Is there a null std::ostream implementation in C++

问题:

I'm looking for a std::ostream implementation that acts like /dev/null. It would just ignore anything that is streamed to it. Does such a thing exist in the standard libraries or Boost? Or do I have to roll my own?

回答1:

If you have boost, then there's a null ostream & istream implementation available in boost/iostreams/device/null.hpp . The gist of it:

#include "boost/iostreams/stream.hpp"
#include "boost/iostreams/device/null.hpp"
...
boost::iostreams::stream< boost::iostreams::null_sink > nullOstream( ( boost::iostreams::null_sink() ) );
...

回答2:

The simplest solution is just to use an unopened std::ofstream. This will result in an error state in the stream, but most outputters won't check this; the usual idiom is to leave the check to the end, after the close (which would put it in code you wrote, where you know that the stream should be invalid).

Otherwise, it's pretty straight forward to implement: just create a streambuf which contains a small buffer, and sets it up in overflow (always returning success). Note that this will be slower than the unopened file, however; the various >> operators will still to all of the conversion (which they don't do if the stream has an error state).

EDIT:

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

It's usual to provide a convenience class derived from istream or ostream as well, which will contain an instance of this buffer which it uses. Something along the lines of:

class NulOStream : private NulStreambuf, public std::ostream
{
public:
    NulOStream() : std::ostream( this ) {}
    NulStreambuf* rdbuf() const { return this; }
};

Or you can just use an std::ostream, passing the address of the streambuf to it.

回答3:

If you set badbit on a stream it won't output anything:

#include <iostream>

int main() {
    std::cout << "a\n";

    std::cout.setstate(std::ios_base::badbit);
    std::cout << "b\n";

    std::cout.clear();
    std::cout << "c\n";
}

Outputs:

a
c

回答4:

I know this is very old thread, but I would like to add this to anyone who is looking for the same solution without boost and and the fastest one.

I combined three different proposals above and one writing directly to /dev/null (so it involves kernel.)

Surprisingly the NullStream that got the most votes performed the worst.

Here are results for 100,000,000 writes:

a) /dev/null : 30 seconds
b) NullStream: 50 seconds
c) badbit    : 16 seconds (the winner in speed, but cannot test for errors!)
d) boost     : 25 seconds (the ultimate winner)

Here is the test code

#include <iostream>
#include <fstream>
#include <time.h>
#include <boost/iostreams/stream.hpp>

class NullStream : public std::ostream {
    class NullBuffer : public std::streambuf {
    public:
        int overflow( int c ) { return c; }
    } m_nb;
public:
    NullStream() : std::ostream( &m_nb ) {}
};

int test( std::ostream& ofs, const char* who ) {
    const time_t t = time(NULL);
    for ( int i = 0 ; i < 1000000000 ; i++ )
        ofs << "Say the same" ;
    std::cout << who << ": " << time(NULL) - t << std::endl;
}

void devnull() {
    std::ofstream ofs;
    ofs.open( "/dev/null", std::ofstream::out | std::ofstream::app );
    test(ofs, __FUNCTION__);
    ofs.close();
}

void nullstream() {
    NullStream ofs;
    test(ofs, __FUNCTION__);
}

void badbit() {
    std::ofstream ofs;
    ofs.setstate(std::ios_base::badbit);
    test(ofs, __FUNCTION__);
}

void boostnull() {
    boost::iostreams::stream< boost::iostreams::null_sink > nullOstream( ( boost::iostreams::null_sink() ) );
    test(nullOstream, __FUNCTION__);
}

int main() {
    devnull();
    nullstream();
    badbit();
    boostnull();
    return 0;
}

EDIT

The fastest solution - where we use badbit - has a downside. If the program checks if the output is successfully written - and I have no idea why the program should not do that - then it will fail because of this badbit. Therefore, the runner up - boost - is the winner.