My problems with my first example

[tomy]

I want to write a desktop calculator using Qt. I designed the first step of the form this way:

I added only one signal & slot using Designer. The Calc.h, Calc.pp and main.cpp are as follows respectively:

#ifndef CALC_H
#define CALC_H

#include <QDialog>

namespace Ui {
class Calc;
}

class Calc : public QDialog
{
    Q_OBJECT

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

private:
    Ui::Calc *ui;
};

#endif // CALC_H

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

Calc::Calc(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Calc)
{
    ui->setupUi(this);
}

Calc::~Calc()
{
    delete ui;

}

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

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);
    Calc* dialog = new Calc;
    dialog -> show();
    return app.exec();
}

Now what codes do I need to add into Calc.h & Calc.cpp to make it worked please?

[tham]

What would you do if this is an CLI program(command line only, no gui)?

They may looks like these

double a = 0, b = 0;
char operation = '+';
std::cin>>a;
std::cin>>operation;
std::cin>>b;
switch(operation){
  case '+':
     std::cout<<(a+b)<<std::endl;
  break;
  //....and so on
}

How would you make this work if it is gui app?

you can do it as following(pseudo codes)

1 : show the result of your input button

connect(ui->oneButton, &QPushButton::clicked, [this]()
{
    ui->lineEdit->setText(ui->lineEit()->text() + "1");
});

2 : store the result of lineEdit when user click on the add button

connect(ui->addButton, &QPushButton::clicked, [this]()
{
    firstNum = ui->lineEdit->text().toDouble();
});

3 : show the result of your input button

4 : show the result of summation when user click on "=" button

connect(ui->resultButton, &QPushButton::clicked, [this]()
{
    auto const curNum = ui->lineEdit->text().toDouble();
    ui->lineEdit->setText(QString::number(firstNum + curNum));
});

These are the simplest way I could think of. There are many things to improve, like support other number, operation, able
to parse long equation(ex : 3 + 4 - 5 * 6 / 33.1), error handles etc.

Something related to resource managament.

This will cause memory leak

Calc* dialog = new Calc;
dialog -> show();

Use it like this(best solution in most cases)

Calc dialog;
dialog.show();

or(not as good as first solution)

std::unique_ptr<Calc> dialog(new Calc); //c++14 support make_unique
dialog -> show();

I strongly suggest you study the concept of RAII and understand the concept of parent--child relationship of Qt, they could
make your life much more easier to live. After you know how they work, you will find out c++ is more easier to manage resource than those languages with gc(like java, c#, js, python etc).

ps : I prefer to create the ui by handcrafting codes when I studied Qt codes, because this help me know better about Qt.
At that time I even do not know what is widget, dialog, mainWindow, what they use for.

[Jeronimo]

@tham parent--child relationship of Qt. I am trying to understand better this and apply like him. But this is not different of other languages. Friendship and inheritance basically.

[tham]

@Jeronimo said in My problems with my first example:

@tham parent--child relationship of Qt. I am trying to understand better this and apply like him. But this is not different of other languages. Friendship and inheritance basically.

Looks like I did not express it clearly, I should said "Object trees & OwnerShip". It is not about the concept of oop but the memory management solution of Qt.

Links :

Object trees & OwnerShip
Parent child relationship in Qt