Add to Qt GUI logger functionality

JonB

@Gaetano03
That a object in main() --- or rather, the QApplication object --- can be accessed from anywhere in a Qt program (because it's a singleton) via qApp macro or qGuiApp macro or QCoreApplication *QCoreApplication::instance(). That means you can use those anywhere in a Qt application, without worrying about the a variable in main().

You could use that to do something like create your own signal (must subclass QAppplication, but that's no problem), which you could connect() to a slot somewhere to update your QPlainText widget. Then even a static C function which is called from the function pointer we discussed can get at a Qt object to emit a signal. Or, you could skip signal/slot if you have, by whatever means, a global pointer to your desired QPlainText, which the static log function can use to set its text directly. Just be careful with coding, but it should be doable!

not sure what happens when it points to a QObject function (the call in the external logger library does not know what to do)?

All the external library knows is the outside world has set g_loggerCallback to the address of some function which takes a std:string (or whatever) as a parameter. That is all it needs to know to be able to call it. What happens at the other end in the function pointed to --- log(std::string message), which is in your Qt program, not the library --- is the other end's problem, so far as the library is concerned. [Which is why it would be nice if it worked and didn't crash :) ]

Gaetano03

@JonB Getting a little bit of shakes now XD

"You could use that to do something like create your own signal (must subclass QAppplication, but that's no problem), which you could connect() to a slot somewhere to update your QPlainText widget. "

The other problem would be that signal and slot should be static as well? can you use connect on static objects?

If I understand well, your first option would be to create a class that inherits Qapplication and get an instance of the Qapplication. Here I define my own signal (static?) which I then emits thorugh the log function defined above (still static). This should be catched by the QPlainText widget, but at this point how? Should I make the slot static as well? Actually it seems I am getting real troubles when I try to use connect with static methods.

Will investigate also second option

Thanks a lot

JonB

@Gaetano03
The only thing which needs to be "static" is the global, non-member function void log(std::string message), to which you set g_loggerCallback = log;, so that the library can invoke it via g_loggerCallback(message).

That can go something like:

void log(std::string message)
{
    emit qApp()->libraryLogged(message);
    // or probably
    MyApplication *myApp = qobject_cast<MyApplication *>(qApp());
    emit myApp->libraryLogged(message);
}

You would need to subclass and use QApplication something like:

class MyApplication : public QApplication
{
signals:
    void libraryLogged(std::string message);
}

Now it remains to have done the connect(). Somewhere in your code which can "see" wherever you create the QTextEdit *logmessageTextEdit = new QTextEdit, or afterwards, and can see the class MyApplication definition, needs e.g.

MyApplication *myApp = qobject_cast<MyApplication *>(qApp());
connect(myApp(), &MyApplication::libraryLogged, this, &ThisClass::onLibraryLogged);

class ThisClass
{
slots:
    void onLibraryLogged(std::string message) { this->logmessageTextEdit->append(message); }
}

Alternatively, if you don't want to use signals/slots, keep some global reference to the logmessageTextEdit pointer to the QTextEdit and just go

void log(std::string message)
{
    if (g_logmessageTextEdit != nullptr)
        g_logmessageTextEdit->append(message);
}

Gaetano03

@JonB said in Add to Qt GUI logger functionality:

Not sure where to declare the log function though

At this point what I did is

I created a QApplication subclass in a .h file (integer_interface.h) which looks like this

class integer_interface : public QApplication
{
    Q_OBJECT
public:
    integer_interface(int &argc, char *argv[]) {}; // empty constructor

signals:
    void libraryLogged(std::string message);

};

then in my logger_widget class, logger_widget.h (the one that contains the QPlainTextWidget) I have the following class definition:

namespace Ui {
class logger_widget; 
}

class logger_widget : public QWidget
{
    Q_OBJECT

public:
    explicit logger_widget(QWidget *parent = nullptr);
    ~logger_widget();

signals:


public slots:
    void onLibraryLogged(std::string message);


private:
    Ui::logger_widget *ui;

    integer::logger::errors err_init;
    integer::logger::integer_logger& logger = integer::logger::integer_logger::get_instance();
    integer_interface *myApp = qobject_cast<integer_interface *>(qApp);

};

and a declaration of the log static function outside the class

static void log(std::string message);

In the logger_widget constructor I have the following two lines to define the connection

integer::logger::g_loggerCallback = log;
connect(myApp,&integer_interface::libraryLogged,this,&logger_widget::onLibraryLogged);

while log and onLibraryLogged implementations look like this

void logger_widget::onLibraryLogged(std::string message)
{
    ui->plainTextEdit_logger->appendPlainText(QString::fromStdString(message));
}

void log(std::string message)
{
    integer_interface *myApp = qobject_cast<integer_interface *>(qApp);
    emit myApp->libraryLogged(message);
}

constructor and function implementations are obviously in the corresponding logger_widget.cpp file

It compiles fine, but I get a runtime error "abort()" and the following message

qt.core.qobject.connect: QObject::connect(integer_interface, logger_widget): invalid nullptr parameter

JonB

@Gaetano03
So I assume that occurs at runtime when it hits

connect(myApp,&integer_interface::libraryLogged,this,&logger_widget::onLibraryLogged);

and indicates one of the two objects are nullptr. Check on line above with e.g.

qDebug() << myApp << this;

?

