QMetaObject use in visual studio (or "asyncronous QSocketNotifier use")

[ChrisW67]

Are you specifically trying to use the low level code for a reason?
AFAICT QTcpSocket will do what you are after without using low level Windows sockets code. When data arrives it will emit readyRead() and that can trigger doSomething(). There is a couple of examples of QTcpSocket use in the docs.

[amdreallyfast]

I tried to use QTcpSocket, and I managed to get it working with Visual Studio (http://www.bogotobogo.com/Qt/Qt5_QTcpSocket.php), but there was not mention of "readyRead()".

I saw "readyRead()" in the docs (http://qt-project.org/doc/qt-5/qiodevice.html#readyRead), but I got the impression that I would have to continuously check it in a loop that wasn't blocked by QApplication::exec().

If you know how to use QTcpSocket asynchronously with QApplication, I'm all ears. I haven't been able to find a single example of such yet.

Also, do you have a link to these docs that you're talking about? The ones with examples? This one (http://qt-project.org/doc/qt-5/QTcpSocket.html) doesn't have any examples. It is just the tip of a very large documentation iceberg.

[anda_skoa]

The document you link to has a number of links to examples, one of them being http://qt-project.org/doc/qt-5/qtnet...t-example.html

You don't call readyRead(), it is a signal just like QSocketNotifier::activated(), so it is connected to a slot that then handles the incoming data.
QTcpSocket just makes the whole setup way easier.

SIGNAL and SLOT are just C-preprocessor macros, the compiler used by QtCreator and the one used by Visual Studio are both equally capable of handling those.

Sure, QtCreator can do some introspection for code completion during editing, but the connection will work regardless of the editor that was used to write it.

Also, for the record, I did find a tutorial on QTcpSocket and managed to get it to work in Visual Studio (http://www.bogotobogo.com/Qt/Qt5_QTcpSocket.php), but there was not mention of "readyRead()".

This is because the example uses blocking I/O, so it calls waitForReadyRead() instead of connecting to readyRead()

Cheers,
_

[amdreallyfast]

...it is a signal just like QSocketNotifier::activated(), so it is connected to a slot...

I'm not following. What is a "signal"? And what is a "slot"? They are preprocessor macros for "qFlagLocation("1"#a QLOCATION)" and "qFlagLocation("2"#a QLOCATION)", respectively.

This is because the example uses blocking I/O, so it calls waitForReadyRead instead of connecting to readRead()

What do you mean, "connecting to"? How would you "connect" to a preprocessor macro? What does that even mean?

---

Added after 42 minutes:

Also, I've been looking at the example you linked to, trying to tear it apart and get it working. I'm running into an error in my "connect(...)" call again, in which it cannot find the function specified in the fourth argument (the one with SLOT(...) surrounding it). This may be a compiler-specific constructor order error.

This is my basic code:

#include <QtCore\QObject>
#include <QtNetwork\QTcpSocket>
#include <QtCore\QDebug>
 
class my_TCP_socket : public QObject
{
public:
   explicit my_TCP_socket(QObject *parent = 0);
 
   ~my_TCP_socket();
 
private:
   QTcpSocket *m_socket_ptr;
 
   void read_it();
};
 
 
 
my_TCP_socket::my_TCP_socket(QObject *parent) :
   QObject(parent)
{
   m_socket_ptr = new QTcpSocket(this);
 
   m_socket_ptr->connectToHost("10.10.10.126", 5);
   if (!(m_socket_ptr->waitForConnected(5000)))
   {
      qDebug() << "Not connected!";
      m_socket_ptr = NULL;
   }
   else
   {
      qDebug() << "Connected!";
   }
 
   this->connect(m_socket_ptr, SIGNAL(readyRead()), this, SLOT(this->read_it(void)));
}
 
void my_TCP_socket::read_it()
{
   QDataStream in(m_socket_ptr);
 
   qint64 bytes_available = m_socket_ptr->bytesAvailable();
   if (bytes_available > 0)
   {
      QString in_data;
      in >> in_data;
 
      qDebug() << in_data;
   }
   else
   {
      qDebug() << "no data";
   }
}

#include <QtCore\QObject>
#include <QtNetwork\QTcpSocket>
#include <QtCore\QDebug>

class my_TCP_socket : public QObject
{
public:
   explicit my_TCP_socket(QObject *parent = 0);

