Calculator on Qt C++. Problem with QString and delete.

[Zloi]

Hi! I'm new in C++ programming. THis is my First GUI application. It's just a very simple Calculator.

 #include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QString>
#include <QMessageBox>
#include<iostream>
using namespace std;
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}


double *a=new double;
double *b=new double;//
double *c=new double;//

void MainWindow::on_pushButton_clicked()
{
    ui->lineEdit->setText(ui->lineEdit->text()+"1");
}

void MainWindow::on_pushButton_2_clicked()
{
ui->lineEdit->setText(ui->lineEdit->text()+"2");
}

void MainWindow::on_pushButton_3_clicked()
{
    ui->lineEdit->setText(ui->lineEdit->text()+"3");
}

void MainWindow::on_pushButton_4_clicked()
{
ui->lineEdit->setText(ui->lineEdit->text()+"4");
}

void MainWindow::on_pushButton_5_clicked()
{
   ui->lineEdit->setText(ui->lineEdit->text()+"5");
}

void MainWindow::on_pushButton_6_clicked()
{
ui->lineEdit->setText(ui->lineEdit->text()+"6");
}

void MainWindow::on_pushButton_8_clicked()
{
     ui->lineEdit->setText(ui->lineEdit->text()+"8");
}

void MainWindow::on_pushButton_7_clicked()
{
     ui->lineEdit->setText(ui->lineEdit->text()+"7");
}

void MainWindow::on_pushButton_9_clicked()
{
ui->lineEdit->setText(ui->lineEdit->text()+"9");
}

void MainWindow::on_pushButton_10_clicked()
{
     ui->lineEdit->setText(ui->lineEdit->text()+"0");
}

void MainWindow::on_pushButton_11_clicked()
{
     ui->lineEdit->setText(ui->lineEdit->text()+"+");
}

void MainWindow::on_pushButton_12_clicked()
{
    ui->lineEdit->setText(ui->lineEdit->text()+"-");
}

void MainWindow::on_pushButton_13_clicked()
{
ui->lineEdit->setText(ui->lineEdit->text()+"*");}

void MainWindow::on_pushButton_14_clicked()
{
  ui->lineEdit->setText(ui->lineEdit->text()+"/");
}

void MainWindow::on_pushButton_15_clicked()

{
    int pos;

string sf, s,s1,s0;

QString str=ui->lineEdit->text();//Getting text from a Lineedit.
pos=(str.indexOf("+",Qt::CaseInsensitive)||(str.indexOf("-",Qt::CaseInsensitive))||(str.indexOf("*",Qt::CaseInsensitive))||(str.indexOf("/",Qt::CaseInsensitive))); //Finding a position of operator.
s=str.toStdString();//Converting QString to a transitional string variable;
sf=s.substr(0,pos+1);
QString st;
st=st.fromStdString(sf);
*a=st.toDouble();
QString sd,st0;
s1=s.substr(pos+1,s.length());
sd=sd.fromStdString(s1);
*b=sd.toDouble();
if (str.contains('+',Qt::CaseInsensitive)){
     *c=*a+*b;
  QString st1=QString::number(*c);
    ui->lineEdit->setText(ui->lineEdit->text()+"= "+st1);

}
else if(str.contains('-',Qt::CaseInsensitive)){
    *c=*a-*b;
  QString st1=QString::number(*c);

            ui->lineEdit->setText(ui->lineEdit->text()+"= "+st1);
}

else if(str.contains('*',Qt::CaseInsensitive))  {
    *c= *a * *b;
    QString st1=QString::number(*c);
            ui->lineEdit->setText(ui->lineEdit->text()+"= "+st1);}
else if(str.contains('/',Qt::CaseInsensitive)) {
  *c=*a/ *b;
  QString st1=QString::number(*c);
       ui->lineEdit->setText(ui->lineEdit->text()+"= "+st1);}


    }

void MainWindow::on_pushButton_16_clicked()
{
    if (!(ui->lineEdit->text().isEmpty())){
ui->lineEdit->setText("");
    }
}

Above is a whole mainwindow.cpp file. Here I got 2 problemes
1) The data, which I get from lineEdit is a QString. But function like substr can not be used to a QString. Is there some similar operation with QString?
2) When I'm tryieng to delete a b and c in here like this

void MainWindow::on_pushButton_16_clicked()
{delete a;
delete b;
delete c;
    if (!(ui->lineEdit->text().isEmpty())){
ui->lineEdit->setText("");
    }

THen the first result is correct, but when I press the pushButton_16 and try to input the next equation, the compiler sys me, that there's an error :
The program has unexpectedly finished.
/home/alexis/Документы/Qt/Projects/build-MyCalculator-Desktop-Release/MyCalculator crashed
What did I wrong?
Once more, excuse me for grammar mistakes )))

[kbw]

1) The data, which I get from lineEdit is a QString. But function like substr can not be used to a QString. Is there some similar operation with QString?

QString::section: http://qt-project.org/doc/qt-5/qstring.html

2) When I'm tryieng to delete a b and c in here like this ...

You're making a rod for your own back.

Instead of doing,

double *a=new double;
double *b=new double;//
double *c=new double;// 

do:

double a;
double b;
double c;

Plus, they should be local variables, not globals.