Passing strings to SLOT function

**ashboi** · 7th September 2013, 04:51

Currently, Iâ€™ve written up an app that executes a client socket Slot at runtime that is used to sending data to the server. Iâ€™m wondering is it possible to pass a text string when a pushbutton is clicked, to a variable in the slot function? Example, when a pushbutton is clicked, a string containing â€œabcdâ€ is passed to the message variable in the socket function, and the socket will send â€œabcdâ€ over to the server.

I was wondering is the following codes would work,

QObject::connect(pushButton,SIGNAL(clicked(),this, SLOT(sendmsg()));
...
...
MainWindow::sendmsg()
{
strings associated with pushbutton...
}
...
...
MainWindow::socket()
{
.......
.......
send(...,sendmsg,...);
.......
.......
}

**Lesiok** · 7th September 2013, 09:05

Something like this.
Slot definition somewhere :

Qt Code:

Switch view

slots :
void receiveMessage( QString message );

slots :
void receiveMessage( QString message );

To copy to clipboard, switch view to plain text mode

signal definition in MainWindow :

Qt Code:

Switch view

signals :
void sendMessage(QString message);

signals :
void sendMessage(QString message);

To copy to clipboard, switch view to plain text mode

and method MainWindow::socket :

Qt Code:

Switch view

MainWindow::socket()
{
.......
.......
emit sendMessage(sendmsg());
.......
.......
}

MainWindow::socket()
{
.......
.......
emit sendMessage(sendmsg());
.......
.......
}

To copy to clipboard, switch view to plain text mode

Of course You must put somewhere this code :

Qt Code:

Switch view

QObject::connect(address_of_MainWindow,SIGNAL(sendMessage(QString)),addres_of_message_receiver, SLOT(receiveMessage( QString message )));

QObject::connect(address_of_MainWindow,SIGNAL(sendMessage(QString)),addres_of_message_receiver, SLOT(receiveMessage( QString message )));

To copy to clipboard, switch view to plain text mode

**anda_skoa** · 7th September 2013, 09:52

If it is a fixed string per button, check QSignalMapper

Cheers,
_

**ashboi** · 7th September 2013, 12:28

I probably should rephrase my question. Currently i have a socket thread(not slot), that executes at runtime when the app starts. the socket thread will wait in the background for a string to be passed to it from the pushbuttons, and when a pushbutton is pressed, example a string containing "abcd" will be pass to the socket thread's "send()" function and finally over to the server side, which is another device. Afterwhich, the socket will remain open until another button is pressed, and repeat the same procedure again.

I don't think the method Lesiok posted can do this, since receiveMessage is another slot within the same app to receive the strings.

Yes, it would be a fixed string per pushbutton, e.g pushbutton1-> "abcd", pushbutton2->
"defg".

**zerokewl** · 7th September 2013, 12:45

Isn't what has been said already a solution?
Using SIGNALS/SLOTS?

Main Thread

Qt Code:

Switch view

signals:
  void sendMessage(QString message);

signals:
  void sendMessage(QString message);

To copy to clipboard, switch view to plain text mode

SocketSender

Qt Code:

Switch view

slots:
  void send(QString message);

slots:
  void send(QString message);

To copy to clipboard, switch view to plain text mode

And when you click a button, set it to

Qt Code:

Switch view

emit sendMessage("ABCD");

emit sendMessage("ABCD");

To copy to clipboard, switch view to plain text mode

That would then EMIT the SIGNAL sendMessage('ABCD') and your SLOT would connect and get the QString value and send it appropriately?

After you create your socket thread, using the CONNECT to link the SIGNALS/SLOTS

Qt Code:

Switch view

connect(this, SIGNAL(sendMessage(QString), socketThread, SLOT(send(QString)));

connect(this, SIGNAL(sendMessage(QString), socketThread, SLOT(send(QString)));

To copy to clipboard, switch view to plain text mode

**ashboi** · 7th September 2013, 13:27

I'm not sure why I'm having an error saying that SIGNAL was not declared.

Qt Code:

Switch view

mainwindow.cpp:14:78: error: macro "SIGNAL" passed 3 arguments, but takes just 1
mainwindow.cpp: In constructor â€˜MainWindow::MainWindow(QWidget*)â€™:
mainwindow.cpp:14:16: error: â€˜SIGNALâ€™ was not declared in this scope
make: *** [mainwindow.o] Error 1

mainwindow.cpp:14:78: error: macro "SIGNAL" passed 3 arguments, but takes just 1
mainwindow.cpp: In constructor â€˜MainWindow::MainWindow(QWidget*)â€™:
mainwindow.cpp:14:16: error: â€˜SIGNALâ€™ was not declared in this scope
make: *** [mainwindow.o] Error 1

To copy to clipboard, switch view to plain text mode

Here is my code

mainwindow.cpp

Qt Code:

Switch view

#include "mainwindow.h"
#include "ui_main.h"
#include "socketthread.h"
#include <QApplication>
 
using namespace std;
 
MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
	connect(this, SIGNAL(sendMessage(QString), SocketThread, SLOT(send(QString)));
}
 
