Child object not showing

[yogeshgokul]

Create the instance of SM in heap. Currently you are creating in stack so its deleting immediately.
Or you can call exec() instead of show();

So now your code should look like this:

mm::mm(QWidget *parent)
        : QDialog(parent)
{
    QHBoxLayout *layout=new QHBoxLayout;
    QPushButton *p = new QPushButton("MM");    
    connect(p,SIGNAL(clicked()),this,SLOT(showMM()));
    layout->addWidget(p);
    setLayout(layout);
    setWindowTitle("MainMenu");
    show(); 
    sm *s = new sm(); // This line is changed
}

mm::mm(QWidget *parent)
        : QDialog(parent)
{
    QHBoxLayout *layout=new QHBoxLayout;
    QPushButton *p = new QPushButton("MM");    
    connect(p,SIGNAL(clicked()),this,SLOT(showMM()));
    layout->addWidget(p);
    setLayout(layout);
    setWindowTitle("MainMenu");
    show(); 
    sm *s = new sm(); // This line is changed
}

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