How to use: istreambuf_iterator

Hi there,

  I think I misunderstanded the use of istreambuf_iterator. I thought
I could use it this way (*). Which would allow me to skip the call to
vector::resize(). Could someone please let me know what is wrong in my
code. My goal is simply to skip the default initialization to 0 of the
std::vector which is compulsary for 'DoMethod1'.

  At least on my OS: Debian & g++-4.2 -03, DoMethod2 is slower by a
factor of 3.

Thanks
-Mathieu

(*)
#include <vector>
#include <iterator>
#include <iostream>
#include <fstream>

void DoMethod1(std::ifstream &in, std::streampos length,
std::vector<char> & v)
{
  in.seekg(0, std::ios::beg);
  v.resize( length );
  in.read(&v[0], length);
}

void DoMethod2(std::ifstream &in, std::streampos length,
std::vector<char> & v)
{
  in.seekg(0, std::ios::beg );
  v.reserve( length );
  std::copy( std::istreambuf_iterator<char>(in),
std::istreambuf_iterator<char>(),
    std::back_inserter(v)) ;
}

int main(int argc, char *argv[])
{
  if( argc < 2 )
    {
    std::cerr << argc << std::endl;
    return 1;
    }
  const char *filename = argv[1];
  time_t start,end;

  std::ifstream in( filename, std::ios::binary ) ;
  in.seekg(0, std::ios::end );
  std::streampos length = in.tellg();

  std::vector<char> v2;
  start = time(0);
  DoMethod2( in, length, v2);
  end = time(0);

  std::cerr << "method2:" << (end - start) << std::endl;

  std::vector<char> v1;
  start = time(0);
  DoMethod1( in, length, v1);
  end = time(0);

  std::cerr << "method1:" << (end - start) << std::endl;

  in.close();

  return 0 ;
}