If myApp == nullptr, you did remember to create an integer_interface, not just a QApplication now, didn't you...??

Everywhere you go integer_interface *myApp = qobject_cast<integer_interface *>(qApp); you might follow that with Q_ASSERT(myApp); to verify all is well at that instant.

P.S.

integer_interface(int &argc, char *argv[]) {}; // empty constructor

Do you have integer_interface(int &argc, char *argv[]) : QApplication(argc, argv) somewhere?

Gaetano03

@JonB
I think the problem might be where I define myApp is not correct? Which is at the moment into the logger_widget class

when I qDebug as you said I get indeed the following value for myApp

QObject(0x0) logger_widget(0x1e04f63bcd0, name="logger_widget")

and the app crashes few seconds after it starts

Should I declare the myApp out of any member class as I did for the log function?

JonB

@Gaetano03

@JonB said in Add to Qt GUI logger functionality:

If myApp == nullptr, you did remember to create an integer_interface, not just a QApplication now, didn't you...??

??

Gaetano03

@JonB

Do you have integer_interface(int &argc, char *argv[]) : QApplication(argc, argv) somewhere?

ye I have it in an integer_interface.cpp, looks like this

#include "integer_interface.h"

integer_interface::integer_interface(int &argc, char *argv[])
    : QApplication(argc, argv)
{

}

??

I have it declared in my logger_widget.h class file (if you also see my previous reply), but was thinking maybe should be declared outside of the class?

Or maybe you meant something else from this

Christian Ehrlicher

@Gaetano03 said in Add to Qt GUI logger functionality:

I have it declared in my logger_widget.h class file (if you also see my previous reply), but was thinking maybe should be declared outside of the class?

You should also instantiate it somewhere.

Gaetano03

@Christian-Ehrlicher

I have it declared and instantiated both in the logger_widget.h class among the private members

private:
    Ui::logger_widget *ui;

    integer::logger::errors err_init;
    integer::logger::integer_logger& logger = integer::logger::integer_logger::get_instance();
    integer_interface *myApp = qobject_cast<integer_interface *>(qApp);

and in the static log function

void log(std::string message)
{
    integer_interface *myApp = qobject_cast<integer_interface *>(qApp);
    emit myApp->libraryLogged(message);
}

as suggested by @JonB

PS
@JonB doing as you suggested as sanity check with the following lines

    integer_interface *myApp = qobject_cast<integer_interface *>(qApp);
    Q_ASSERT(myApp);

gives a bunch of errors not sure I understand:
logger_widget.h:42:14: C++ requires a type specifier for all declarations

logger_widget.h:42:5: Expected ')'
0:0: :42:5: note: to match this '('

logger_widget.h:42:14: Duplicate member 'myApp'
logger_widget.h:41:24: previous declaration is here

JonB

@Gaetano03
Look, can you please show your main() function now. I have a funny feeling your still have QApplication a(argc, argv); there, and what do you think you actually need instead of that?

JonB

@Gaetano03 said in Add to Qt GUI logger functionality:

integer_interface *myApp = qobject_cast<integer_interface *>(qApp);
Q_ASSERT(myApp);

If that is in a .h where you are declaring member variables in a class definition you cannot add arbitrary C++ statements to execute..... Put the qobject_cast<> in the constructor, where you can verify it.

Gaetano03

@JonB
you have the right funny feeling, just realized how silly I was.

Everything works fine now and I get stuff printed on my QPlainText, at least for now XD.

Thanks a lot, really!

I really appreciate

JonB

@Gaetano03
Yeah, do you get the point? You can subclass QApplication to add whatever you want, but then you must create a SubclassedApplication not the original base class QApplication to use it. Just like any other subclassing.

qApp() macro returns QApplication *-type pointer, for whatever (single) QApplication instance has been created. If you derived and created the derived instance, qApp won't return that derived type (though it will return the correct derived instance). So to call methods/access variables you defined in your subclass that's why we qobject_cast<SubclassedApplication *>(qApp).

Gaetano03

@JonB
Yes I got the point, thanks also for this explanation!! That helps a lot understanding what I am doing behind the lines!!!

I have to admit that I worked so much on the library that I forgot for a while a main calling it and launching the app was even existing XD