   ~my_TCP_socket();

private:
   QTcpSocket *m_socket_ptr;

   void read_it();
};



my_TCP_socket::my_TCP_socket(QObject *parent) :
   QObject(parent)
{
   m_socket_ptr = new QTcpSocket(this);

   m_socket_ptr->connectToHost("10.10.10.126", 5);
   if (!(m_socket_ptr->waitForConnected(5000)))
   {
      qDebug() << "Not connected!";
      m_socket_ptr = NULL;
   }
   else
   {
      qDebug() << "Connected!";
   }

   this->connect(m_socket_ptr, SIGNAL(readyRead()), this, SLOT(this->read_it(void)));
}

void my_TCP_socket::read_it()
{
   QDataStream in(m_socket_ptr);

   qint64 bytes_available = m_socket_ptr->bytesAvailable();
   if (bytes_available > 0)
   {
      QString in_data;
      in >> in_data;

      qDebug() << in_data;
   }
   else
   {
      qDebug() << "no data";
   }
}

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Those functions may not exist yet when "connect(...)" is called in the constructor, although I may just be doing it wrong because this

int method_index = this->metaObject()->indexOfMethod("init()");

int method_index = this->metaObject()->indexOfMethod("init()");

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just returns -1, and that's after initialization.

Again, any feedback would be helpful.

---

Added after 20 minutes:

Followup: Should have noted that the attempt connect the "read_it()" function resulted in this error at runtime:

Object::connect: No such slot QObject::this->read_it(void) in *path redacted*

Object::connect: No such slot QObject::this->read_it(void) in *path redacted*

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I have tentatively discovered that the Q_OBJECT macro needs to be declared at the top of the "private" section of the class, but that is producing a linker error. I put that into a new thread. The question seemed different enough from this thread.

Please advise still. The examples are not working outside QtCreator.

[ChrisW67]

I'm not following. What is a "signal"? And what is a "slot"?

The concepts are documented: Signals & Slots. Assistant, which ships with Qt, is your friend. Searching the index for "examples" is also instructive.

The general principals of Network Programming specifically Using TCP with QTcpSocket and QTcpServer. Fortune Client Example is a receiver for a binary payload but it demonstrates the asynchronous use of QTcpSocket.

[amdreallyfast]

I don't seem to be getting across that I am not using QtCreator. The explanations and examples in QtAssistant and every piece of documentation that I have found so far assume that I am using the QtCreator IDE. I am not.

I already got the code working in QtCreator. I didn't want to use that because the rest of my program was build in Visual Studio. The documentation that I have seen for signals and slots completely falls apart when you try to run it in Visual Studio because the Meta-Object Compiler is not a built-in thing.

Do you know how I could get MOC working in Visual Studio? I have found no such thing yet. This documentation (http://qt.developpez.com/doc/4.7/moc/), which is about using MOC, again assume QtCreator.

[ChrisW67]

I don't seem to be getting across that I am not using QtCreator.

We do not seem to be communicating that C++ is C++ (and Qt idiom is Qt idiom) regardless of which editor you use to write it. Visual Studio and Qt Creator are glorified editors that can run a build process on the source they edit.

The explanations and examples in QtAssistant and every piece of documentation that I have found so far assume that I am using the QtCreator IDE. I am not.

The examples in Qt Assistant, which is not part of Qt Creator, are IDE agnostic (unless you happen to be reading the manual for the Qt Creator IDE itself of course). The documents I specifically linked you to make absolutely no mention of Qt Creator and do not rely on it in any way. You can equally well use Notepad and the command line.


If you are using VS Express then, no, there is no point and click method of getting moc support (It must be possible to make it work but you need to understand how applications are built). If you are using a full VS version then the Qt VS Add-in should look after this for you. I do not use VS with Qt so I cannot help with the specifics.
What is the Qt Visual Studio add-in?
Qt Downloads, Click Show Downloads button, and scroll to "Other Downloads" group.

[amdreallyfast]

...C++ is C++ (and Qt idiom is Qt idiom) regardless of which editor you use to write it.

Alright. I never picked that up. Everything that I've been taught and everything that I have learned through school in computer science tells me that it's all implementation dependent. If everyone follows the standard, then I'm ok and I can use it, but if anything fails, then nearly any and every explanation is an option, and that leaves me overwhelmed. I am intentionally asking absurd questions to try to figure out what is happening, and I'm sorry if that is irritating, but I don't know any other way to ping other people for little-used information.

