C / C++ / MFC

Re: How do I access private variable of an object ?

17-Jun-21 0:35

Naughty.

CPallini17-Jun-21 1:32

CPallini

17-Jun-21 1:32

But true.

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

Re: How do I access private variable of an object ?

Member 1496877117-Jun-21 6:36

Re: How do I access private variable of an object ?

17-Jun-21 6:36

Sorry, I posted this is wrong forum.
But to the comedians - do not quit you day job.

In the mean time I'll looking at usage of "friend".

Richard MacCutchan17-Jun-21 6:40

Re: How do I access private variable of an object ?

17-Jun-21 6:40

Member 14968771 wrote:
In the mean time I'll looking at usage of "friend".

Don't waste your time, since it will not make any difference. You cannot declare your class to be a friend of some other class in the hope of accessing its private variables. If that was possible then the millions of applications would be open to hacking. Do what I suggested above and make use of the information provided in the documentation: that is what it is there for..

CPallini17-Jun-21 7:43

CPallini

17-Jun-21 7:43

It was serious, actually. If you the designer of the class made a member private then you have two ways to access it: the publicly exposed interface (for instance the get/set methods) or a dirty hack on its bare representation in memory. Typically you cannot use friend because you are not the designer of the class.

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

Re: How do I access private variable of an object ?

Magnus Forslund15-Jul-21 2:19

Magnus Forslund

15-Jul-21 2:19

Private members are only accessible via public functions.
Try int start_X = QPoint.SetX(dimentions.x2);

Pass a function with parameter

Member 1496877115-Jun-21 10:35

15-Jun-21 10:35

Reason for this post is
I am trying to learn not just "cut and paste".

In the following code I am trying to put a function with parameter into "addAction " function It fails with two errors I am trying to decipher and learn what I am doing wrong.
I tested

Run_DDD_OPTION(testWidget); and it runs fine

<pre>    QWidget *testWidget = new QWidget();
    testWidget->setWindowTitle("test widget window ");
    Run_DDD_OPTION(testWidget); runs fine 

    QAction *CAT_DDD_OPTION = CATMenu->
    addAction(tr("&Device Discovery Dialog (modified Bluetooth scanner ) "),
    this,
    &MainWindow::Run_DDD_OPTION(testWidget)
    );
    
   this runs as expected when no parameters are passed 

    QAction *CAT_DDD_OPTION_TAB = CATMenu->
    addAction(tr("&Device Discovery Dialog (modified Bluetooth scanner ) "),
    this,
    &MainWindow::Run_TAB
    );

There are too many "addAction" functions , different objects, so all I have to go by are these errors. (It woudl probably help if I knew what "addAction" expects as parameters.

<pre>/media/qe/TSET_QT_LABEL/QT_PROGRAMS/JUNE  15  WORKCOPY 2/CAT/mdi/mainwindow.cpp:1157: error: lvalue required as unary ‘&’ operand
../../../JUNE  15  WORKCOPY 2/CAT/mdi/mainwindow.cpp: In member function ‘void MainWindow::createActions()’:
../../../JUNE  15  WORKCOPY 2/CAT/mdi/mainwindow.cpp:1157:32: error: lvalue required as unary ‘&’ operand
 1157 |     &MainWindow::Run_DDD_OPTION(testWidget)
      |      ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~

/media/qe/TSET_QT_LABEL/QT_PROGRAMS/JUNE  15  WORKCOPY 2/CAT/mdi/mainwindow.cpp:1155: error: cannot take the address of an rvalue of type 'void'

Greg Utas15-Jun-21 11:38

Greg Utas

15-Jun-21 11:38

This looks suspicious to me:

C++

&MainWindow::Run_DDD_OPTION(testWidget)

It looks like you're trying to create a pointer to whatever Run_DDD_OPTION returns. However, you also say

Run_DDD_OPTION(testWidget); // runs fine

which either means that it returns void or you didn't care what it returned. My guess is the former, which would explain all the error messages. If it returns void, you can't take its address, and there is no lvalue (=a memory address).

EDIT: If you want to pass the function Run_DDD_OPTION as an argument to addAction, just write

C++

addAction(tr("&Device Discovery Dialog(modified Bluetooth scanner)"),
   this,
   MainWindow::Run_DDD_OPTION
   );

You may need the & in front of the function name--I haven't done this in a long time, so I have to look it up. --It looks like it's optional. If it doesn't work without it, try it with it.

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modified 15-Jun-21 17:48pm.

Member 1496877115-Jun-21 13:30

15-Jun-21 13:30

Thanks
I think you may have answered my question.

The line is just to test validity of the function.

Run_DDD_OPTION(testWidget); // runs fine

addAction(tr("&Device Discovery Dialog(modified Bluetooth scanner)"),
   this,
   MainWindow::Run_DDD_OPTION
   );

The above code works IF "&" is prefixed ,
HOWEVER I like to make that parameter of "addAction" variable, more versatile.

And that is why I am having issues, hence my post.

&MainWindow::Run_DDD_OPTION

So my question would be
if this works without parameter
why do I need to return a pointer ?
I presume it would be same as the passed parmeter type.

Run_DDD_OPTION

Now again , I just do not want to "cut and paste" without knowing why.
Thanks for your contribution.

Greg Utas15-Jun-21 13:39

Greg Utas

15-Jun-21 13:39

I don't understand how you want to make addAction's last parameter "more versatile". You'll have to explain that.

