How to use methods of an object from a different class?

[bask185]

I have a serial object in my mainwindow.cpp in which I can succesfully write and send bytes to an arduino.

I can open a new window with a keyboard:

This keyboard is ofcourse made in a different class. If I press a button like the 'a'. I want to serial.write("a"); But as you can guess it. I cannot call any serial method because the serial object only exists in the scope of the main window.

I am happy with 1 of the following 2 solutions: I either want to be able to call serial methods in the keyboard class ór I want to call a function in the mainwindow class from out the keyboard class with which I send a character or string as to be written arguement.

I do not know how do to any of this. So how can I fix this?

[bask185]

@J.Hilk said in How to use methods of an object from a different class?:

@bask185 said in How to use methods of an object from a different class?:

Keyboard *keyboard;

I think I found the mixup,

You have Keyboard as a priavte member of your mainwindow.h, as it should be, and as a global Variable too.

I think the instance of your keyboard, that you show and have input of, is not the one you connected your Signal from.

Change that.

Application Output:  hello world   "a"   // we have a winner!
                     hello world  "A" // < caps works too :D

My thank is great Y'all

@jsulm said in How to use methods of an object from a different class?:

I guess you want to send the string/character entered by user over serial bus, right?

was it that obvious :P?

In that case you should change
serial->write("c");

to
serial->write(c.toLatin1()); // If you only use ASCII characters

Application Output:  "\x06\x04\x01""a"  // < proper respons of the arduino for receiving the 'a'. Tnx ;)

sigh though... in processing or arduino or whatever I usually send.. just integers or longs. Vague Qt syntax with & * :: private public and what if I want to send a long variable over serial port would I need to characterize all 10 numbers and send 10 bytes instead of 4????

@Eeli-K said in How to use methods of an object from a different class?:

@bask185 I didn't know you know so little about C++ and Qt. It's true that C++ is in many ways a low-level language. Qt takes it a bit higher, but certainly it's still means more work for certain things than many other languages. But on the other hand C++ is on the most popular and used general purpose languages in the world, and unlike with processing, you can call yourself a real programmer if you master C++ and Qt. Every language has its place and has pros and cons. If you want to just get one program done in your life C++ isn't the right language for you, otherwise it pays back what you put into learning it.

we had a few lessons in C++ in my first year in visual studios but even there we would not use :: privates and publics, (the latter 2 we got with java). And as said before I only learned how to use pointers with arrays and not with functions.

But regardless, I almost have my bachelor degree in electrical engineering and learning a new programming language from scrap is what we do.

[bask185]

The code:
Keyboard class

#include "keyboard.h"
#include "ui_keyboard.h"
#include <QtSerialPort>
#include "mainwindow.h"


bool caps = true, shift = true;

Keyboard::Keyboard(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Keyboard)

{
    ui->setupUi(this);
}

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

void Keyboard::on_a_clicked()
{
   //  mainwindow::send('a');  //  or
   //  serial.write("a");
}

MainWindow:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPixmap>
#include <QDebug>
#include <QPalette>
#include <QtSerialPort>
#include "keyboard.h"
QSerialPort *serial;
void readCommand();

char lettres[5][40][16];     // initializes all to be used variables;
int index1 = 0;
int index2 = 0;
int colom[5];
int row[5];
int colorIndex[5];

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    serial = new QSerialPort();
    serial->setPortName("COM4");
    serial->open(QIODevice::ReadWrite);
    serial->setBaudRate(QSerialPort::Baud115200);
    serial->setDataBits(QSerialPort::Data8);
    serial->setParity(QSerialPort::OddParity);
    serial->setStopBits(QSerialPort::OneStop);
    //serial.setFlowControl(QSerialPort::NoFlowControl);
    //serial.open(QIODevice::ReadWrite);

    QPixmap logo(":/resources/img/logo.jpg");
    int w = ui->label->width();
    int h = ui->label->height();
    ui->label->setPixmap(logo.scaled(w,h,Qt::KeepAspectRatio));

    for(int i=0;i<5;i++) {
        for (int j=0;j<40;j++) {
            for (int k=0;k<16;k++){
                lettres[i][j][k] =' ';   // initializes the letters array
            }
        }
    }
   connect(serial, &QSerialPort::readyRead, this, &MainWindow::serialReceived);
}

