QObect::connect call does not compile

CynaCons

Hello,

When calling QObject::connect I face problems that I cannot undertsand, even though I have used the signal/slots mechanisms many times witout issues.

I was in the process of writing a very simple TCP Client. At some point I added slots with names like "connect" and I think thats when things started to go wrong.

Here are the things that I checked before asking for help:

Class header file is including QObject and header file is included in the class source file
Class inherit from QObject
Class uses the Q_OBJECT
The project .pro file includes "network"

I checked many other things but here are the most obvious. Complete code can be found below

TcpClient.h

#ifndef TCPCLIENT_H
#define TCPCLIENT_H

#include <QObject>
#include <QTcpSocket>

class TcpClient : public QObject
{
    Q_OBJECT

public:
    explicit TcpClient();
    /* Received from the HMI. Connect to the server */
    void myFunctionToConnect(QString IP_received);
    /* Received from the HMI. Write on the socket */
    void send_text(QString text);

public slots:
    /* From the socket. Emit signal to the HMI */
    void connection_OK();
    /* From the socket. Read the socket and emit signal to the HMI */
    void read();

signals:
    void to_HMI_connection_OK();
    void to_HMI_update_text(QString);

private:
    QString IP;
    int port;
    QTcpSocket socket;
};

#endif // TCPCLIENT_H

TcpClient.c

#include "tcpclient.h"

#include <QTextStream>


TcpClient::TcpClient()
{
    port = 2000;
    /* Signal emitted when the client is connected to a server */
    QObject::connect(&socket, SIGNAL(connected()), this, SLOT(connection_OK()));

    /* Signal emitted when data have is received and is ready to be read */
    QObject::connect(&socket, SIGNAL(readyRead()), this, SLOT(read()));
}

void TcpClient::myFunctionToConnect(QString IP_received)
{
    IP = IP_received;
    socket.connectToHost(IP, port);
}

void TcpClient::send_text(QString text)
{
    /* Create a stream and associate it to the socket */
    QTextStream stream(&socket);
    /* Write in the stream the text input from the HMI */
    stream << text << endl;
}

void TcpClient::connection_OK()
{
    /* Send signal to HMI to update the connection status */
    emit to_HMI_connection_OK();
}

void TcpClient::read()
{
    QString line;
    while(socket.canReadLine()){
        /* Read from the socket the received line */
        line = socket.readLine();
        /* Send signal to HMI to update the received text from the socket */
        emit to_HMI_update_text(line);
    }
}

The two not-compiling lines are:
QObject::connect(&socket, SIGNAL(connected()), this, SLOT(connection_OK()));
and
QObject::connect(&socket, SIGNAL(readyRead()), this, SLOT(read()));

This is what I get from the console when compiling (only an extract):

..\SocketClientServer\tcpclient.cpp: In constructor 'TcpClient::TcpClient()':
..\SocketClientServer\tcpclient.cpp:10:79: error: no matching function for call to 'TcpClient::connect(QTcpSocket*, const char*, TcpClient*, const char*)'
     QObject::connect(&socket, SIGNAL(connected()), this, SLOT(connection_OK()));
                                                                               ^
In file included from C:/Qt/5.8/mingw53_32/include/QtCore/QObject:1:0,
                 from ..\SocketClientServer\tcpclient.h:4,
                 from ..\SocketClientServer\tcpclient.cpp:1:
C:/Qt/5.8/mingw53_32/include/QtCore/qobject.h:222:43: note: candidate: template<class Func1, class Func2> static QMetaObject::Connection QObject::connect(const typename QtPrivate::FunctionPointer<Func>::Object*, Func1, const typename QtPrivate::FunctionPointer<Func2>::Object*, Func2, Qt::ConnectionType)
     static inline QMetaObject::Connection connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal,
                                           ^
C:/Qt/5.8/mingw53_32/include/QtCore/qobject.h:222:43: note:   template argument deduction/substitution failed:
C:/Qt/5.8/mingw53_32/include/QtCore/qobject.h: In substitution of 'template<class Func1, class Func2> static QMetaObject::Connection QObject::connect(const typename QtPrivate::FunctionPointer<Func>::Object*, Func1, const typename QtPrivate::FunctionPointer<Func2>::Object*, Func2, Qt::ConnectionType) [with Func1 = const char*; Func2 = const char*]':
..\SocketClientServer\tcpclient.cpp:10:79:   required from here
C:/Qt/5.8/mingw53_32/include/QtCore/qobject.h:222:43: error: no type named 'Object' in 'struct QtPrivate::FunctionPointer<const char*>'
C:/Qt/5.8/mingw53_32/include/QtCore/qobject.h:254:13: note: candidate: template<class Func1, class Func2> static typename QtPrivate::QEnableIf<((int)(QtPrivate::FunctionPointer<Func2>::ArgumentCount) >= 0), QMetaObject::Connection>::Type QObject::connect(const typename QtPrivate::FunctionPointer<Func>::Object*, Func1, Func2)
             connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, Func2 slot)
             ^
C:/Qt/5.8/mingw53_32/include/QtCore/qobject.h:254:13: note:   template argument deduction/substitution failed:
..\SocketClientServer\tcpclient.cpp:10:79: note:   candidate expects 3 arguments, 4 provided
     QObject::connect(&socket, SIGNAL(connected()), this, SLOT(connection_OK()));

I have spent more than two hours on this, please tell me it's not obvious =)

mchinand

I think you need to add the QObject constructor to your constructor:

TcpClient::TcpClient()
: QObject()
{
//rest of your constructor goes here
}

kshegunov

It appears the overload resolution got all scrambled up with the template instantiation. You should migrate to the pointer-to-member syntax anyway, so I'd suggest that to be the first thing you do:

QObject::connect(&socket, &QTcpSocket::connected, this, &TcpClient::connection_OK);
QObject::connect(&socket, &QTcpSocket::readyRead, this, &TcpClient::read);

As a side note, you should make it a habit to support the QObject tree through the constructors of your derived classes, and to set the appropriate parent for your members:

TcpClient::TcpClient(QObject * parent)
    : QObject(parent), port(2000), socket(this)
{
}

Vinod Kuntoji

@CynaCons ,

I tried your example in Qt 5.7(msvc). It is compiling.

CynaCons

Adding the QObject constructor to my class's constructor and adding the pointer-to-member syntax suggested by Kshegunov fixed the issue.

Thanks guys and good day.