simple tcp socket program fails when QPushButton click!

Mandar Khire

Hi,
I am learning simple tcp socket program in qt so I use program which I found in INTERNET.
When I run it as it is, then I can see socket is created & message transfered properly.
But when I try to use it with GUI ie. I create one QWidget which has one QPushButton & when this button click my aim is to open socket & transfer message. But I failed. I doesnt know where I am wrong.
For reference code is as below:-
Headers

client.h

#ifndef CLIENT_H
#define CLIENT_H
// client.h
#include <QtNetwork>
#include <QObject>
#include <QString>
#include <QTcpSocket>

class Client: public QObject
{
    Q_OBJECT
public:
    Client(QObject* parent = 0);
    ~Client();
    void start(QString address, quint16 port);
public slots:
    void startTransfer();
private:
    QTcpSocket client;
};
#endif // CLIENT_H

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QObject>
#include "client.h"
#include "server.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private slots:
    void on_pushButton_clicked();

private:
    Ui::MainWindow *ui;


};

#endif // MAINWINDOW_H

server.h

#ifndef SERVER_H
#define SERVER_H
#include <QtNetwork>
#include <QObject>
#include <QTcpServer>
#include <QTcpSocket>


class Server: public QObject
{
    Q_OBJECT
public:
    Server(QObject  * parent = 0);
    ~Server();
public slots:
    void acceptConnection();
    void startRead();
private:
    QTcpServer server;
    QTcpSocket* tcpSocket;
    QString recievedmessage;
};
#endif // SERVER_H

Sources

client.cc

#include "client.h"
#include <QHostAddress>


Client::Client(QObject* parent): QObject(parent)
{
    connect(&client, SIGNAL(connected()),this, SLOT(startTransfer()));
}

Client::~Client()
{
    client.close();
}

void Client::start(QString address, quint16 port)
{
    QHostAddress addr(address);
    client.connectToHost(addr, port);
}

void Client::startTransfer()
{
    QString message = "hello World Server";
     qDebug() << "client transfer message = "<< message;
    int message_length = message.length();
    client.write(message.toStdString().c_str(),message_length);
    //  client.write("Hello World Mandar12345", 23);

}

server.cc

#include "server.h"
#include <iostream>
using namespace std;

Server::Server(QObject *parent): QObject(parent)
{
    connect(&server, SIGNAL(newConnection()),
            this, SLOT(acceptConnection()));
    server.listen(QHostAddress::Any, 8888);  //8888
}

Server::~Server()
{
    //    server.close();
}

void Server::acceptConnection()
{
    tcpSocket = server.nextPendingConnection();

    connect(tcpSocket, SIGNAL(readyRead()),
            this, SLOT(startRead()));

}

void Server::startRead()
{
    char buffer[1024] = {0};
    //  QTcpSocket* socket = qobject_cast< QTcpSocket* >(sender());
    //  tcpSocket = qobject_cast< QTcpSocket* >(sender());
    //  qDebug() << tcpSocket->readAll();
    tcpSocket->read(buffer, tcpSocket->bytesAvailable());
    recievedmessage = "message = " + QString::fromStdString(buffer);
    cout << buffer << endl;
    qDebug() << "recievedmessage = "<< recievedmessage;
    tcpSocket->close();
}

main.cpp

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

int main(int argc, char** argv)
{
    QApplication app(argc, argv);

    //    Client client;
    //    client.start("127.0.0.1", 8888); 
    //    Server server;
    MainWindow w;
    w.show();
    return app.exec();
}

MainWindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "client.h"
#include "server.h"
#include <iostream>

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

}

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

void MainWindow::on_pushButton_clicked()
{
    Client client;
    client.start("127.0.0.1", 8888);
    Server server;
}

Need a guidance!

mrjj

Hi
Its a classic error where your variable runs out of scope and get deleted.
You must move the variable to mainwindow.h as a member so it will live
as long as mainwindow.

void MainWindow::on_pushButton_clicked()
{
Client client; <<< local variable
client.start("127.0.0.1", 8888);
Server server; <<< local variable
} //here server+client is deleted.

Mandar Khire

Thanks mrjj,
Now I understand why variable should kept in header file.
Thanks.

Solved simple tcp socket program fails when QPushButton click!