Custom C++ cout class - output to both console and log file

Question

I'm working on a program that makes heavy use of "cout << strSomething;" to log information to the console. I need to modify the program so that all console output goes to both the console AND a file. Although I can modify the "cout <<" in our code, there are several large third party libraries that also use "cout <<"; those libraries cannot be modified due to their licenses - so modifying all references to "cout <<" is not a solution. Also, the use of "wtee.exe" isn't possible due to the manner in which the command lines are executed.

I am using Visual Studio 2008. I've seen the posting at Google Groups: redirect cout to file, which appears to do EXACTLY what I want to do. The only problem is that the code won't compile. I get C2248 errors "cannot access protected member" on the ->overflow() and ->sync() method calls.

Would anyone know how to get this code to compile? Or an alternate way of redirecting cout to both console and file simultaneously?

Answer 1

The boost::iostreams::tee_device is made for this

#include <boost/iostreams/stream.hpp>
#include <boost/iostreams/tee.hpp>

#include <fstream>
#include <iostream>

int
main()
{
    typedef boost::iostreams::tee_device<std::ostream, std::ofstream> Tee;
    typedef boost::iostreams::stream<Tee> TeeStream;

    std::ofstream file( "foo.out" );
    Tee tee( std::cout, file );

    TeeStream both( tee );

    both << "this goes to both std::cout and foo.out" << std::endl;

    return 0;
}

sample invocation:

samm$ ./a.out
this goes to both std::cout and foo.out
samm$ cat foo.out
this goes to both std::cout and foo.out
samm$ 

Answer 2

This easily extends to additional streams.

OstreamFork.hpp -- Distribute data to 2 streams simultaneously

#include <iomanip>
#include <fstream>
#include <iostream>
using namespace std ;

class ostreamFork           // Write same data to two ostreams
{
public:
  ostream& os1 ;
  ostream& os2 ;

  ostreamFork( ostream& os_one , ostream& os_two )
  : os1( os_one ) ,
    os2( os_two )
  {}

 } ;

                          // For data: int, long , ...
 template <class Data>
 ostreamFork& operator<<( ostreamFork& osf , Data d )
 {
   osf.os1 << d ; 
   osf.os2 << d ;
   return osf ;
 }
                        // For manipulators: endl, flush
 ostreamFork& operator<<( ostreamFork& osf , ostream& (*f)(ostream&)  )
 {
   osf.os1 << f ; 
   osf.os2 << f ;
   return osf ;
 }

                            // For setw() , ...
template<class ManipData>
 ostreamFork& operator<<( ostreamFork& osf , ostream& (*f)(ostream&, ManipData )  )
 {
   osf.os1 << f ; 
   osf.os2 << f ;
   return osf ;
 }

TestOstreamFork.cpp:

#include "stdafx.h"
#include <fstream>
  using namespace std ;
#include "ostreamFork.hpp"

int main(int argc, char* argv[])
{
  ofstream file( "test2.txt" ) ;
  ostreamFork osf( file , cout ) ;

  for ( int i = 0 ; i < 10 ; i++ )
  {
    osf << i << setw(10) << " " << 10*i << endl  ;
  }

    return 0 ;
}

Output to both cout and test2.txt:

0          0
1          10
2          20
3          30
4          40
5          50
6          60
7          70
8          80
9          90