C++: Redirecting STDOUT

In my application, I want to redirect t开发者_JAVA技巧he 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.

Invoking a callback function whenever something’s written to the underlying standard output stream is hard: it requires overriding low-level, system-specific functions (on POSIX systems, it would require at least overriding write, and the way this is called might depend on the standard library implementation, so it might be different between e.g. glibc and musl).

But depending on what exactly you’re after you can solve this in C++ without resorting to low-level OS specific functions, by manipulating the C++ stream buffers directly.

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

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

auto buf = callback_streambuf(MyHandler);
auto pold_buffer = std::cout.rdbuf(&buf);

std::cout << "Hello" << std::cout;

// Restore original buffer:
std::cout.rdbuf(pold_buffer);

However, what this implementation won’t do is call your callback function exactly twice. Instead, the number of calls will depend on several factors, but it generally will not depend on the number of stream insertions (<<), and there is no way around that!

For the specific example above, the callback is called once, with the data "Hello\n".

The following minimal implementation illustrates how to get a streambuf to call your handler:

class callback_streambuf : public std::streambuf {
public:
    callback_streambuf(std::function<void(char const*, std::streamsize)> callback) : callback(callback) {}

protected:
    std::streamsize xsputn(char_type const* s, std::streamsize count) {
        callback(s, count);
        return count;
    }

private:
    std::function<void(char const*, std::streamsize)> callback;
};

This implementation has several caveats. For instance, it does the wrong thing when attempts are made to use it as an input stream. It doesn’t override overflow (since I don’t think this is ever called, although I find conflicting information about this on the internet; at any rate, adding overflow would be trivial). I didn’t implement synchronisation, so the callback will be called concurrently from multiple threads. Furthermore, there is no error handling, since the callback does not return a success status. I also had to change the signature of the callback to

void MyHandler(char const* data, std::streamsize count);

The second parameter is required since data isn’t a string, it’s a raw char buffer, so there is no way to determine its length intrinsically, and MyHandler can do nothing useful with the data without knowing its length.

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

Check out How to buffer stdout in memory and write it from a dedicated thread

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

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.
    }
}

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