How correctly to call a function from another file?

[stampede]

The problem is that you declare a local object in the function's scope:

void DialogForm::on_pbClearLineEdit_clicked()
{
    MainWindow m; // <-- this is allocated on the stack and will be deleted at the end of the scope
    m.clear(); // 
}

void DialogForm::on_pbClearLineEdit_clicked()
{
    MainWindow m; // <-- this is allocated on the stack and will be deleted at the end of the scope
    m.clear(); // 
}

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You should use a signal from the dialog class to inform the MainWindow that the "clear" button was pressed. Declare a signal in your Dialog class and connect it to `MainWindow::clear` slot.