I need some exercises for each chapter

mrjj

Hi

According to C++ conventions what I wrote seems quite right:

connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(setText("1")));
This is wrong for the SLOT macro.

The new code you show is correct.
connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot(bool))); << no "1" just the type

void Calculator::myslot(bool){
lineEdit -> setText("1"); // correct. for normal Qt , it would be ui->lineEdit->setText.
}

jsulm

@tomy You should check what connect(...) returns. My guess is that the connection fails because in the connect you have:

SLOT(myslot(bool))

but your slot does not have any parameter:

void Calculator::myslot()

So, connect(...) cannot find slot myslot(bool)

tomy

Thanks to all.
@jsulm
So apparently you disagree with mrjj. I write what he says but the code shows nothing. I think your assumption is correct because myslot() has no parameters.

Now the question is this: How to connect the buttons (1, 2 & +) to a slot so that, that slot prints out the values of these buttons onto the lineEdit please?

Based on what I'd learned form C++ I thought the code below would work for that purpose:

in the Calculator.h:

...
private slots:
   void myslot(const QString&);
};

And in Calculator.cpp:

...
connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot("1")));
 connect(twoButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot("2")));
 connect(addButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot("+")));
}

void Calculator::myslot(const QString& str){
 lineEdit -> setText(str);
}

But this does not work. I have a good experience in C++ but I'm a novice in Qt.
Now what would be your solution for that purpose please?

jsulm

@tomy Where did I disagree with @mrjj?
You should read more carefully what I wrote.
The problem with your code is that you provide a wrong slot signature in connect. Again:

SLOT(myslot(bool))

there is no such slot! Your slot has this signature (no bool parameter):

void Calculator::myslot()

So, either change

SLOT(myslot(bool))
to
SLOT(myslot())

or

void Calculator::myslot()
to
void Calculator::myslot(bool)

tomy

@jsulm

OK & thanks a bunch but still it doesn't work!

tomy

I appreciate your time that you all dedicate to help me but is it that hard to write a function that has a string parameter so that when we call it by strings it prints out the sent string onto the lineEdit?

jsulm

@tomy If you say "does not work" you should say what does not work or what happens.
You should really read about signals/slots.

You could use lambdas (C++11) and new connect syntax like this:

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

tomy

This post is deleted!

tomy

I meant using neither:

connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot("1")));

void Calculator::myslot(const QString& str){
lineEdit -> setText(str);
}
Nor:

connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot(bool)));

void Calculator::myslot(bool){
lineEdit -> setText("1");
}
And Nor:

connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot()));

void Calculator::myslot(){
lineEdit -> setText("1");
}
the program works!

That is when I each time using the above methods run the code and click on the buttons, nothing will be printed out onto the lineEdit, that is clicking the buttons do nothing.

Now you say I should read about:
lambdas (C++11) and new connect syntax like this:
connect(oneButton, &QPushButton::clicked, { lineEdit->setText("1"); });

BUT THESE ARE VERY ADVANCED FOR ME. AT THE BEGINING OF THIS THREAD I SAID I NEED SOME PROPER EXERCISE FOR WHAT I HAVE LEARNED THROUGH THESE 2 CHAPTERS OF THAT BOOK, NOT ADVANCED TOPICS THAT ARE IN THE NEXT CHAPTERS OF THE BOOK.

jsulm

@tomy I already suggested to check what connect(...) returns to see whether it works or not. Did you try it?

qDebug() << connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot(bool)));

it should print true if it succeeded.

This connect call is wrong:

connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot(bool)));

lineEdit is QLineEdit, right? QLineEdit does not have a slot named "myslot(bool)". This slot is in your Calculator class, right?
Then it should be:

connect(oneButton,SIGNAL(clicked(bool)), this, SLOT(myslot(bool)));

connect() will print a warning if it cannot do the connection - you can check "Application output" tab in QtCreator.
Furthermore you can try to debug: set a break point inside the slot run your app with debugger (F5 in QtCreator), press the button and see whether the slot is called or not.

You wrote "I have a good experience in C++", so I assumed you're familiar with lambdas as they were introduced in C++11. And in your current use case they are very handy.

tomy

@jsulm said in I need some exercises for each chapter:

@tomy I already suggested to check what connect(...) returns to see whether it works or not. Did you try it?
qDebug() << connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot(bool)));
it should print true if it succeeded.

