The most requested next C++ app you guys asked for was a calculator, so here it is. We’ll make a C++ GUI Calculator app in one video. Along the way we’ll learn a lot! We’ll cover a topic which confuses many people which is how to setup event handling with Signals and Slots. We’ll also cover how to work with widgets in many ways, setting up an interface, stylesheets, casting, regular expressions and much more.

All of the heavily commented code follows the video below.

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Code from the Video

DESIGN

Drop QLineEdit in window -> Name it Display -> Preferred & Expanding
Right Click Window -> Layout -> Layout in Grid 
Create QPushButtons 5 wide and 4 deep -> Preferred & Preferred
7 8 9 / M+
4 5 6 * M-
1 2 3 + M
AC 0 +/- - =
Name them Button0 - Button9, Add, Subtract, Multiply, Divide, Equals, ChangeSign, Clear, MemAdd, MemClear, MemGet
Line Edit Stylesheet
QLineEdit {
     background-color: gray; border: 1px solid gray;
	color: #ffffff;}
10. PushButton Stylesheet
QPushButton {
     background-color: #C0C0C0; border: 1px solid gray;
	border-radius: 0px;
	padding: 5px;
}
QPushButton:pressed {
    background-color: #A9A9A9; border: 1px solid gray;
	border-radius: 0px;
	padding: 5px;
}

————— CALC.H —————

#ifndef CALC_H
#define CALC_H

#include <QMainWindow>

// Use the standard UI namespace which is tied to the .ui file
namespace Ui {
class Calc;
}

class Calc : public QMainWindow
{
    // Declares our class as a QObject which is the base class
    // for all Qt objects
    // QObjects handle events
    Q_OBJECT

public:
    
    // Declare a constructor and by passing 0 we state this widget
    // has no parent
    explicit Calc(QWidget *parent = 0);
    ~Calc();

private:
    Ui::Calc *ui;

    // These slots are executed when a signal is
    // submitted (Ex. Number button is clicked)
private slots :
    void NumPressed();
    void MathButtonPressed();
    void EqualButtonPressed();
    void ChangeNumberSign();
};

#endif // CALC_H

————— END CALC.H —————


————— CALC.CPP —————

#include "calc.h"
#include "ui_calc.h"

// Holds current value of calculation
double calcVal = 0.0;

// Will define if this was the last math button clicked
bool divTrigger = false;
bool multTrigger = false;
bool addTrigger = false;
bool subTrigger = false;

// Constructor
Calc::Calc(QWidget *parent) :

    // Call the QMainWindow constructor
    QMainWindow(parent),

    // Create the UI class and assign it to the ui member
    ui(new Ui::Calc)
{

    // Setup the UI
    ui->setupUi(this);

    // Put 0.0 in Display
    ui->Display->setText(QString::number(calcVal));

    // Will hold references to all the number buttons
    QPushButton *numButtons[10];

    // Cycle through locating the buttons
    for(int i = 0; i < 10; ++i){
        QString butName = "Button" + QString::number(i);

        // Get the buttons by name and add to array
        numButtons[i] = Calc::findChild<QPushButton *>(butName);

        // When the button is released call num_pressed()
        connect(numButtons[i], SIGNAL(released()), this,
                SLOT(NumPressed()));
    }

    // Connect signals and slots for math buttons
    connect(ui->Add, SIGNAL(released()), this,
            SLOT(MathButtonPressed()));
    connect(ui->Subtract, SIGNAL(released()), this,
            SLOT(MathButtonPressed()));
    connect(ui->Multiply, SIGNAL(released()), this,
            SLOT(MathButtonPressed()));
    connect(ui->Divide, SIGNAL(released()), this,
            SLOT(MathButtonPressed()));

    // Connect equals button
    connect(ui->Equals, SIGNAL(released()), this,
            SLOT(EqualButtonPressed()));

    // Connect change sign
    connect(ui->ChangeSign, SIGNAL(released()), this,
            SLOT(ChangeNumberSign()));


}

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

void Calc::NumPressed(){

    // Sender returns a pointer to the button pressed
    QPushButton *button = (QPushButton *)sender();

    // Get number on button
    QString butVal = button->text();

    // Get the value in the display
    QString displayVal = ui->Display->text();

    if((displayVal.toDouble() == 0) || (displayVal.toDouble() == 0.0)){

        // calcVal = butVal.toDouble();
        ui->Display->setText(butVal);

    } else {
        // Put the new number to the right of whats there
        QString newVal = displayVal + butVal;

        // Double version of number
        double dblNewVal = newVal.toDouble();

        // calcVal = newVal.toDouble();

        // Set value in display and allow up to 16
        // digits before using exponents
        ui->Display->setText(QString::number(dblNewVal, 'g', 16));

    }
}

void Calc::MathButtonPressed(){

    // Cancel out previous math button clicks
    divTrigger = false;
    multTrigger = false;
    addTrigger = false;
    subTrigger = false;

    // Store current value in Display
    QString displayVal = ui->Display->text();
    calcVal = displayVal.toDouble();

    // Sender returns a pointer to the button pressed
    QPushButton *button = (QPushButton *)sender();

    // Get math symbol on the button
    QString butVal = button->text();

    if(QString::compare(butVal, "/", Qt::CaseInsensitive) == 0){
        divTrigger = true;
    } else if(QString::compare(butVal, "*", Qt::CaseInsensitive) == 0){
        multTrigger = true;
    } else if(QString::compare(butVal, "+", Qt::CaseInsensitive) == 0){
        addTrigger = true;
    } else {
        subTrigger = true;
    }

    // Clear display
    ui->Display->setText("");

}

void Calc::EqualButtonPressed(){

    // Holds new calculation
    double solution = 0.0;

    // Get value in display
    QString displayVal = ui->Display->text();
    double dblDisplayVal = displayVal.toDouble();

    // Make sure a math button was pressed
    if(addTrigger || subTrigger || multTrigger || divTrigger ){
        if(addTrigger){
            solution = calcVal + dblDisplayVal;
        } else if(subTrigger){
            solution = calcVal - dblDisplayVal;
        } else if(multTrigger){
            solution = calcVal * dblDisplayVal;
        } else {
            solution = calcVal / dblDisplayVal;
        }
    }

    // Put solution in display
    ui->Display->setText(QString::number(solution));

}

void Calc::ChangeNumberSign(){

    // Get the value in the display
    QString displayVal = ui->Display->text();

    // Regular expression checks if it is a number
    // plus sign
    QRegExp reg("[-+]?[0-9.]*");

    // If it is a number change the sign
    if(reg.exactMatch(displayVal)){
        double dblDisplayVal = displayVal.toDouble();
        double dblDisplayValSign = -1 * dblDisplayVal;

        // Put solution in display
        ui->Display->setText(QString::number(dblDisplayValSign));
    }

}


————— END CALC.CPP —————


————— MAIN.CPP —————

#include "calc.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Calc w;
    w.show();

    return a.exec();
}

————— END MAIN.CPP —————