Mirror console output to file in c++ - c++

Question

In C++, is there a smart way to mirror output from stdout to both the console and the file?
I'm hoping there is a way to do it like in this question.
Edit: It would be nice to be able to do this with just the standard libraries (ie: no boost)..

Answer 1

Alternatively, just start your program so it's piped to the tee command.

Answer 2

You could try a Tee Device provided by Boost.Iostreams.
A Tee device directs output to multiple streams. As far as I know, you can chain them to reach theoretically infinite output devices from one write call.
This answer shows an example for how to do exactly what you want.

Answer 3

You can do this by creating a class that extends std::streambuf, and has a std::ofstream member. After overriding the std::streambuf::overflow and std::streambuf::sync member functions, you'll be all set.
Most of the code below comes from here. The stuff I've added ("ADDED:") for file-mirroring is pointed out. This might be overly complex as I'm at work and can't pore over it fully to simplify it, but it works - the bonus of doing it this way (instead of just using a std::streambuf* is that any code (say you have an external library) that writes to std::cout will write to your file.
mystreambuf.h
#ifndef MYSTREAMBUF_H
#define MYSTREAMBUF_H
template <typename charT, typename traits = std::char_traits<charT> >
class mystreambuf : public std::basic_streambuf<charT, traits>
{
public:
// The size of the input and output buffers.
static const size_t BUFF_SIZE = 1024;
typedef traits traits_type;
typedef typename traits_type::int_type int_type;
typedef typename traits_type::pos_type pos_type;
typedef typename traits_type::off_type off_type;
// You can use any method that you want, but here, we'll just take in a raw streambuf as a
// slave I/O object. xor_char is what each character is xored with before output.
explicit mystreambuf(std::streambuf* buf)
: out_buf_(new charT[BUFF_SIZE])
{
// ADDED: store the original cout stream and open our output file
this->original_cout = buf;
outfile.open("test.txt");
// Initialize the put pointer. Overflow won't get called until this buffer is filled up,
// so we need to use valid pointers.
this->setp(out_buf_, out_buf_ + BUFF_SIZE - 1);
}
// It's a good idea to release any resources when done using them.
~mystreambuf() {
delete [] out_buf_;
// ADDED: restore cout, close file
std::cout.rdbuf(original_cout);
outfile.flush();
outfile.close();
}
protected:
// This is called when there are too many characters in the buffer (thus, a write needs to be performed).
virtual int_type overflow(int_type c);
// This is called when the buffer needs to be flushed.
virtual int_type sync();
private:
// Output buffer
charT* out_buf_;
// ADDED: tracking the original std::cout stream & the file stream to open
std::streambuf* original_cout;
std::ofstream outfile;
};
#endif
mystreambuf.cpp
// Based on class by perfectly.insane from http://www.dreamincode.net/code/snippet2499.htm
#include <fstream>
#include <iostream>
#include <streambuf>
#include "mystreambuf.h"
// This function is called when the output buffer is filled.
// In this function, the buffer should be written to wherever it should
// be written to (in this case, the streambuf object that this is controlling).
template <typename charT, typename traits>
typename mystreambuf<charT, traits>::int_type
mystreambuf<charT, traits>::overflow(typename mystreambuf<charT, traits>::int_type c)
{
charT* ibegin = this->out_buf_;
charT* iend = this->pptr();
// Reset the put pointers to indicate that the buffer is free
// (at least it will be at the end of this function).
setp(out_buf_, out_buf_ + BUFF_SIZE + 1);
// If this is the end, add an eof character to the buffer.
// This is why the pointers passed to setp are off by 1
// (to reserve room for this).
if(!traits_type::eq_int_type(c, traits_type::eof())) {
*iend++ = traits_type::to_char_type(c);
}
// Compute the write length.
int_type ilen = iend - ibegin;
// ADDED: restore cout to its original stream, output to it, output to the file, then set cout's stream back to this, our streambuf)
std::cout.rdbuf(original_cout);
out_buf_[ilen] = '\0';
std::cout << out_buf_;
outfile << out_buf_;
std::cout.rdbuf(this);
return traits_type::not_eof(c);
}
// This is called to flush the buffer.
// This is called when we're done with the file stream (or when .flush() is called).
template <typename charT, typename traits>
typename mystreambuf<charT, traits>::int_type
mystreambuf<charT, traits>::sync()
{
return traits_type::eq_int_type(this->overflow(traits_type::eof()),
traits_type::eof()) ? -1 : 0;
}
int main(int argc, char* argv[]) {
mystreambuf<char> filter(std::cout.rdbuf());
std::cout.rdbuf( &filter );
std::cout << "Hello World" << std::endl;
return 0;
}
hope this helps; cheers