In this case, the problem is in hunting down the build commands to moc.exe (http://qt-project.org/doc/qt-4.8/moc...d-line-options), and figuring out how to include them in the right order to get the rest of the compilation steps to jive.

Thanks for the feedback so far.

I'll post my findings when I solve them. It may have been easier to just learn Win32 threading.

[ChrisW67]

Can I suggest you start by ignoring Visual Studio, open a Qt command prompt (usually a shortcut for this in the Start menu), establish your Microsoft command line environment, something like:

C:\Program Files\Microsoft SDKs\Windows\v7.1\bin\SetEnv.cmd /x86

C:\Program Files\Microsoft SDKs\Windows\v7.1\bin\SetEnv.cmd /x86

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and follow:
Getting Started Programming with Qt
Where you see "make" as a command run "nmake" (the Microsoft make utility)

By the time you get down to "Using a QMainWindow" you will have a PRO file that generates a Makefile containing moc invocations. You will see these in the output when you run nmake.

[amdreallyfast]

Issue is resolved. I explained it to myself in the reply below. Maybe it will help someone else too.


Thanks for the feedback. I still had to figure it out for myself, but a few of the supplied links provided new ideas when I looked at them the second (or third) time.

I am running Visual Studio 2013 Express, so to get MOC to work, I had to do manually adjust the command line that built the VS project. In case any other confused person discovers this, I'll just post all my demo code and give myself a thorough explanation.

Misc Note: The following two additional include directories were specified in project properties->Configuration Properties->C/C++->Addition Include Directories:

C:\Qt\4.8.5\include
$(ProjectDir)

C:\Qt\4.8.5\include
$(ProjectDir)

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Explanation:
In order to use SLOT or SIGNAL in Visual Studio without the Qt plugin, such as if you are using VS Express as I am, you need to invoke the MOC system yourself. Examine the header file below for details on how SLOT is used, but SIGNAL, Q_INVOKABLE, and other meta-system tags are used in the same way.

The MOC system will generate .cpp files (they appear to be C-based files, so they may actually be C files that can work without a fuss in a C++ compiler (??is this correct??)), not binary files, so they cannot be linked in later and must be run through the compiler from the beginning. Therefore, the files in need of the meta system tags need to be run through MOC before the rest of the compilation begins.

(1) Right-click your project (NOT the solution) that contains the file that needs the MOC tags (in my case, the project that contained my_TCP_socket.h, shown below), select properties.

(2) Configuration Properties->Custom Build Step->General

(3) Leave "Execute After" blank.

(4) Under "Execute Before", select "PreBuildEvent" from the drop down menu. This tells Visual Studio to attempt to execute the text in "Command Line" before the pre-build event, which is the earliest build phase in the Visual Studio program builder (http://msdn.microsoft.com/en-us/library/e85wte0k.aspx).

(5) In "Command Line", follow this outline: *location of moc executable* *path of file to run through MOC* -o *path of where you want the output file to go (hence the "-o" option)
In my case, this was as follows:

C:\Qt\4.8.5\bin\moc.exe $(ProjectDir)\my_TCP_socket.h -o $(ProjectDir)\tmp\moc_my_TCP_socket.cpp

C:\Qt\4.8.5\bin\moc.exe $(ProjectDir)\my_TCP_socket.h -o $(ProjectDir)\tmp\moc_my_TCP_socket.cpp

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Note: It is necessary to include the MOC-generated .cpp files from a source file. It is generally shenaniganry to #include a source file (.c or .cpp) at all because it will cause "X is already defined" errors if it happens to get included more than once. It is perfectly fine to declare things more than once, but it is not ok to define them more than once. Class method definitions in a header file can get away with it because they are special. Each compilation unit (that is, source file) is compiled once on its own into an object file, along with all the stuff that was dumped into it via #includes, and then all the resulting object files are linked together. If there is a function definition that appears more than once during this linking stage, such as non-class functions generated by MOC that were included in more than one compilation unit, then there will be "X is already defined" errors during linking.

