Inherited class is not shown - c++

Question

I have 2 classes: first of them is inheritor of QWidget and the second one is inheritor of the first class. When i launch my program i get 2 windows of first class. Why? And another question. Second::Second(QWidget *pwgt): First(pwgt) - is this string correct? i.e. should i send pwgt to constructor of the first class?
firstclass.h
#ifndef FIRSTCLASS_H
#define FIRSTCLASS_H
#include <QtGui>
class First: public QWidget
{
Q_OBJECT
protected:
QLabel *firstText;
QPushButton *firstButton;
public:
First(QWidget *pwgt = 0);
};
#endif // FIRSTCLASS_H
firstclass.cpp
#include "firstclass.h"
First::First(QWidget *pwgt)
{
firstText = new QLabel("First Class Text");
firstButton = new QPushButton("First Class Button");
QVBoxLayout *lay = new QVBoxLayout;
lay->addWidget(firstText);
lay->addWidget(firstButton);
this->setLayout(lay);
}
secondclass.h
#ifndef SECONDCLASS_H
#define SECONDCLASS_H
#include <QtGui>
#include "firstclass.h"
class Second: public First
{
Q_OBJECT
private:
QLabel *secondText;
QPushButton *secondButton;
public:
Second(QWidget *pwgt = 0);
public slots:
void changeText();
};
#endif // SECONDCLASS_H
secondclass.cpp
#include "secondclass.h"
Second::Second(QWidget *pwgt): First(pwgt)
{
secondText = new QLabel("Second Class Text");
secondButton = new QPushButton("Second Class Button");
QVBoxLayout *lay = new QVBoxLayout;
lay->addWidget(secondText);
lay->addWidget(secondButton);
connect(secondButton, SIGNAL(clicked()), this, SLOT(changeText()));
this->setLayout(lay);
}
void Second::changeText()
{
firstText->setText("From second class");
}
main.cpp
#include "firstclass.h"
#include "secondclass.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
First first;
Second second;
first.show();
second.show();
return a.exec();
}

Answer 1

The problem is that you set the layout with setLayout(lay); in the First class' constructor. This is correct, especially if you create objects of the First class. However when you create objects of the Second class, the First class constructor still called. In the following code:
First first; // Calls the First class constructor
Second second; // Calls both First and Second class constructors
As Qt documentation states that
If there already is a layout manager installed on this widget, QWidget won't let you install another.
This means that once you set the layout in the First class constructor, the attempt to set it again in the Second class ignored. As a result you see the first layout in both cases.
WRT your second question: yes, usually you have to pass the argument to the base class constructor. You even had to pass the parent to the QWidget class in your First class constructor:
First::First(QWidget *pwgt) : QWidget(pwgt) {}

Answer 2

You call setLayout twice on your second widget - once in the First constructor and then again in the Second constructor. But the second one will be ignored: http://qt-project.org/doc/qt-4.8/qwidget.html#setLayout
Why are you inheriting like this anyway, if they're supposed to look completely different.