This is the Calculator.cpp now:

#include <QtWidgets>
#include "calculator.h"

Calculator::Calculator(QWidget *parent)
    :QDialog(parent)
{
    setupUi(this);

   qDebug() << connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot()));
}

void Calculator::myslot(){
    lineEdit -> setText("1");
}

I ran it (Ctrl+R) but it show no errors and runs successfully. How to know what it has printed?

This connect call is wrong:
connect(oneButton,SIGNAL(clicked(bool)), lineEdit, SLOT(myslot(bool)));
lineEdit is QLineEdit, right? QLineEdit does not have a slot named "myslot(bool)". This slot is in your Calculator class, right?
Then it should be:
connect(oneButton,SIGNAL(clicked(bool)), this, SLOT(myslot(bool)));

OK, I changed the connect to:

connect(oneButton,SIGNAL(clicked(bool)), this, SLOT(myslot()));

But no changes again.

connect() will print a warning if it cannot do the connection - you can check "Application output" tab in QtCreator.

Application output is in the bottom, I checked it in both cases (above). There is no warning message there.

Furthermore you can try to debug: set a break point inside the slot run your app with debugger (F5 in QtCreator), press the button and see whether the slot is called or not.

I also tried this option. I pressed F5 for the two above cases. In the Application output this has been printed:
Debugging starts
Debugging has finished

You wrote "I have a good experience in C++", so I assumed you're familiar with lambdas as they were introduced in C++11. And in your current use case they are very handy.

Yes, I have a good experience in C++ but it's not 11. But do you the book I read for Qt? Do you know at what level (of Qt) is the reader of it?
I've passed the first two chapters and can start chapter 3, but, I don't like to deceive myself, rather, I want to examine myself to see whether I have learned the contents good enough or not.

jsulm

@tomy said in I need some exercises for each chapter:

connect(oneButton,SIGNAL(clicked(bool)), this, SLOT(myslot()));

You should print the connect() return value:

qDebug() << connect(oneButton,SIGNAL(clicked(bool)), this, SLOT(myslot()));

The output should be in "Application Output" tab in QtCreator when you start the application.

To debug you need first to set a breakpoint inside your slot and then start with F5, then press the button.

Another thing to check is whether "oneButton" is really the button you see when you start your application. Usually all UI objects are accessed via ui->oneButton. But you decided to do it differently. Since I don't have your complete code I cannot check this.

"But do you the book I read for Qt? Do you know at what level (of Qt) is the reader of it?" - no I don't. I just try to help you. Lambdas are not Qt, they are pure C++11. The only new Qt thing for you is the new connect() syntax. In my opinion it is not wrong to learn something new during exercises even if it was not yet covered by the book. But if you don't want to then just ignore the suggested lambda based solution.

tomy

First I know you want to help me and for that I appreciate you and thank you.

You also are right about learning new things during solving a given exercise but I think the way is improper. It's probably my fault either.

Anyway, we all (in this thread) want to solve the issue. I have a suggestion! Design a simplest form that is possible to do a single work please. That is, a form with one button and one lineEdit, when you run it and click on the button it will be printed on the lineEdit.
To sum up, a form named (say) "test" with a test.h, test.cpp and a main.cpp file in the most simple style/method (that is, suitable for low level beginner of Qt).

I think that way most of the issues will be solved.

jsulm

@tomy This works for me just fine.
ButtonEdit.pro file:

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = ButtonEdit
TEMPLATE = app


SOURCES += main.cpp\
        mainwindow.cpp

HEADERS  += mainwindow.h

FORMS    += mainwindow.ui

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private slots:
    void mySlot();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    connect(ui->pushButton, SIGNAL(clicked(bool)), this, SLOT(mySlot()));
}

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

void MainWindow::mySlot()
{
    ui->lineEdit->setText("+");

main.cpp

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

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

    return a.exec();
}

mainwindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <widget class="QPushButton" name="pushButton">
    <property name="geometry">
     <rect>
      <x>10</x>
      <y>10</y>
      <width>99</width>
      <height>27</height>
     </rect>
    </property>
    <property name="text">
     <string>+</string>
    </property>
   </widget>
   <widget class="QLineEdit" name="lineEdit">
    <property name="geometry">
     <rect>
      <x>10</x>
      <y>50</y>
      <width>113</width>
      <height>27</height>
     </rect>
    </property>
   </widget>
  </widget>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

tomy

@jsulm
Thank you. Would you please take a screenshot of the design page (mainwindow.ui) and send it?

jsulm

@tomy I could, but there is nothing special: just a button and a line edit bellow the button.

tomy

It couldn't help. But I thank you and appreciate your time. The thread is finished.