Re: Working directly on output stream

From:

red floyd <no.spam@here.dude>

Newsgroups:

comp.lang.c++

Date:

Thu, 03 Jan 2008 17:31:51 GMT

Message-ID:

<b_8fj.1562$pA7.1123@newssvr25.news.prodigy.net>

mathieu wrote:

Hi there,

  I am trying to rewrite this very slow function (due to AFAIK the
extra copy):

void DoAction(std::istream &is, std::ostream &os)
{
      uint16_t c;
      while( is.read((char*)&c,2) )
        {
        c =
          (c >> (16 - 12 - 1)) & pmask;
        os.write((char*)&c, 2 );
        }
}

  I was thinking of doing:

os.rdbuf( is.rdbuf() )

  and then working directly with a ostreambuf_iterator, but I cannot
read from a ostreambuf_iterator can I ?

Don't reassign the output streambuf. Use transform, or a loop on the
streambuf iterators.

i.e.:

struct convert
{
    uint16_t operator()(const uint16 c) const
    {
       return (c >> shift) & pmask);
    }
    convert(uint16_t& pmask_) : pmask(pmask_) { }
private:
    static const shift = (16 - 12 - 1);
    uint16_t pmask;

};

void DoAction(std::istream& is, std::ostream& os)
{
     std::transform(std::istreambuf_iterator<uint16_t>(is),
                    std::istreambuf_iterator<uint16_t>(),
                    std::ostreambuf_iterator<uint16_t>(os));
}

or

void DoAction(std::istream& is, std::ostream& os)
{
     std::istreambuf_iterator<uint16_t> in(is);
     const istream_iterator<uint16_t> end;
     std::ostreambuf_iterator<uint16_t> out(os);

    while (in != end)
       *out++ = ((*in++) >> (16 - 12 - 1)) & pmask;
}

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