anyway, I suggest you refuse this approach.

no, my code is working fine. I have a separate combo box member variable which is actually displayed on the GUI. The static variable is just used as i require to pass the address of the variable to a different class through a static function. Since a static member function can only access static member variables, so i had to use an extra variable...

in the class constructor, i just assign the normal combo box pointer to the static combo box variable...in the GUI, the normal combo box is displayed.

[spirit]

I guess it works fine, because you use layouts.

[montylee]

yes i am using layouts but as i said the combo box being shown on the GUI is a normal member variable.
The static variable just stores the same address as the normal variable...so it's just a copy...

Thanks for your help!

[spirit]

in the class constructor, i just assign the normal combo box pointer to the static combo box variable....

this can lead to memory leak, because when you init static variable you create a new comboboex and then (according to your words) you reset variable with a new object, but don't destroy old.

[montylee]

i am not allocating new memory for the static combo box, i am just assigning the same address to the static variable, so there is no extra memory allocation. Here's my code:

sm_pTestCombo = m_pTestCombo;

sm_pTestCombo = m_pTestCombo;

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here, sm_pTestCombo is the static member variable and m_pTestCombo is the normal member variable.

[spirit]

what about this

QComboBox *Test::m_comboBox = new QComboBox();

QComboBox *Test::m_comboBox = new QComboBox();

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?

[spirit]

if you use code like this

QComboBox *Test::m_comboBox = 0;

QComboBox *Test::m_comboBox = 0;

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then ok.

[montylee]

i think that code was posted by you. I am not allocating any new memory