Log issues

[Cobra91151]

Hi! I have created the log program and want to collect some logs and display it in QTextBrowser without saving to the file. The problem is I get error:

log.obj:-1: error: LNK2001: unresolved external symbol "private: static class QTextBrowser * Log::textBrowser" (?textBrowser@Log@@0PAVQTextBrowser@@A)

static QTextBrowser *textBrowser;
static void writeToLog(QString log)  {
       textBrowser->append(log);
}

How to fix this issue? Thanks.

[koahnig]

@Cobra91151

Did you somewhere assign something to textBrowser?

The initialization is missing.

[mawh1960]

Have you included widgets in your .pro file?

QT += widgets

[Cobra91151]

@mawh1960

Yes. I have it included in .pro file.

[Cobra91151]

@koahnig

In the Log class constructor I have initialized - textBrowser = new QTextBrowser(this);

[Chris Hennes]

The most common time I run into this error is when I declare a static variable in my header file, but forget to create it in my code. For example, in your code above you have:
static QTextBrowser *textBrowser;
You've probably got that as a member of your Log class defined in your .h file, so in your .cpp file, you need:
QTextBrowser *Log::textBrowser;

[Cobra91151]

@Chris-Hennes

I have changed code in .cpp file (in constructor):

QTextBrowser *Log::textBrowser;
textBrowser->installEventFilter(this);
connect(textBrowser, &QTextBrowser::textChanged, this, &Log::checkLogTextBrowser);

but I get error: C2655: 'Log::textBrowser': definition or redeclaration illegal in current scope?

[Chris Hennes]

The creation of the QTextBrowser does not belong in your constructor, it must be a standalone line in your .cpp file.

[Cobra91151]

@Chris-Hennes

Thanks. I have fixed it but when I use Log::writeToLog("Some data"); in for example Test.cpp, the app crashes.

Code:
Log.h
static void writeToLog(QString logMessage);

Log.cpp

void Log::writeToLog(QString logMessage)
{
    textBrowser->append(logMessage);
}

The same crash when I open Log also exists. Any suggestions? Thanks.

Update:
textBrowser->installEventFilter(this);

It crashes on this line:

connect(textBrowser, &QTextBrowser::textChanged, this, &Log::checkLogTextBrowser);

[Chris Hennes]

Your static variable is probably uninitialized. It looks to me like you are trying to create something like a Singleton -- are you familiar with that design pattern? My guess is that you are trying to call writeToLog without first initializing the QTextBrowser. Remember that by using a static member function you do not have an object to work with, and therefore it's very possible that your class's constructor is never called.

[Chris Hennes]

In fact, nowhere in any of the code you've entered here do you ever actually instantiate a QTextBrowser. Where is your allocation statement (e.g. textBrowser = new QTextBrowser;)?

[Cobra91151]

@Chris-Hennes

Thanks. Now it works but the same as it was without static, when I use Log::writeToLog("Some data"); to write to QTextBrowser in for example Test window and close it, then open Log window to check data, QTextBrowser is empty.

To open Log I use:

Log *appLog = new Log(this);
appLog->show();

[Chris Hennes]

How/where are you allocating the QTextBrowser?

[Cobra91151]

@Chris-Hennes

I have initialized textBrowser = new QTextBrowser(); in constructor and in function below because constructor is not called when I call Log::writeToLog("Some data") in Test.cpp and app crashes.

void Log::writeToLog(QString logMessage)
{
    textBrowser = new QTextBrowser();
    textBrowser->append(logMessage);
}

But the result is the same (QTextBrowser is empty), I think because of two initialization.

[Chris Hennes]

Definitely do not make a new QTextBrowser every time you call writeToLog, that defeats the purpose of making it static. You need to instantiate it one time in any given program run. Start by initializing it to null (or Q_NULLPTR), then in writeToLog check to see if it exists already. If it does not, then allocate it and set its parameters as appropriate, so something like:

QTextBrowser *Log::textBrowser = null;

...

void Log::writeToLog(QString logMessage)
{
    if (!textBrowser) {
        textBrowser = new QTextBrowser;
        textBrowser->installEventFilter(this);
        connect(textBrowser, &QTextBrowser::textChanged, this, &Log::checkLogTextBrowser);
    }
    textBrowser->append(logMessage);
}

[Chris Hennes]

(Note that my example code won't actually work as written because writeToLog is static, so you don't have a "this" pointer... those two lines will need to be in your constructor, along with appropriate checks to create textBrowser there if needed.)

[Cobra91151]

@Chris-Hennes

Thanks for code example. It now works. One more question, since Log::writeToLog(QString logMessage) is a static function I can't use this, I get error: log.cpp:116: error: C2355: 'this': can only be referenced inside non-static member functions or non-static data member initializers. So I need to get the instance of the Log class. Any ideas?

[Chris Hennes]

Yes, remove those two lines from the if statement in writeToLog, but keep them in an identical if statement in your constructor.

[Cobra91151]

@Chris-Hennes

I have added also check in constructor, because it crashes when open Log without adding anything to it.

   if (!textBrowser) {
        textBrowser = new QTextBrowser();
        textBrowser->installEventFilter(this);
        connect(textBrowser, &QTextBrowser::textChanged, this, &Log::checkLogTextBrowser);
    } else {
        textBrowser->installEventFilter(this);
        connect(textBrowser, &QTextBrowser::textChanged, this, &Log::checkLogTextBrowser);
    }

Also I have notices that connect(textBrowser, &QTextBrowser::textChanged, this, &Log::checkLogTextBrowser); is not working. I have some buttons which is disabled and when new log data is added, those buttons should be enabled, but it still disabled.

void Log::checkLogTextBrowser()
{
     if (textBrowser->document()->isEmpty()) {
         saveLog->setEnabled(false);
         saveLogAs->setEnabled(false);
         openLog->setEnabled(false);
         clearLog->setEnabled(false);
         deleteLog->setEnabled(false);
         sendLog->setEnabled(false);
     } else {
         saveLog->setEnabled(true);
         saveLogAs->setEnabled(true);
         openLog->setEnabled(false);
         clearLog->setEnabled(true);
         deleteLog->setEnabled(false);
         sendLog->setEnabled(false);
     }
}

I can use some trigger as bool checkButtonsState to check when log is added and then set it true to enable buttons? But why connect is not working?

[Chris Hennes]

Make sure to call checkLogTextBrowser() directly in your constructor as well, to handle anything that was logged before you opened it.