bool check status from subclass

[ebsaith]

Good day,

Created 2 classes (mainWindow & qlabel)
I have a variable of type bool in mainWindow class
How do I check its status from mainWindow's subclass(qlabel)

main.cpp
void MainWindow::on_action_yes_triggered()
{
    flag3 = true;
}

main.cpp
void MainWindow::on_action_yes_triggered()
{
    flag3 = true;
}

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in qlabel class

if(MainWindow.flag3 == true)
    {
       //do function
    }

if(MainWindow.flag3 == true)
    {
       //do function
    }

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Errors: expected primary expression before '.'
where am i going wrong