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


void MainWindow::serialReceived()
{
    QByteArray b = serial->readAll();
    qDebug() << b;
}

void MainWindow::send(char c)    // send function which I'd like to call from Keyboard
{
  serial->write("c");
  qDebug() << c;
}

[Eeli K]

The two classes are unnecessarily tightly coupled, as far as I can see the keyboard class doesn't have to know the main window. Just write a signal in it, emit it in on_button_clicked() and connect it in the mainwindow to the slot mainwindow::send (which of course must be a slot).

[bask185]

@Eeli-K said in How to use methods of an object from a different class?:

Just write a signal in it, emit it in on_button_clicked() and connect it in the mainwindow to the slot mainwindow::send

?? and now in English??

I have this in mainwindow

void MainWindow::send(char c)
{
  connect(keyboard, &Keyboard::on_a_clicked(), this &MainWindow::send(c));
  serial->write("c"); // I am aware that this does not send the value of variable c just "c"
  qDebug() << c;
}

and in Keyboard I do

void Keyboard::on_a_clicked()
{
   QString c;
   c = ui->a->text();
   MainWindow::send('a');
}

in main window I get 2 errors

C:\Users\sknippels\Documents\GUI\mainwindow.cpp:65: error: cannot call member function 'void Keyboard::on_a_clicked()' without object
   connect(keyboard, &Keyboard::on_a_clicked(), this &MainWindow::send(c));
                                             ^

and

C:\Users\sknippels\Documents\GUI\mainwindow.cpp:65: error: invalid operands of types 'MainWindow*' and 'void' to binary 'operator&'
   connect(keyboard, &Keyboard::on_a_clicked(), this &MainWindow::send(c));
                                                                        ^

I understand less than nothing of this vague syntax. And I also never use pointers, I would only use a pointer for use with arrays but because there are those things called for-loops... I just don't do pointers.

EDIT. I made some changes, I also noticed a typo. I move this line to the constructor (and removed the typo)

connect(keyboard, &Keyboard::on_a_clicked, this, &MainWindow::send);

and now I have 3 error messages in qobjectdefs_impl.h

C:\Qt\5.8\mingw53_32\include\QtCore\qglobal.h:732: error: static assertion failed: The slot requires more arguments than the signal provides.
 #define Q_STATIC_ASSERT_X(Condition, Message) static_assert(bool(Condition), Message)
                                               ^

C:\Qt\5.8\mingw53_32\include\QtCore\qglobal.h:732: error: static assertion failed: Signal and slot arguments are not compatible.
 #define Q_STATIC_ASSERT_X(Condition, Message) static_assert(bool(Condition), Message)
                                               ^

C:\Qt\5.8\mingw53_32\include\QtCore\qobjectdefs_impl.h:76: error: no type named 'Car' in 'struct QtPrivate::List<>'
         typedef typename List_Append<List<typename L::Car>,typename List_Left<typename L::Cdr, N - 1>::Value>::Value Value;
                                                                                                                      ^

For me this is total Chinese :(

[J.Hilk]

@bask185
hi,

I'll try to help:

Your custom Keyboard, as it is made in a new class, has to be initialized somewhere.

I asume something like this in your MainWindow constructor:

//Something like this
Keyboard kBoard;

// or
Keyboard *kBoard = new Keyboard();

// or 
Keyboard *kBoard = new Keyboard(anyParent);

kBoard is in this case the object.

in your KeyBorad header you need a SIGNAL

signals:
void keyPressed(QString key);

change your on_click function:

void Keyboard::on_a_clicked()
{
   QString c;
   c = ui->a->text();
   emit KeyPressed(c);
}

Move

connect(keyboard, &Keyboard::on_a_clicked(), this &MainWindow::send(c));

from the SLOT send(c) to right behind the object creation and change it to the following to remove the error:

Keyboard *kBoard = new Keyboard();
connect(keyboard, &Keyboard::KeyPressed, this &MainWindow::send);

and you should be done :)

[bask185]

@J.Hilk said in How to use methods of an object from a different class?:

@bask185
hi,

I'll try to help:

Your custom Keyboard, as it is made in a new class, has to be initialized somewhere.

