C++: Redirecting STDOUT

In my application, I want to redirect the output that would normally go to the stdout stream to a function I define. I read that you can redirect stdio to a file, so why not to a function?

For example:

void MyHandler( const char* data );

//<<Magical redirection code>>

printf( "test" );
std::cout << "test" << std::endl;

//MyHandler should have been called with "test" twice, at this point

• How can I achieve this / similar behaviour?

@Konrad Rudolph is right, you can totally do this, easily, at least for cout/cerr/clog. You don't even need your own streambuf implementation, just use an ostringstream.

// Redirect cout.
streambuf* oldCoutStreamBuf = cout.rdbuf();
ostringstream strCout;
cout.rdbuf( strCout.rdbuf() );

// This goes to the string stream.
cout << "Hello, World!" << endl;

// Restore old cout.
cout.rdbuf( oldCoutStreamBuf );

// Will output our Hello World! from above.
cout << strCout.str();

Same thing works for cerr and clog, but in my experience that will NOT work for stdout/stderr in general, so printf won't output there. cout goes to stdout, but redirecting cout will not redirect all stdout. At least, that was my experience.

If the amount of data is expected to be small, the freopen/setbuf thing works fine. I ended up doing the fancier dup/dup2 thing redirecting to a pipe.

Update: I wrote a blog post showing the dup2 method I ended up using, which you can read here. It's written for OS X, but might work in other Unix flavors. I seriously doubt it would work in Windows. Cocoa version of the same thing here.

All the other answers are wrong. You can do this, and you don’t need to resort to freopen, nor to any other C or nonstandard functions.

Instead, you need to create your own std::streambuf implementation, i.e. your own stream buffer.

Once you have that, you can redirect the cout stream by switching the buffer:

your_stream_buffer new_buffer;
streambuf* old_buffer = cout.rdbuf(&new_buffer);

cout << "Hello"; // This will be redirected to `new_buffer`.

// Restore original buffer:
cout.rdbuf(old_buffer);

Since I’ve never done that myself I cannot tell you exactly how the implementation of streambuf has to look like. My advice is to have a look at the documentation and do a dummy implementation that prints diagnostics for all implemented functions. This should help figuring out how the class works.

Alternatively, have a look at a C++ reference book of your choice that describes the streambuf class.

It's possible to disable stdin/stdout by dereferencing its pointer:

FILE fp_old = *stdout;  // preserve the original stdout
*stdout = *fopen("/dev/null","w");  // redirect stdout to null
HObject m_ObjPOS = NewLibraryObject(); // call some library which prints unwanted stdout
*stdout=fp_old;  // restore stdout

Answer: Yes you can, via a dup. freopen will only reopen stdout to a file, as you talked about.

Check out https://stackoverflow.com/questions/955962/how-to-buffer-stdout-in-memory-and-write-it-from-a-dedicated-thread

The std::cout object has a fixed meaning, and that is to output to the standard out stream. The user of your program gets to control where standard out is connected to, not you. What you can do is decide whether you wish to write to a file, to standard out or to any other output stream. So in your code you switch what stream you write to.

Again, the point of writing to the standard out stream is to give the user a flexibility in choosing where the output goes to. You're not supposed to redirect standard out; this is something the user is supposed to have the freedom to do.

Another thing is that you shouldn't mix C IO and C++ IO in a C++ program. Choose which IO library you wish to work with and stick to it.

That said, you can in C++ quite elegantly switch streams for a function to take input from by templating the handler function on the template parameters of std::basic_istream<>. Then the function will read its input from the input stream independently of the real kind of stream it's working with. Here's an example:

#include<iostream>
#include<fstream>
#include<string>

template<class Ch, class Tr>
void dodge_this(std::basic_istream<Ch, Tr>& in)
{
    // in is an input stream. read from it as you read from std::cin.
}

int main(int argc, char* argv[])
{
    if( std::string(argv[1]) == "cin" ) {
        dodge_this(std::cin);
    } else if( std::string(argv[1]) == "file" ) {
        std::ifstream file("input.txt");
        dodge_this(file);
    } else {
        dodge_this(dev_null_stream);  // i just made that up. you get the idea.
    }
}

The following is not the best implementation, but it should work for most output...

using std::cout;
using std::endl;
using std::string;
using std::stringstream;

class Handler {
        public:
                Handler() {
                };
                virtual ~Handler() {
                };
                void operator<<(string str) {
                        this->stream << str;
                };
                string print(void) {
                        return this->stream.str();
                }
        private:
                stringstream stream;
};

int main(int argc, char ** argv) {
        Handler handle;
        handle << argv[1];
        cout << handle.print() << endl;
        return 0;
}

I just put a small main function underneath for you to try out and see if it works the way you want it to..

Another option is to place your handler class calls into the inherited streambuf class. I had a requirement to redirect cout to a Win GUI edit control in a dialog box that may be of some use. Here is the class code:

//-------------------------------- DlgStringbuf Definition -----------------------     
class DlgStringbuf : public std::stringbuf
{
public:
  DlgStringbuf(void) : _hwndDlg(NULL), _editControlID(0), _accum(""), _lineNum(0) {}

  void SetDlg(HWND dlg, int editControlID)
    { _hwndDlg = dlg; _editControlID = editControlID; }
  void Clear(void)
    { _accum.clear(); _lineNum = 0; }

protected:
  virtual std::streamsize xsputn(const char* s, std::streamsize num) 
  {
    std::mutex m;
    std::lock_guard<std::mutex> lg(m);

    // Prepend with the line number
    std::string str(s, (const uint32_t)num);
    str = std::to_string(_lineNum) + ": " + str + "\r\n";

    // Accumulate the latest text to the front
    _accum = str + _accum;

    // Write to the Win32 dialog edit control.
    if(_hwndDlg != NULL)
      SetDlgItemTextW(_hwndDlg, _editControlID, (LPCWSTR)(std::wstring(_accum.begin(), _accum.end())).c_str());

    _lineNum++;
    return(num);
  }//end xsputn.

private:
  std::string     _accum;
  HWND            _hwndDlg;
  int             _editControlID;
  uint32_t        _lineNum;

};//end DlgStringbuf.

//-------------------------------- DlgStream Definition ------------------------------
class DlgStream : public std::ostream
{
public:
  DlgStream(void) : std::ostream(&_sbuff) {}

  void SetDlg(HWND dlg, int editControlID)
    { _sbuff.SetDlg(dlg, editControlID); }

  void Clear(void)
    { _sbuff.Clear(); }

private:
  DlgStringbuf  _sbuff;
};

...and in the WinMain, someplace after the dialog box and its edit control is created:

  // Redirect all cout usage to the activity dlg box.
  // Save output buffer of the stream - use unique pointer with deleter that ensures to restore
  // the original output buffer at the end of the program.
  auto del = [&](streambuf* p) {  cout.rdbuf(p); };
  unique_ptr<streambuf, decltype(del)> origBuffer(cout.rdbuf(), del);

  // Redirect the output to the dlg stream.
  _dlgStream.SetDlg(hwndActivityDlg, IDC_EDIT_ACTIVITY);
  _dlgStream.copyfmt(cout);
  cout.rdbuf(_dlgStream.rdbuf());

  cout << "this is from cout";

You can use sprintf to write to a character array and then read the value:

char buf[1024];
sprintf(buf, "test");
MyHandler(buf);

there are also snprintf and a few others depending on platform