Here is my custom TCP socket class declared in "my_TCP_socket.h":

#pragma once
 
#include <QtCore\QObject>
#include <QtNetwork\QTcpSocket>
#include <QtCore\QDebug>
 
// if this header file gets included by more than one source file, then there will be an "X is already defined" error, so it is a very bad plan to include the MOC-generated code here
// Note: The commented out #include was kept in expressly for this comment.
//#include "tmp\moc_my_TCP_socket.cpp"
 
class my_TCP_socket : public QObject
{
public:
   explicit my_TCP_socket(QObject *parent = 0);
 
   ~my_TCP_socket();
 
private:
   // http://qt-project.org/doc/qt-4.8/qobject.htm#Q_OBJECT
   // "The Q_OBJECT macro must appear in the private section of a class definition 
   // that declares its own signals and slots or that uses other services provided 
   // by Qt's meta-object system."
   Q_OBJECT
 
   QTcpSocket *m_socket_ptr;
 
   // http://qt-project.org/doc/qt-4.8/qobject.html#Q_INVOKABLE
   // "Apply this macro to definitions of member functions to allow them to be 
   // invoked via the meta-object system. The macro is written before the return 
   // type..."   
   // Note: This applies to Q_SLOT and Q_SIGNAL as well, even though the 
   // documentation at the link does not expressly say so (as far as I saw).
   Q_SLOT void read_it();
};

#pragma once

#include <QtCore\QObject>
#include <QtNetwork\QTcpSocket>
#include <QtCore\QDebug>

// if this header file gets included by more than one source file, then there will be an "X is already defined" error, so it is a very bad plan to include the MOC-generated code here
// Note: The commented out #include was kept in expressly for this comment.
//#include "tmp\moc_my_TCP_socket.cpp"

class my_TCP_socket : public QObject
{
public:
   explicit my_TCP_socket(QObject *parent = 0);

   ~my_TCP_socket();

private:
   // http://qt-project.org/doc/qt-4.8/qobject.htm#Q_OBJECT
   // "The Q_OBJECT macro must appear in the private section of a class definition 
   // that declares its own signals and slots or that uses other services provided 
   // by Qt's meta-object system."
   Q_OBJECT

   QTcpSocket *m_socket_ptr;

   // http://qt-project.org/doc/qt-4.8/qobject.html#Q_INVOKABLE
   // "Apply this macro to definitions of member functions to allow them to be 
   // invoked via the meta-object system. The macro is written before the return 
   // type..."   
   // Note: This applies to Q_SLOT and Q_SIGNAL as well, even though the 
   // documentation at the link does not expressly say so (as far as I saw).
   Q_SLOT void read_it();
};

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Here is my_TCP_socket.cpp:

#include "my_TCP_socket.h"
 
#include <iostream>
using std::cout;
using std::endl;
 
#include <QtCore\QMetaMethod>
 
#include "tmp\moc_my_TCP_socket.cpp"
 
my_TCP_socket::my_TCP_socket(QObject *parent) :
   QObject(parent)
{
   m_socket_ptr = new QTcpSocket(this);
 
   m_socket_ptr->connectToHost("10.10.10.126", 5);
   if (!(m_socket_ptr->waitForConnected(5000)))
   {
      qDebug() << "Not connected!";
      m_socket_ptr = NULL;
   }
   else
   {
      qDebug() << "Connected!";
   }
 
   // just showing that the file exists
   int method_index = this->metaObject()->indexOfMethod("read_it()");
   cout << "index of method read_it() is '" << method_index << "'" << endl;
   QObject::connect(m_socket_ptr, SIGNAL(readyRead()), this, SLOT(read_it()));
}
 
my_TCP_socket::~my_TCP_socket()
{
   if (NULL != m_socket_ptr)
   {
      m_socket_ptr->close();
   }
}
 
void my_TCP_socket::read_it()
{
   //QDataStream in(m_socket_ptr);
 
   if (m_socket_ptr->waitForReadyRead(3000))
   {
      qint64 bytes_available = m_socket_ptr->bytesAvailable();
      if (bytes_available > 0)
      {
         qDebug() << "Reading '" << bytes_available << "' bytes";
         qDebug() << m_socket_ptr->readAll();
         //QString in_data;
         //in >> in_data;
 
         //qDebug() << in_data;
      }
      else
      {
         qDebug() << "no data";
      }
   }
}

#include "my_TCP_socket.h"

#include <iostream>
using std::cout;
using std::endl;

#include <QtCore\QMetaMethod>

#include "tmp\moc_my_TCP_socket.cpp"

my_TCP_socket::my_TCP_socket(QObject *parent) :
   QObject(parent)
{
   m_socket_ptr = new QTcpSocket(this);

   m_socket_ptr->connectToHost("10.10.10.126", 5);
   if (!(m_socket_ptr->waitForConnected(5000)))
   {
      qDebug() << "Not connected!";
      m_socket_ptr = NULL;
   }
   else
   {
      qDebug() << "Connected!";
   }