MainWindow::~MainWindow()
{
    	delete ui;
}
 
void MainWindow::on_pushButton_clicked()
{
    	emit sendMessage("ABCD");
}

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

using namespace std;

MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
	connect(this, SIGNAL(sendMessage(QString), SocketThread, SLOT(send(QString)));
}

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

void MainWindow::on_pushButton_clicked()
{
    	emit sendMessage("ABCD");
}

To copy to clipboard, switch view to plain text mode

mainwindow.h

Qt Code:

Switch view

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
 
namespace Ui
{
    class MainWindow;
}
 
class MainWindow : public QMainWindow {
    	Q_OBJECT
 
public:
    	MainWindow(QWidget *parent = 0);
    	~MainWindow();
 
signals:
        void sendMessage(QString message);
 
private slots:
	void on_pushButton_clicked();
 
private:
	Ui::MainWindow *ui;
 
};
 
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui
{
    class MainWindow;
}

class MainWindow : public QMainWindow {
    	Q_OBJECT

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

signals:
        void sendMessage(QString message);

private slots:
	void on_pushButton_clicked();

private:
	Ui::MainWindow *ui;

};

#endif // MAINWINDOW_H

To copy to clipboard, switch view to plain text mode

socketthread.cpp

Qt Code:

Switch view

void SocketThread::run()
{
.....
.....
string_in = send(); // not sure how to connect the Qstring value here
.....
.....
}

void SocketThread::run()
{
.....
.....
string_in = send(); // not sure how to connect the Qstring value here
.....
.....
}

To copy to clipboard, switch view to plain text mode

socketthread.h

Qt Code:

Switch view

#ifndef SOCKET_H
#define SOCKET_H
#include <QThread>
 
class SocketThread : public QThread
{
     	Q_OBJECT
 
	public slots:
	void send(QString message);
 
	private:
     	void run();
};
#endif // SOCKET_H

#ifndef SOCKET_H
#define SOCKET_H
#include <QThread>

class SocketThread : public QThread
{
     	Q_OBJECT

	public slots:
	void send(QString message);

	private:
     	void run();
};
#endif // SOCKET_H

To copy to clipboard, switch view to plain text mode

**anda_skoa** · 7th September 2013, 13:48

Your error indicates that you are using SIGNAL wrong and indeed there is a closing parentheses missing.
And it is using the wrong name, the signal's name is sendMessage.

so over all it should be

Qt Code:

Switch view

SIGNAL(sendMessage(QString))

SIGNAL(sendMessage(QString))

To copy to clipboard, switch view to plain text mode

Or, as I said, even simpler using a QSIgnalMapper

Qt Code:

Switch view

{   	
	ui->setupUi(this);
 
 QSignalMapper *mapper = new QSignalMapper(this;
 
 mapper->setMapping(ui->pushButton, "ABCD");
 connect(ui->pushButton, SIGNAL(clicked()), mapper, SLOT(map()));
 
	connect(mapper, SIGNAL(mapped(QString)), SocketThread, SLOT(send(QString)));
}

{   	
	ui->setupUi(this);

 QSignalMapper *mapper = new QSignalMapper(this;

 mapper->setMapping(ui->pushButton, "ABCD");
 connect(ui->pushButton, SIGNAL(clicked()), mapper, SLOT(map()));

	connect(mapper, SIGNAL(mapped(QString)), SocketThread, SLOT(send(QString)));
}

To copy to clipboard, switch view to plain text mode

Assuming SocketThread is still owned by the main thread and has not been moved "into itself", you will also either require a Qt::QueuedConnection for the connect or mutex safe-guarding in the slot.

All in all I would advise to first make it work without threads. QTcpSocket works event based for a reason!

Cheers,
_

**ashboi** · 7th September 2013, 14:30

Ah, missed that ')'.

Now I'm having another error at

Qt Code:

Switch view

connect(this, SIGNAL(sendMessage(QString)), SocketThread, SLOT(send(QString)));

connect(this, SIGNAL(sendMessage(QString)), SocketThread, SLOT(send(QString)));

To copy to clipboard, switch view to plain text mode

and I can't seem to figure out whats wrong with it.

Qt Code:

Switch view

mainwindow.cpp: In constructor â€˜MainWindow::MainWindow(QWidget*)â€™:
mainwindow.cpp:15:58: error: expected primary-expression before â€˜,â€™ token
make: *** [mainwindow.o] Error 1

mainwindow.cpp: In constructor â€˜MainWindow::MainWindow(QWidget*)â€™:
mainwindow.cpp:15:58: error: expected primary-expression before â€˜,â€™ token
make: *** [mainwindow.o] Error 1

To copy to clipboard, switch view to plain text mode

On another note, my socket client is using zmq as a transport layer, and it executes at runtime when the app's GUI starts, currently, the I have the socket using 'cout' to check for errors/feeback from the server, and wait for messages to send over to the server. Initially, I was hoping that I can do something like, when a button is clicked, the strings "abcd" would be passed to the socket, and printed in the terminal, just like if a user were to manually type in "abcd" into the terminal, to show its actually working.

