Re: std::vector::reserve and std::ifstream::read

From:

Michael Doubez <michael.doubez@free.fr>

Newsgroups:

comp.lang.c++

Date:

Tue, 24 May 2011 05:06:42 -0700 (PDT)

Message-ID:

<2982f8dd-bbe1-450c-aab1-607466509700@j28g2000vbp.googlegroups.com>

On 24 mai, 13:51, mathieu <mathieu.malate...@gmail.com> wrote:

On May 24, 1:01 pm, Michael Doubez <michael.dou...@free.fr> wrote:

On 24 mai, 11:16, gwowen <gwo...@gmail.com> wrote:

On May 24, 9:58 am, mathieu <mathieu.malate...@gmail.com> wrote:

Dear all,

  I do not understand how to use the STL vector class with the
ifstream class. I would like to reserve a chunk of memory (no
initialization is required) and fill it with values from a file. As
far as I understand vector::reserve requires a subsequent call to
push_back or insert. However I do not see how I can do this in the
following example:

#include <fstream>
#include <vector>

int main(int argc, char *argv[])
{
  const char *filename = argv[1];
  std::ifstream is( filename );
  const size_t l = 512;
  std::vector<char> v;
  v.reserve( l ); // need push_back or insert

If you reserve() space, you can't read or write to that space until
you also adjust the size (as push_back() will, or resize()). The
reserve() may well speed your code up.

char tmp;
is.read(&tmp,1);
v.push_back(tmp);

// the C++/STL like solution will use an istream_iterator and a
back_inserter
// I've got to say, I don't care for it...
vector<char> V;
copy(istream_iterator<char>(is), istream_iterator<char>(),
back_inserter(V));

Or simply:
v.assign(stream_iterator<char>(is), istream_iterator<char>());

It does not work for me. istream_iterator does not implement +()
operator:

Yes, they are not random access input iterator.

v.assign( std::istream_iterator<char>(is),
std::istream_iterator<char>(is)+l);

You want to read char by char ?

char c;

If you want formatted input then:
is>>c;
If you want unformatted input then:
is.get(c);

And you loop:
while( is.get(c) ) {
v.push_back(c);
}

--
Michael