How to access to a var in the main window (QWidget) from an other window (QWidget)???

[andreaQt]

In my project i have this 4 classes in separated .h and .cpp file:

MainWindow::MainWindow(QWidget *parent): QWidget (parent){   
    built_sphere_window   = new BuiltSphere;
    built_cylinder_window = new BuiltCylinder;
    glWidgetWindow = new GlWidget;
....}
 
BuiltCylinder::BuiltCylinder(QWidget *parent) : QWidget(parent){
    glWidgetCylinder = new GlWidget;
....}
 
BuiltSphere::BuiltSphere(QWidget *parent) : QWidget(parent)
{
    glWidgetSphere = new GlWidget;
....}
 
GlWidget::GlWidget(QWidget *parent) : QGLWidget(QGLFormat(/* Additional format options */), parent){
    alpha = 0;      //previous =>25
    beta = 0;       //previous =>-25
    distance = 6;   //previous =>2.5
    initializeWindow();
....}

MainWindow::MainWindow(QWidget *parent): QWidget (parent){   
    built_sphere_window   = new BuiltSphere;
    built_cylinder_window = new BuiltCylinder;
    glWidgetWindow = new GlWidget;
....}

BuiltCylinder::BuiltCylinder(QWidget *parent) : QWidget(parent){
    glWidgetCylinder = new GlWidget;
....}

BuiltSphere::BuiltSphere(QWidget *parent) : QWidget(parent)
{
    glWidgetSphere = new GlWidget;
....}

GlWidget::GlWidget(QWidget *parent) : QGLWidget(QGLFormat(/* Additional format options */), parent){
    alpha = 0;      //previous =>25
    beta = 0;       //previous =>-25
    distance = 6;   //previous =>2.5
    initializeWindow();
....}

To copy to clipboard, switch view to plain text mode 

from the "mainwindow" i do this calls:

void MainWindow::built_sphere(){
    built_sphere_window->setWindowTitle("Built Sphere");
    built_sphere_window->setWindowModality(Qt::ApplicationModal);
    built_sphere_window->num_sphere = num_element_inserted->at(1)+1;
    built_sphere_window->show();
}
void MainWindow::built_cylinder(){
    built_cylinder_window->setWindowTitle("Built Cylinder");
    built_cylinder_window->setWindowModality(Qt::ApplicationModal);
    built_cylinder_window->num_cyl = num_element_inserted->at(0)+1;
    built_cylinder_window->show();
}

void MainWindow::built_sphere(){
    built_sphere_window->setWindowTitle("Built Sphere");
    built_sphere_window->setWindowModality(Qt::ApplicationModal);
    built_sphere_window->num_sphere = num_element_inserted->at(1)+1;
    built_sphere_window->show();
}
void MainWindow::built_cylinder(){
    built_cylinder_window->setWindowTitle("Built Cylinder");
    built_cylinder_window->setWindowModality(Qt::ApplicationModal);
    built_cylinder_window->num_cyl = num_element_inserted->at(0)+1;
    built_cylinder_window->show();
}

To copy to clipboard, switch view to plain text mode 

I have several object to build but at this time just the previous 2.
when i open one of this window i to read the data from the respective var & i'm right to visualize the number of the new object that i want to build.
My question is:
how i can set correctly the signal to send to mainwindow the number that i want only at push to ok button in the build window?
in mainwindow.cpp i think i have to set (i write the call for one build element):

connect (built_sphere_window, SIGNAL (sig_update_num(const int)), this, SLOT (update_value(const float)));

connect (built_sphere_window, SIGNAL (sig_update_num(const int)), this, SLOT (update_value(const float)));

To copy to clipboard, switch view to plain text mode 

and in the respective slot update the num_element_inserted.
in builtsphere.h:

signals:
     void sig_update_num(const int);

signals:
     void sig_update_num(const int);

To copy to clipboard, switch view to plain text mode 

in builtsphere.cpp:

emit sig_update_num(num_sphere+1)

emit sig_update_num(num_sphere+1)

To copy to clipboard, switch view to plain text mode 

I think that the code write like this work, my finally question is:
if i want to read/write a value that is stored in a variable in the main window or in all of other classes from one of it is better do a "QSharedMemory+signal in a class like shareddata.h" to do the job or with the code that i write above?
At this time i do my application in one thread (i want to test it first that run correctly & do the job that i want), next step is do multithread to optimize the performance in multicore processor; so i think that in multithread is better to build a class shareddata.h where i can use the lock() & unlock() functions, is the right way to do the job?