I asume something like this in your MainWindow constructor:

//Something like this
Keyboard kBoard;

// or
Keyboard *kBoard = new Keyboard();

// or 
Keyboard *kBoard = new Keyboard(anyParent);

kBoard is in this case the object.

in your KeyBorad header you need a SIGNAL

signals:
void keyPressed(QString key);

change your on_click function:

void Keyboard::on_a_clicked()
{
   QString c;
   c = ui->a->text();
   emit KeyPressed(c);
}

Move

connect(keyboard, &Keyboard::on_a_clicked(), this &MainWindow::send(c));

from the SLOT send(c) to right behind the object creation and change it to the following to remove the error:

Keyboard *kBoard = new Keyboard();
connect(keyboard, &Keyboard::KeyPressed, this &MainWindow::send);

and you should be done :)

Tnx I am closer but I got one error message left.
in

Keyboard *keyboard = new Keyboard();
   connect(keyboard, &Keyboard::KeyPressed, this &MainWindow::send);

I get

C:\Users\sknippels\Documents\GUI\mainwindow.cpp:52: error: invalid use of member (did you forget the '&' ?)
    connect(keyboard, &Keyboard::KeyPressed, this &MainWindow::send);
                                                               ^

it complains about the & eventhough it is there.. :/

[J.Hilk]

@bask185

mmh,
did you declare send as a slot in your mainwindow cpp?

private slots:
//public slots:
     void send(QString s);

Edit:

just noticed, your Send-Function wants a char and you send a QString that will most likly not work.

change one or the other. I would suggest chaning the Mainwindow::Send to accept QString, makes it easier.

[jsulm]

@bask185 said in How to use methods of an object from a different class?:

Keyboard *keyboard = new Keyboard();
connect(keyboard, &Keyboard::KeyPressed, this &MainWindow::send);

Aren't you missing a comma?

Keyboard *keyboard = new Keyboard();
   connect(keyboard, &Keyboard::KeyPressed, this, &MainWindow::send);

[jsulm]

@J.Hilk said in How to use methods of an object from a different class?:

//Something like this
Keyboard kBoard;

Please do NOT do this in the constructor: kBoard would be a local variable and would disappear when the constructor finishes!

[J.Hilk]

@jsulm @bask185
Oh, sorry about that.
ApparentlyI missed that when I wrote my answer.

I tried to cover all bases, just in case, but that's an error my from my part.

[bask185]

@jsulm said in How to use methods of an object from a different class?:

Aren't you missing a comma?

That is what I thought as well, but no. Adding comma will give errors

@jsulm said in How to use methods of an object from a different class?:

Please do NOT do this in the constructor: kBoard would be a local variable and would disappear when the constructor finishes!

at first I used Keyboard *keyboard = new Keyboard(); with those strange pointer things. Now I declare keyboard with the rest of my variables and I initialize it in the constructor.

@J.Hilk said in How to use methods of an object from a different class?:

ust noticed, your Send-Function wants a char and you send a QString that will most likly not work

I also noticed that and changed it already.

There are a few more things I found out. I was using 'keyboard' at first, but that turns red so it might interfere. I swapped out 'keyboard' for Kboard. But my way of decalaring it in the variable section and initializing it in the constructor suddenly upset the compiler so I went back to the constructor and now I have:

Keyboard *Kboard = new Keyboard();
   connect(Kboard, &Keyboard::KeyPressed, this &MainWindow::send);

also in keyboard.h I changed private by public because I was getting an error message there at first.

