Main:

#include "base.h"
#include <QApplication>
#include <QLabel>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Base w;
    w.show();
    return a.exec();
}

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.

Base.h

#ifndef BASE_H
#define BASE_H
#include <QPushButton>
#include <QMainWindow>
#include <QVBoxLayout>

class Base : public QWidget
{
    Q_OBJECT

public:
    explicit Base(QWidget *parent = 0);
    ~Base();

public slots:
    void AddWidget();

private:
    QVBoxLayout *Vlayout;
};

#endif

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.

Base.cpp

#include "base.h"
#include <QApplication>
#include <QPushButton>
#include <QLabel>
#include <QLayout>
#include <QDialog>
#include <QHBoxLayout>

Base::Base(QWidget *parent)
    : QWidget(parent)
{
    QHBoxLayout* Hlayout=new QHBoxLayout(this);
    Vlayout=new QVBoxLayout(this);

    QPushButton *m=new QPushButton("exit",this);
    QPushButton *l=new QPushButton("+",this);

    Hlayout->addWidget(l);
    Hlayout->addWidget(m);

    Vlayout->addLayout(Hlayout);

    connect(l,SIGNAL(clicked()),this,SLOT(AddWidget()));
    connect(m,SIGNAL(clicked()),qApp,SLOT(quit()));

    m->setSizePolicy(QSizePolicy::Expanding,QSizePolicy::Expanding);
    l->setSizePolicy(QSizePolicy::Expanding,QSizePolicy::Expanding);
}

Base::~Base()
{}

void Base::AddWidget(){

    Vlayout->addWidget(new QLabel("Added",this));
    Vlayout->update();

    /*QDialog *dialog= new QDialog(this);
    QVBoxLayout *t= new QVBoxLayout(dialog);
    t->addWidget(new QLabel("QLabel added successfully",dialog));
    dialog->show();*/
}