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Member 1496877115-Jun-21 15:23

15-Jun-21 15:23

Simple, the app will use several GUI interfaces - in Qt terminology "widgets".
There is lots of commonality and I like to reuse my code.
So passing "widget" as a parameter to addAction would be nice.

Besides - I need to get over this "passing function as a parameter with parameters ".
I have a feeling QT makes its own syntax...

Should that be

function ( passFunction, passFunctionparamater ); // pointer , pointer

Regarding Constructor Calling in C++

Greg Utas16-Jun-21 0:22

Greg Utas

16-Jun-21 0:22

I haven't used Qt, so I don't know if I can help. It sounds like a widget is an object, but you're only passing functions and parameters. Would passing an object help?

It's even possible to pass a class's member function[^] as a parameter. I've not used this capability, so you'd have to read up on it if that's what you want.

The problem with passing a function and its parameters is that different functions take different parameters. To deal with that, I think you'd need std::function[^]. Again, that's not something I've used, but now you can look for more details.

My go-to site for C++ is here[^]. It's a bit formal, so sometimes you need to look elsewhere for explanations and examples that are easier to understand. But it will provide you with the right terminology to use when investigating something.

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Member 1522917415-Jun-21 3:08

Member 15229174

15-Jun-21 3:08

class A
{
public:
int a;
};

class B:public class A
{
public:
int b;
}

class C: public class B
{
public:
B obj;
}

void main()
{
C obj1(10,20);
}

How do you write constructor for all three class so that the main will get called/?

David Crow15-Jun-21 3:29

David Crow

15-Jun-21 3:29

Member 15229174 wrote:
...so that the main will get called/?

main() is called by the OS/framework, not you.

If, instead, you wanted to know how to call the base class constructor, how about something like:

class C : public B
{
  public:
    C(int x, int y) : B(x, y)
    { 
    }

    // B obj;
};

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Member 1522917415-Jun-21 3:55

Member 15229174

15-Jun-21 3:55

Actually an interviewer asked me this question and told that class C contains the object of B class and through main C obj1(10,20) should be passed. Now you have to write constructor in all three class so that in main() function, it will not give an error.

Richard MacCutchan15-Jun-21 5:02

15-Jun-21 5:02

Since all the member variables are public you only need a constructor in C. And it would still work without that.

Maximilien15-Jun-21 5:01

Maximilien

15-Jun-21 5:01

you C class does not have a constructor with 2 parameters.

CI/CD = Continuous Impediment/Continuous Despair

Advice on writing a GUI application that can run on very old Windows operating systems without Service Packs and Frameworks

CPallini15-Jun-21 23:39

CPallini

15-Jun-21 23:39

Actually class C constructor should have had four (Yes, 4, since it contains four variables) parameters. Anyway...
Try

C++

#include <iostream>
using namespace std;
class A
{
public:
  int a;
  A(int a):a(a){}
};

class B: public A
{
public:
  int b;
  B(int a, int b): A(a), b(b){}
};


class C: public B
{
public:
  B obj;
  C(int a, int b): B(a, b), obj(b, a){} // swapped the arguments of 'obj' just to make it a little different
  friend ostream & operator << (ostream & os, const C & c); // overload the insertion operator just to show this object content
};

ostream & operator << (ostream & os, const C & c)
{
  os << "a = " << c.a << ", b = " << c.b << ", obj.a = " << c.obj.a << ", obj.b = " << c.obj.b;
  return os;
}

int main()
{
  C obj1(10,20);
  cout << obj1 << endl;
}

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

arnold_w14-Jun-21 8:32

arnold_w

14-Jun-21 8:32

Some of my employer's customers have old Windows-computers (some are from the 90's) and they are not allowed to connect to the Internet and they have no Service Packs or Frameworks installed. I need to write a simple GUI application that does some UART-communication that can run on these old computers and I would greatly appreciate some advice.
1. What API would you recommend I use? I've heard of an API called "Win32", is that the one I should use?
2. What GUI library would you recommend I choose?
3. Can I program in C++ or am I restricted to C?
4. Is Visual Studio 2005 a good choice for an IDE or would you recommend something else?
5. Anything else I need to be aware of?

Re: Advice on writing a GUI application that can run on very old Windows operating systems without Service Packs and Frameworks

Victor Nijegorodov14-Jun-21 8:43

Victor Nijegorodov

14-Jun-21 8:43

Win32 API
VC++ MFC application
C++
VC++6.0

Re: Advice on writing a GUI application that can run on very old Windows operating systems without Service Packs and Frameworks

Mircea Neacsu14-Jun-21 8:45

Mircea Neacsu

14-Jun-21 8:45

1. What API would you recommend I use? I've heard of an API called "Win32", is that the one I should use?

Most probably. At that time there weren't many other choices.

2. What GUI library would you recommend I choose?

You don't necessarily need a GUI library. Win32 was quite capable of making a simple GUI app.

3. Can I program in C++ or am I restricted to C?

You can use C++.

4. Is Visual Studio 2005 a good choice for an IDE or would you recommend something else?

VS 2019 is WAY better and you can target 32-bit Win32 applications.

5. Anything else I need to be aware of?

Link everything statically. You don't want to depend on runtime DLLs. Keep it simple, keep it small.

Mircea

Re: Advice on writing a GUI application that can run on very old Windows operating systems without Service Packs and Frameworks

Richard MacCutchan14-Jun-21 8:51