Printing to nowhere with ostream

问题:

I'd like to send data to nowhere, I mean that I don't want to print data in console nor in file, but I need some std::ostream object. How to do that?

回答1:

I've used:

std::ostream bitBucket(0);

recently without problems, although it was flagged as having some potential problems if you looked at it from a certain angle (see the link below).

Aside: From what I understand (and I'm not entirely sure of this), that call above eventually ends up calling basic_ios::init(0) and, because that's a NULL pointer being passed in, it sets the stream state, as returned by the rdstate() function, to the badbit value.

This in turn prevents the stream from outputting any more information, instead just tossing it away.

The following program shows it in action:

#include <iostream>

int main (void) {
    std::ostream bitBucket(0);
    bitBucket << "Hello, there!" << std::endl;
    return 0;
}

The page where I got it from also had this as a probably-cleaner solution (slightly modified to remove the duplication of my first solution above):

#include <iostream>

class null_out_buf : public std::streambuf {
    public:
        virtual std::streamsize xsputn (const char * s, std::streamsize n) {
            return n;
        }
        virtual int overflow (int c) {
            return 1;
        }
};

class null_out_stream : public std::ostream {
    public:
        null_out_stream() : std::ostream (&buf) {}
    private:
        null_out_buf buf;
};

null_out_stream cnul;       // My null stream.

int main (void) {
    std::cout << std::boolalpha;

    //testing nul

    std::cout << "Nul stream before: " << cnul.fail() << std::endl;
    cnul << "Goodbye World!" << std::endl;
    std::cout << "Nul stream after: " << cnul.fail() << std::endl;
}

回答2:

The simplest solution is just to output to an unopened std::ofstream (or any other output stream in an error state). This will result in the stream being permanently in an error state. This could be an advantage (<< operators will skip the formatting), but if any code that you can't control checks for errors, and does something particular if they occur, you'll likely have problems.

Otherwise, it's pretty simple to implement a null stream; the only streambuf function you really have to override is overflow. Something like the following should do the trick:

class NulStreambuf : public std::streambuf
{
    char                dummyBuffer[64];
protected:
    virtual int         overflow( int c )
    {
        setp( dummyBuffer, dummyBuffer + sizeof( dummyBuffer ) ) ;
        return (c == EOF) ? '\0' : c ;
    }
};

(The buffer will avoid some unnecessary virtual function calls. On some platforms, this makes a significant difference.)

Then create an output stream which uses it:

class NulOStream : public NulStreambuf, public std::ostream
{
public:
    NulOStream() : std::ostream( this ) {}
};

(The use of inheritance, rather than containment, ensures that the streambuf is fully constructed before being passed to the ostream. This generally isn't necessary in practice, but the standard doesn't seem to authorize passing a not yet constructed streambuf to the constructor of ostream.)

回答3:

Simplest solution: Use a std::stringstream.

#include <sstream>
#include <iostream>

void func(std::ostream& o){
    o << "blatest\n";
}

int main(){
    std::stringstream black_hole;
    func(std::cout);
    func(black_hole);
}

The stringstream will contain the output, but if you don't use it, it's the same as if it was never filled.

回答4:

Some suggestions here http://bytes.com/topic/c/answers/589209-std-null-stream

A good answer from that site

Use ordinary std::fstream, open it only for writing to required file "/dev/null". It should work.

If you really want to create own stream, just derive it from basic_ostream and simply define your own operator<< to be function which only returns stream reference. You will have to write dummy 'write' and 'put' method too (and all other methods for output).

In fact,

#include <streambuf>
#include <ostream>

template <class cT, class traits = std::char_traits<cT> >
class basic_nullbuf: public std::basic_streambuf<cT, traits> {
typename traits::int_type overflow(typename traits::int_type c)
{
return traits::not_eof(c); // indicate success
}
};

template <class cT, class traits = std::char_traits<cT> >
class basic_onullstream: public std::basic_ostream<cT, traits> {
public:
basic_onullstream():
std::basic_ios<cT, traits>(&m_sbuf),
std::basic_ostream<cT, traits>(&m_sbuf)
{
init(&m_sbuf);
}

private:
basic_nullbuf<cT, traits> m_sbuf;
};

typedef basic_onullstream<char> onullstream;
typedef basic_onullstream<wchar_t> wonullstream;

From http://bytes.com/topic/c/answers/428285-null-ostream

回答5:

Since nobody mentioned it, if it's about suppressing std or error output, you can simply close the corresponding file descriptors (e.g. fclose (stdout) or fclose (stderr)).
That will shup up everything, including things like printf or fprintf (stderr, ...)
So you will indeed keep using the usual cout or cerr, but they will be turned into bit buckets.