Question

I am looking for a way to branch (tee) the input read from an istream (cin, in my case) out to a log file (clog/ofstream/etc), while still using the input for processing.

I have read about boost::tee_device, and it is very similar to my requirements. Unfortunately, it is implemented as an ostream, and thus solves a similar problem from “the other side of the pipe”.

I attempted to write an istream (adaptor) class which forwards the input functions on to a wrapped input stream (cin), and also sends what was read to the log file.

This works fine for basic types which call operator>>(…) directly, however, I have run into issues with some more advanced usage of the input stream, for example, for operator>>(std::string), and the std::string getline function.

Is there any easier way to do this (possibly via rdbuf() manipulation)?

Thanks!

Edit: I could change my code all over the place to something like: cin >> value; clog << value; — but that would be a significant and ugly change. I would also prefer to have an easy way to turn logging off. Thus, I would like a way to model this as an istream “filter” and then simply replace all references to cin with this istream “logger”.

Ideal Solution:

class log_istream : public std::istream
{
public:
    log_istream( std::istream & in , std::ostream & out );

    /* ... istream forwarding functions ... */

private:
    std::istream & in_;
    std::ostream & out_;     
};

int main() {
    log_istream logger( std::cin , std::ofstream("logfile.out") );

    logger >> value; // this implies infile >> value and logfile << value
    getline(logger,my_string); // this also implies logfile.writeline(value)
    // etc
}

etc.

Answer 1

Final Answer:

#ifndef TEE_ISTREAM_H_
#define TEE_ISTREAM_H_

/*****************************************************************************/

#include <boost/iostreams/tee.hpp>
#include <boost/iostreams/invert.hpp>
#include <boost/iostreams/device/file.hpp>
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/tr1/memory.hpp>
#include <iostream>

/*****************************************************************************/

namespace bio = boost::iostreams;

/*****************************************************************************/

class tee_source : public bio::source {
public:
    tee_source( std::istream & in, const std::string & filename )
        : in_(in), log_file_(filename, std::ios::app), tee_(bio::tee(log_file_), 1)
    { }

    std::streamsize read(char* s, std::streamsize n)
    {
        return tee_.read(in_,s,n);
    }

private:
    std::istream &                               in_;
    bio::file                                    log_file_;
    bio::inverse< bio::tee_filter< bio::file > > tee_;
};

/*****************************************************************************/

typedef bio::filtering_istream                tee_istream_t;
typedef std::tr1::shared_ptr< tee_istream_t > tee_istream_ptr_t;

/*****************************************************************************/

inline tee_istream_ptr_t make_tee_istream( std::istream & in, const std::string & filename )
{
    return tee_istream_ptr_t( new tee_istream_t( tee_source( in , filename ), 0 ) );
}

/*****************************************************************************/

#endif