Layouts, constructors, QPainter - my bigger project in C++ and Qt

Hi,
I am trying to understand layout systems and QPainter class in better way so I am trying to write my own bigge rproject - fractal generator. In this thread I am going to updating my problems with this project so at the beginning i have some.
Here we have a code:

juliaset.cpp

Code:

#include "juliaset.h"
 
JuliaSet::JuliaSet(QWidget *parent) :
    QWidget(parent),nMax(30), rMax(2.0),W(600),H(600)
{
    this -> setFixedSize(600,600);
    this -> setStyleSheet("background-color:yellow;");
 
    c = complex<double> (-0.61, 0.4);
}
//*****************************************************
void JuliaSet::paintEvent(QPaintEvent *event){
 
    painter = new QPainter(this);
    painter -> setPen(QColor(255,255,0) );
 
    unsigned short int xe,ye;
 
        for(x0=-1.5;x0<=1.5;x0+=0.01)
         {
             for(y0=-1.5;y0<=1.5;y0+=0.01)
              {
                  z = complex<float>(x0,y0);  //change cordinates x and y in complex number
                     for(int k=1;k<=nMax;k++)  //iteration
                      {
                        z = pow(z,2)+c;
                          if(abs(z)>rMax) //disnatnce longer than 2 then stop
                            break;
                         else if(abs(z)<=rMax && k ==nMax) //last iteration and abs(z)<2 then draw pixel
                          {
                            xe = 200*x0 + 300;  //screen coordinates
                            ye = 200*y0 + 300;
                            painter -> drawPoint(xe,ye);
                          }
                       }
                  }
          }
}

mainwindow.cpp

Code:

#include "mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QWidget(parent)
{
    this -> setStyleSheet("background-color: rgb(35, 5, 5);");
 
    unsigned short pos = 0; //help variable for use vector of QLabels and vector of spin boxes
 
    this -> setWindowTitle("Fractal Generator                  v. 1.0");
 
    QGridLayout *grid = new QGridLayout(this);
    grid->setSpacing(5);
 
        for(int j=0; j<3; j++){ //columns
            ptrSpinBox = new QSpinBox();
            ptrSpinBox -> setFixedSize(60,20);
            ptrSpinBox ->setRange(0, 255); //r,g,b value [0;255]
            vctrSpinBoxes.push_back(ptrSpinBox);
            ptrSpinBox = NULL;  //creating spin box and saving into vector
 
       if(j%3 == 0) //first spin box is R, second one is G third one is B
        ptrQLabelRGB = new QLabel("R");
         else if(j%3 == 1)
          ptrQLabelRGB = new QLabel("G");
           else if(j%3 == 2)
            ptrQLabelRGB = new QLabel("B");
 
         ptrQLabelRGB ->setFixedSize(10,20);
         vctrLabels.push_back(ptrQLabelRGB);
         ptrQLabelRGB = NULL; //creating comment R,G,B of spin box and saving into vector
 
         grid -> addWidget(vctrLabels[pos], 5,2*j);
         grid -> addWidget(vctrSpinBoxes[pos], 5, 2*j+1);
         pos++;
        }
 
    spacer = new QSpacerItem(100, 20, QSizePolicy::Minimum, QSizePolicy::Expanding);
    grid -> addItem(spacer,5,6);
 
    ptrFractalWindow = new JuliaSet();
    grid -> addWidget(ptrFractalWindow, 0, 0,5,20);
 
        for(int j=7; j<10; j++){ //columns
            ptrSpinBox = new QSpinBox();
            ptrSpinBox -> setFixedSize(60,20);
            ptrSpinBox ->setRange(0, 255); //r,g,b value [0;255]
            vctrSpinBoxes.push_back(ptrSpinBox);
            ptrSpinBox = NULL;  //creating spin box and saving into vector
 
       if(j%3 == 1) //first spin box is R, second one is G third one is B
        ptrQLabelRGB = new QLabel("R");
         else if(j%3 == 2)
          ptrQLabelRGB = new QLabel("G");
           else if(j%3 == 0)
            ptrQLabelRGB = new QLabel("B");
 
         ptrQLabelRGB ->setFixedSize(10,20);
         vctrLabels.push_back(ptrQLabelRGB);
         ptrQLabelRGB = NULL; //creating comment R,G,B of spin box and saving into vector
 
         grid -> addWidget(vctrLabels[pos], 5,2*j);
         grid -> addWidget(vctrSpinBoxes[pos], 5, 2*j+1);
         pos++;
        }
 
    ptrFractalColor = new QLabel("Fractal Color");
    ptrFractalColor -> setFixedSize(120,20);
    ptrBackgroundColor = new QLabel("Background Color");
    ptrBackgroundColor -> setFixedSize(120,20);
 
    grid -> addWidget(ptrFractalColor, 7,2,1,3);
    grid -> addWidget(ptrBackgroundColor, 7,16,1,3);
 
    generateButton = new QPushButton("Generate");
    generateButton -> setFixedSize(120,50);
 
    grid -> addWidget(generateButton, 8,6,2,5);
 
    realSlider = new QSlider(Qt::Horizontal,0);
    imagSlider = new QSlider(Qt::Horizontal,0);
 
    grid -> addWidget(realSlider, 9,2,1,3);
    grid -> addWidget(imagSlider, 9,16,1,3);
 
    ptrRealPartC = new QLabel("Real(c)");
    ptrImagPartC = new QLabel("Imag(c)");
 
    grid -> addWidget(ptrRealPartC, 10,3,1,3);
    grid -> addWidget(ptrImagPartC, 10,17,1,3);
}
//*************************************************************************
MainWindow::~MainWindow()
{
   for(unsigned short i=0; i<vctrSpinBoxes.size(); i++){  //deleting pointers
       delete vctrSpinBoxes[i];
   }
 
   for(unsigned short i=0; i<vctrLabels.size(); i++){  //deleting pointers
       delete vctrLabels[i];
   }
 
   delete ptrFractalColor;
   delete ptrBackgroundColor;
   delete grid;
}

So I've 2 questions for you.
1) As you can main window is widqet class. Don't worry about setting buttons, spin boxes etc. in my grid layout, I need to fixed it. But there is one thing which makes me crazy - constructor and his size. I was trying to only create object in constructor and define private method createMenu() where I can set them in layout like this:

Code:

void MainWindow::createMenu(){
 grid -> addWidget(generateButton, 8,6,2,5);
}

But it doesnt work when I've used this method in constructor - my program can't run. How to make it more elegant and set this menu in another function?

2) The second one is drawing on QWidget. Is it possible to have Widget with QPainter and other classes which will have draw() method? Or my solution is correct and if I have 4 classes for 4 different fractals I need to derived them from QWidget?

Rafal