How to Add Number to Outher Like 1 & 1 = 11 In QT C++

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Hi Guys
I'm New In QT And C++.
I Design Calculator Using C++ And QT
I Using QLcdNumber In QT And I Want To Know How I Can Add Number To Outher Every Click On Button.
Ex:
I Press Button 1
And Then I Press Button 2
Then QLcdNumber = 12
I Try To Use The + But It Calculate Them.

This My Code In Button 1 ( Event Clicked )

const int one = 1;
  int newvalue;

  if (ui->viewlcdnumber->value() == 0)
  {
      ui->viewlcdnumber->display(one);
  }
  else
  {
      newvalue = one + ui->viewlcdnumber->value();
      ui->viewlcdnumber->display(newvalue);
  }