**zerokewl** · 7th September 2013, 14:31

you forgot a ) here

Qt Code:

Switch view

MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
	connect(this, SIGNAL(sendMessage(QString), SocketThread, SLOT(send(QString)));
}

MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
	connect(this, SIGNAL(sendMessage(QString), SocketThread, SLOT(send(QString)));
}

To copy to clipboard, switch view to plain text mode

it should be

Qt Code:

Switch view

MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
	connect(this, SIGNAL(sendMessage(QString)), SocketThread, SLOT(send(QString)));
}

MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
	connect(this, SIGNAL(sendMessage(QString)), SocketThread, SLOT(send(QString)));
}

To copy to clipboard, switch view to plain text mode

I wrote this as you Edited your last update fixing the missing bracket..

Can you please post your Constructor with new code?

**ashboi** · 7th September 2013, 14:39

Sure thing, here it is

Qt Code:

Switch view

MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
	connect(this, SIGNAL(sendMessage(QString)), SocketThread, SLOT(send(QString)));
}

MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
	connect(this, SIGNAL(sendMessage(QString)), SocketThread, SLOT(send(QString)));
}

To copy to clipboard, switch view to plain text mode

**zerokewl** · 7th September 2013, 14:52

Where is SocketThread instantiated from ?

Should you have something like?

Qt Code:

Switch view

MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
        SocketThread *socketThread = new SocketThread();
	connect(this, SIGNAL(sendMessage(QString)), socketThread, SLOT(send(QString)));
}

MainWindow::MainWindow(QWidget *parent) :
    	QMainWindow(parent),
    	ui(new Ui::MainWindow)
{   	
	ui->setupUi(this);
        SocketThread *socketThread = new SocketThread();
	connect(this, SIGNAL(sendMessage(QString)), socketThread, SLOT(send(QString)));
}

To copy to clipboard, switch view to plain text mode

**ashboi** · 7th September 2013, 15:17

socketThread is instantiated from main.cpp, I guess it would be easier if it was instantiated in mainwindow.cpp instead. No more errors on this.

But I have a question on how should I pass the string into my socket slot function? I tried

Qt Code:

Switch view

string_in = send();

string_in = send();

To copy to clipboard, switch view to plain text mode

but results in an argument not being provided.

Currently this is what I have

socketthread.cpp

Qt Code:

Switch view

void SocketThread::run()
{
...connect....
.................
while(1){
string string_in = ""; // I would like to pass the string into string_in
....
.......
}
exec();
}

void SocketThread::run()
{
...connect....
.................
while(1){
string string_in = ""; // I would like to pass the string into string_in
....
.......
}
exec();
}

To copy to clipboard, switch view to plain text mode

**zerokewl** · 7th September 2013, 15:33

The connect signal

Qt Code:

Switch view

connect(this, SIGNAL(sendMessage(QString)), SocketThread, SLOT(send(QString)));

connect(this, SIGNAL(sendMessage(QString)), SocketThread, SLOT(send(QString)));

To copy to clipboard, switch view to plain text mode

Sends the Emitted QString to the socketThread::send(QString message).
Once you emit the signal and it is recieved from the socketThread::send(QString message) you could do this.

Qt Code:

Switch view

class socketThread{
    private:
        QString message;
}
 
void socketThread::send(QString message){
    this->message = message;
}

class socketThread{
    private:
        QString message;
}

void socketThread::send(QString message){
    this->message = message;
}

To copy to clipboard, switch view to plain text mode

And then inside your run

Qt Code:

Switch view

void SocketThread::run()
{
...connect....
.................
while(1){
string string_in = message.toStdString()  //convert QString to string
....
.......
}

void SocketThread::run()
{
...connect....
.................
while(1){
string string_in = message.toStdString()  //convert QString to string
....
.......
}

To copy to clipboard, switch view to plain text mode

**Lesiok** · 7th September 2013, 18:18

Originally Posted by ashboi

Currently this is what I have

socketthread.cpp

Qt Code:

Switch view

void SocketThread::run()
{
...connect....
.................
while(1){
string string_in = ""; // I would like to pass the string into string_in
....
.......
}
exec();
}

void SocketThread::run()
{
...connect....
.................
while(1){
string string_in = ""; // I would like to pass the string into string_in
....
.......
}
exec();
}

To copy to clipboard, switch view to plain text mode

With this run() method You never receive any signal because thread event loop is never started. What are You doing in this "never ending" loop ?

Thread: Passing strings to SLOT function

Thread Tools

Search Thread

Display

Hybrid View

Passing strings to SLOT function

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

The following user says thank you to anda_skoa for this useful post:

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

The following user says thank you to zerokewl for this useful post:

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

Re: Passing strings to SLOT function

Similar Threads

Passing argument to slot.

Passing data via signal/slot

Passing an integer to a slot

Passing QsqlRecords in a function

passing arguments in SLOT

Tags for this Thread

Bookmarks

Bookmarks

Posting Permissions