Using << operator to write to both a file and cout

Question

I\'d like to overload << operator to write the value it takes to a file and cout. I have tried to do it with following code, but couldn\'t succeed it. It just writes t

Answer 1

The usual way of doing this is to use a filtering streambuf, which forwards to both of the target streambufs. Something like the following should do the trick:

class LoggingStreambuf : public std::streambuf
{
    std::streambuf* myPrinciple;
    std::ostream*   myOwner;
    std::filebuf    myLogging;
protected:
    int overflow( int ch ) override
    {
        myLogging->sputc( ch );
        return myPrinciple->sputc( ch );
    }
public:
    LoggingStreambuf( std::streambuf* principal, std::string const& logFileName )
        : myPrinciple( principal )
        , myOwner( nullptr )
        , myLogging( logFileName )
    {
    }

    LoggingStreambuf( std::ostream& principal, std::string const& logFileName )
        : myPrinciple( principal.rdbuf() )
        , myOwner( &principal )
        , myLogging( logFileName )
    {
        myOwner.rdbuf( this );
    }

    ~LoggingStreambuf()
    {
        if ( myOwner != nullptr ) {
            myOwner.rdbuf( myPrinciple );
        }
    }
};

(This particular code supposes that the file output is a log file, a secondary output on which errors should be ignored. Other error handling strategies can also be implemented, and of course, there's no reason that you couldn't provide two arbitrary std::streambuf*, and create a new std::ostream outputting through an instance of the custom streambuf.)

To use this class, as it is written:

LoggingStreambuf logger( std::cout, "logfile.txt" );
//  And output to std::cout as usual...

More generally, of course: anytime you want to do something special with regards to the data sink or source of a stream, you implement a new std::streambuf, since this is the class which handles sinking and sourcing of data for the streams.