But I am still having this error in the connect function:

C:\Users\sknippels\Documents\GUI\mainwindow.cpp:51: error: invalid use of member (did you forget the '&' ?)
    connect(Kboard, &Keyboard::KeyPressed, this &MainWindow::send);
                                                             ^

how my code looks like atm:

Keyboard *Kboard = new Keyboard();  // mainwindow.cpp constructor
   connect(Kboard, &Keyboard::KeyPressed, this &MainWindow::send);

void MainWindow::send(QString c)  // send function in mainwindow.ccp
{
  serial->write("c"); // still does not send the value of c, just "c"
  qDebug() << c;
}

void Keyboard::on_a_clicked() // push button function in keyboard.cpp
{
   QString c;
   c = ui->a->text();
   emit KeyPressed(c);
   }

public slots: // keyboard.h
    void KeyPressed(QString c); // also tried KeyPressed(QString); and KeyPressed(); no effect
}

private slots: //mainwindow.h
    void serialReceived(); // one of many other functions
    void send(QString c);

I think we have all the pieces of this strange C++ puzzle now

[jsulm]

@bask185 You're still missing the comma, please add it as it is needed. And if you then still get an error then please post it here: just saying there is an error does not help.

[J.Hilk]

@bask185

additionally to what @jsulm said,

public slots: // keyboard.h
    void KeyPressed(QString c); // also tried KeyPressed(QString); and KeyPressed(); no effect

ought to be a signal

signals: // keyboard.h
    void KeyPressed(QString c); // also tried KeyPressed(QString); and KeyPressed(); no effect

not a slot

[bask185]

@jsulm said in How to use methods of an object from a different class?:

@bask185 You're still missing the comma, please add it as it is needed. And if you then still get an error then please post it here: just saying there is an error does not help.

Keyboard *Kboard = new Keyboard();
   //connect(Kboard, &Keyboard::KeyPressed, this &MainWindow::send);
   connect(Kboard, &Keyboard::KeyPressed, this, &MainWindow::send);

V

C:\Users\sknippels\Documents\GUI\mainwindow.cpp:52: error: undefined reference to `Keyboard::KeyPressed(QString)'

C:\Users\sknippels\Documents\GUI\keyboard.cpp:76: error: undefined reference to `Keyboard::KeyPressed(QString)'

C:\Users\sknippels\Documents\build-GUI-Desktop_Qt_5_8_0_MinGW_32bit-Debug\debug\moc_keyboard.cpp:251: error: undefined reference to `Keyboard::KeyPressed(QString)'

collect2.exe:-1: error: error: ld returned 1 exit status

[jsulm]

@bask185 Please read what @J-Hilk wrote

[bask185]

@jsulm said in How to use methods of an object from a different class?:

@bask185 Please read what @J-Hilk wrote

I did and changed it and I can finally atleast build it. only pressing the 'a' on the keyboard does not do anything.

I know that void Keyboard::on_a_clicked() gets executed using Qdebug I can also tell c has the correct value.

But in mainwindow void MainWindow::send(QString c) does not get executed. I added qDebug() << "hello world"; but nothing happens :(

[jsulm]

@bask185 Can you show the whole code where

Keyboard *Kboard = new Keyboard();
   //connect(Kboard, &Keyboard::KeyPressed, this &MainWindow::send);
   connect(Kboard, &Keyboard::KeyPressed, this, &MainWindow::send);

is embedded?

[bask185]

@jsulm said in How to use methods of an object from a different class?:

@bask185 Can you show the whole code where

Keyboard *Kboard = new Keyboard();
   //connect(Kboard, &Keyboard::KeyPressed, this &MainWindow::send);
   connect(Kboard, &Keyboard::KeyPressed, this, &MainWindow::send);

is embedded?
I don't know what you mean??

I moved Keyboard *Kboard; to the global variables of mainwindow but it seems that does not make a difference. Button 'a' of the keyboard still does not trigger the send function in the main window

[jsulm]

@bask185 Can you please post your code? Else I don't know what is wrong.

[J.Hilk]

@bask185

what @jsulm meant, is, can you post the whole content between the brackets {} where

Keyboard *Kboard = new Keyboard();
   //connect(Kboard, &Keyboard::KeyPressed, this &MainWindow::send);
   connect(Kboard, &Keyboard::KeyPressed, this, &MainWindow::send);

is copied out of.