QTextStream interpreted constant by gcc

champax

Hello everyone. I'm new on Qt, and I'm working on Ubuntu 18 Qt5.15.2. I use gcc for debogging.
As part of my project, I wanted to develop my own logger. In this context, I create a Loggin object, which instantiate a LoggerLog object, witch record in real time data in a log file. The Loggin object create e LoggerTerminal object too, which displays with coloring in real time the equivalent in the terminal.

These 2 instances inherit from a virtual class named Logger.

In the write method of LoggerLog, I use a QTextSteam created by my constructor. This QTextStream is itself defined as a variable and not a constant. However, when debugging, I see that the compiler defines it as a constant, which prevents me from using it beyond the constructor.

Sorry, it's a lot of files extracts, but I simplified them to the maximum.

logger.h

#ifndef LOGGER_H
#define LOGGER_H

//Qt += core
#include <QString>

//SL
#include <string>

enum importance_level:  quint8 {critical, v_important, important, normal};
enum type_event:        quint8 {error, alert, success, failure, instruction};

class Logger
{
  public:
    virtual ~Logger(){}
    virtual void write( const QString&,
                        const importance_level level = importance_level::normal,
                        const type_event type = type_event::instruction) const = 0;
    virtual QString get_level(importance_level level) const 
    {
      switch(level)
      {
        case importance_level::normal:
          return "";
        
        case importance_level::important:
          return "IMPORTANT: ";
      }
    }
    virtual QString get_type(type_event type) const
    {
      switch(type)
      {
        case type_event::instruction:
          return "INSTRUCTION: ";
        
        case type_event::failure:
          return "FAILURE: ";
      }
    }

  protected:
    const QString m_T_format = "hh:mm:ss";
    const QString m_DT_format = "ddd d MMMM yyyy hh:mm:ss";
};

#endif // LOGGER_H

loggerLog.h

#ifndef LOGGERLOG_H
#define LOGGERLOG_H

// QT += core
#include <QtCore>
#include <QFile>
#include <QTextStream>
#include <QDataStream>

//virtual class
#include "logger.h"

class LoggerLog : public Logger
{
    public:
        LoggerLog(const QString name_logFile);   
        virtual ~LoggerLog();
        virtual void write( const QString& msg,
                            const importance_level level = importance_level::normal,
                            const type_event type = type_event::instruction) const;
        
    private:
        QFile m_logFile;
        QTextStream m_out;
};

#endif //LOGGERLOG_H

loggerLog.cpp

#include "loggerLog.h"

LoggerLog::LoggerLog(const QString name_logFile)
    : m_logFile(name_logFile), m_out(&m_logFile)
{
    m_logFile.open(QFile::WriteOnly | QFile::Truncate);
    m_out   << " =============================================== \n"
            << "    Begin Output log :\n"
            << "         - " << QDateTime::currentDateTime().toString(m_DT_format) << '\n'
            << " =============================================== \n\n";
}

LoggerLog::~LoggerLog()
{
    m_out   << '\n'
            << " =============================================== \n"
            << "    End   Output log :\n"
            << "         - " << QDateTime::currentDateTime().toString(m_DT_format) << '\n'
            << " =============================================== \n\n";
}


void LoggerLog::write(  const QString& msg,
                        const importance_level level,
                        const type_event type) const
{
    m_out   << ' ' << QTime::currentTime().toString(m_T_format) << ":   - " 
            << get_level(level) << get_type(type) << msg << '\n';
}

my terminal output :

$ ./compile_logger.bash 
../../src/log/loggerLog.cpp: In member function ‘virtual void LoggerLog::write(const QString&, importance_level, type_event) const’:
../../src/log/loggerLog.cpp:42:13: error: no match for ‘operator<<’ (operand types are ‘const QTextStream’ and ‘QString’)
     m_out   << QTime::currentTime().toString(m_T_format) << ":   - "
     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you have any idea of the reason of this interpretation, or a solution to avoid this QTextStream to pass constant, it would be very nice. Thank's in advance !!

Champax

Christian Ehrlicher

Your write() function is const so the members are const too - basic c++. Make you function non-const.

sierdzio

@champax said in QTextStream interpreted constant by gcc:

void LoggerLog::write( const QString& msg,
const importance_level level,
const type_event type) const

Your method is const but you are trying to modify a member variable m_out inside of it.

Change write() to be non-const and it will work.