   // just showing that the file exists
   int method_index = this->metaObject()->indexOfMethod("read_it()");
   cout << "index of method read_it() is '" << method_index << "'" << endl;
   QObject::connect(m_socket_ptr, SIGNAL(readyRead()), this, SLOT(read_it()));
}

my_TCP_socket::~my_TCP_socket()
{
   if (NULL != m_socket_ptr)
   {
      m_socket_ptr->close();
   }
}

void my_TCP_socket::read_it()
{
   //QDataStream in(m_socket_ptr);

   if (m_socket_ptr->waitForReadyRead(3000))
   {
      qint64 bytes_available = m_socket_ptr->bytesAvailable();
      if (bytes_available > 0)
      {
         qDebug() << "Reading '" << bytes_available << "' bytes";
         qDebug() << m_socket_ptr->readAll();
         //QString in_data;
         //in >> in_data;

         //qDebug() << in_data;
      }
      else
      {
         qDebug() << "no data";
      }
   }
}

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Lastly, here is main.cpp:

#include "my_TCP_socket.h"
 
#include <QtCore/QCoreApplication>
#pragma comment (lib, "C:/Qt/4.8.5/lib/QtCored4.lib")
#pragma comment (lib, "C:/Qt/4.8.5/lib/QtNetworkd4.lib")
 
 
int main(int argc, char **argv)
{
   int app_ret_val = 0;
   QCoreApplication app(argc, argv);
   my_TCP_socket S;
 
   app_ret_val = app.exec();
 
   return app_ret_val;
}

#include "my_TCP_socket.h"

#include <QtCore/QCoreApplication>
#pragma comment (lib, "C:/Qt/4.8.5/lib/QtCored4.lib")
#pragma comment (lib, "C:/Qt/4.8.5/lib/QtNetworkd4.lib")


int main(int argc, char **argv)
{
   int app_ret_val = 0;
   QCoreApplication app(argc, argv);
   my_TCP_socket S;
   
   app_ret_val = app.exec();

   return app_ret_val;
}

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[anda_skoa]

There is really no point in calling waitForReadyRead() in the slot connected to the socket's readyRead() signal.

In the worst case this makes your program wait again instead of reading the data that is already there.

Cheers,
_

[amdreallyfast]

There is really no point in calling waitForReadyRead() in the slot connected to the socket's readyRead() signal.

O rly? I was wondering about that. I had an issue in which two transmissions of data would somehow, in the same time frame, be read instead of one. Only 25 bytes is coming from my microcontroller, but when I put in the waitForReadyRead(...), it read 50 bytes.

I should note that the waitForReadyRead(...) call was necessary to retrieve data at all when I ran the program in Visual Studio recently. I must have inadvertently "crossed some wires", so to speak. I'll try to do without, although I'd like to know how that function works and what it is linked such that it read twice.

[amdreallyfast]

I made a mistake when describing the setup for moc. As it is, it will not run in a fresh project.

I had to manually create the "/tmp/" folder that I described in the command line for the custom build step because moc.exe does not seem to create folders that do not exist in the specified path (I had to run moc.exe manually to figure that one out). After I manually create the folder, I had to run moc.exe manually on each file that contained a meta-object keyword, such as Q_OBJECT, SLOT, and SIGNAL, to create the moc-generated source file.

THEN the Visual Studio build would run as described. This isn't as pretty and easy as doing it in QtCreator. I have to manually help it along with the initial setup, and then it works.

EDIT: I know that my custom build step is running because I can see the created/modified time in Windows Explorer, but it is apparently not finishing before the preprocessor comes along. This may be a symptom of the same problem that requires me to run moc.exe manually before I can build my Visual Studio project. Apparently, something is happening in the preprocessor before the PreBuildEvent, even before moc.exe finishes. Or maybe Visual Studio launches moc.exe in its own thread, then continues on in the build step before moc.exe finishes. I don't know. Whatever is going on, if I add a function labeled with Q_SIGNAL, then it must not have a definition because the moc-generated source file will define the function labeled as a signal. The linker will flip out at this unless it is already present in the moc-generated source file, so I had to run moc.exe manually once I added this function before the build would complete.