customize cout

Question

How can I derive a class from cout so that, for example, writing to it

new_cout << \"message\";

would be equivalent to

Answer 1

Further from Mykola's response, I have the following implementation in my code. The usage is

         LOG_DEBUG("print 3 " << 3); 

prints

         DEBUG (f.cpp, 101): print 3 3 

You can modify it to use FUNCTION along/in place of LINE and FILE

/// Implements a simple logging facility. 
class Logger
{
        std::ostringstream os_;
        static Logger* instance_;
        Logger();
public:
        static Logger* getLogger();
        bool isDebugEnabled() const;
        void log(LogLevelEnum l, std::ostringstream& os, const char* filename, int lineno) const;
        std::ostringstream& getStream()
        { return os_; }
};

void Logger::log(LogLevelEnum l, std::ostringstream& os, const char* filename, int lineno) const
{
        std::cout << logLevelEnumToString(l) << "\t(" << fileName << ": " << lineno << ")\t- " << os.str();
        os.str("");
}

#define LOG_common(level, cptext) do {\
        utility::Logger::getLogger()->getStream() << cptext; \
        utility::Logger::getLogger()->log(utility::level, utility::Logger::getLogger()->getStream(), __FILE__, __LINE__);  \
} while(0); 

enum LogLevelEnum {
        DEBUG_LOG_LEVEL,
        INFO_LOG_LEVEL,
        WARN_LOG_LEVEL,
        ERROR_LOG_LEVEL,
        NOTICE_LOG_LEVEL,
        FATAL_LOG_LEVEL
};

#define LOG_DEBUG(cptext)    LOG_common(DEBUG_LOG_LEVEL, cptext)
#define LOG_INFO(cptext)     LOG_common(INFO_LOG_LEVEL , cptext)
#define LOG_WARN(cptext)     LOG_common(WARN_LOG_LEVEL , cptext)
#define LOG_ERROR(cptext)    LOG_common(ERROR_LOG_LEVEL, cptext)
#define LOG_NOTICE(cptext)   LOG_common(NOTICE_LOG_LEVEL, cptext)
#define LOG_FATAL(cptext)    LOG_common(FATAL_LOG_LEVEL, cptext)

const char* logLevelEnumToString(LogLevelEnum m)
{
        switch(m)
        {
                case DEBUG_LOG_LEVEL:
                        return "DEBUG";
                case INFO_LOG_LEVEL:
                        return "INFO";
                case WARN_LOG_LEVEL:
                        return "WARN";
                case NOTICE_LOG_LEVEL:
                        return "NOTICE";
                case ERROR_LOG_LEVEL:
                        return "ERROR";
                case FATAL_LOG_LEVEL:
                        return "FATAL";
                default:
                        CP_MSG_ASSERT(false, CP_TEXT("invalid value of LogLevelEnum"));
                        return 0;
        }
}