Compose output streams

Question

I'd like to compose two (or more) streams into one. My goal is that any output directed to cout, cerr, and clog also be outputted into a file, along with the original stream. (For when things are logged to the console, for example. After closing, I'd like to still be able to go back and view the output.)

I was thinking of doing something like this:

class stream_compose : public streambuf, private boost::noncopyable
{
public:
    // take two streams, save them in stream_holder,
    // this set their buffers to `this`.
    stream_compose;

    // implement the streambuf interface, routing to both
    // ...

private:
    // saves the streambuf of an ios class,
    // upon destruction restores it, provides
    // accessor to saved stream
    class stream_holder;

    stream_holder mStreamA;
    stream_holder mStreamB;
};

Which seems straight-forward enough. The call in main then would be something like:

// anything that goes to cout goes to both cout and the file
stream_compose coutToFile(std::cout, theFile);
// and so on

I also looked at boost::iostreams, but didn't see anything related.

Are there any other better/simpler ways to accomplish this?

Answer 1

I would write a custom stream buffer that just forwards data to the buffers of all your linked streams.

#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>
#include <functional>

class ComposeStream: public std::ostream
{
    struct ComposeBuffer: public std::streambuf
    {
        void addBuffer(std::streambuf* buf)
        {
            bufs.push_back(buf);
        }
        virtual int overflow(int c)
        {
            std::for_each(bufs.begin(),bufs.end(),std::bind2nd(std::mem_fun(&std::streambuf::sputc),c));
            return c;
        }

        private:
            std::vector<std::streambuf*>    bufs;

    };  
    ComposeBuffer myBuffer;
    public: 
        ComposeStream()
            :std::ostream(NULL)
        {
            std::ostream::rdbuf(&myBuffer);
        }   
        void linkStream(std::ostream& out)
        {
            out.flush();
            myBuffer.addBuffer(out.rdbuf());
        }
};
int main()
{
    ComposeStream   out;
    out.linkStream(std::cout);
    out << "To std::cout\n";

    out.linkStream(std::clog);
    out << "To: std::cout and std::clog\n";

    std::ofstream   file("Plop");
    out.linkStream(file);
    out << "To all three locations\n";
}

Answer 2

You mention having not found anything in Boost.IOStreams. Did you consider tee_device?