how to remove variable sharing between objects?

Question

Hi i am new to object oriented programming. here I intent to make three animation plots

subwindow_movie *child_Movie = new subwindow_movie;
child_Movie->setGeometry(20,100,620,560);
child_Movie->setplot(0);
child_Movie->show();

subwindow_movie *child_Movie1 = new subwindow_movie;
child_Movie1->setGeometry(680,100,620,560);
child_Movie1->setplot(1);
child_Movie1->show();


subwindow_movie *child_Movie2 = new subwindow_movie;
child_Movie2->setGeometry(325,350,620,560);
child_Movie2->setplot(2);
child_Movie2->show();

but the problem is that they all share same value of setplot i,e when the third subwindow_movie is created child_movie0 and child_movie1 both setplot values become 2; how to get rid of that... below is the set value function inside the class

#include "subwindow_movie.h"
#include "ui_subwindow_movie.h"

#include "plot.h"

#include <qapplication.h>

#include <qmainwindow.h>

int movie;

subwindow_movie::subwindow_movie(QWidget *parent) :

    QDialog(parent),

    ui(new Ui::subwindow_movie)

{

    ui->setupUi(this);

}

subwindow_movie::~subwindow_movie()
{
    delete ui;
}

void subwindow_movie::setplot(int value)

{
     if (value ==0)

     {   d_plot0 = new Plot( this );

         movie=0;}

     else if (value ==1)

     {   d_plot1 = new Plot( this );

         movie=1;}

     else if (value ==2)

     {   d_plot2 = new Plot( this );

         movie=2;}

}


void subwindow_movie::on_movie_slider_valueChanged(int value)

{
    if (movie ==0)

    {  d_plot0->setAlpha(value);

       }
    else if (movie ==1)

    {  d_plot1->setAlpha(value);

    }
    else if (movie ==2)

    {  d_plot2->setAlpha(value);

    }
}

the real problem is int movie which changes for with the creation of new child_movie causes the child_movie0 to run movie2. i dont want the movie variable changed for the child_movie0 with the creation of child_movie1.

Answer 1

With

int movie;

there is only one instance of the movie variable (it is a global variable). Each time you set its value, the previous value is overwritten.

Make movie a member variable instead:

class subwindow_movie {
    int movie;

public:

    ...
}

Then, movie is available as separate instance for each subwindow_movie object you are creating. Inside the subwindow_movie methods, you can still access the variable as before.

Essentially, you should never use global variables - see also http://www.learncpp.com/cpp-tutorial/42-global-variables: Why global variables are evil