Re: Implementing writing of a matrix to 2 destinations

dor_amit@hotmail.com wrote:

  > I am trying to implement serialization of a matrix to stdout
  > and to a file. Requirements are:

  > a) Each serialization uses 2 delimiters: a column delimiter
  > and a row delimiter.

  > b) The pair of delimiters for file serialization differs from
  > that of stdout serialization.

  > I've tried this:

  > #include <vector>
  > #include <string>
  > #include <iostream>
  > #include <algorithm>
  > #include <iterator>
  > #include <fstream>
  > #include <stdlib.h>

  > using namespace std;

  > typedef vector<int> IntVec;
  > typedef vector<IntVec> IntMat;
  >
  > namespace std
  > {
  > ostream& operator<<(ostream& o,const IntVec& v)
  > {
  > copy(v.begin(),v.end(),ostream_iterator<IntVec::value_type>(o,"
  > "));
  > }
  > ostream& operator<<(ostream& o,const IntMat& m)
  > {
  >
  > copy(m.begin(),m.end(),ostream_iterator<IntMat::value_type>(o,"\n"));
  > }
  > ofstream& operator<<(ofstream& o,const IntVec& v)
  > {
  >
  > copy(v.begin(),v.end(),ostream_iterator<IntVec::value_type>(o,"|"));
  > }
  > ofstream& operator<<(ofstream& o,const IntMat& m)
  > {
  > // problem: ostream_iterator is instatiated, leading to this
  > calling
  > // the serialization to STDOUT
  >
  > copy(m.begin(),m.end(),ostream_iterator<IntMat::value_type>(o,"$$$\n"));
  > }
  > };

There are two problems here: the first is that formally, you are
not allowed to define functions in namespace std. Most of the
time, you will get away with it; in this case, you just might
not, since an implementation could have conceivably defined
operator<< over vector<int> for some internal purposes. It is,
at any rate, very bad practice, since if accepted, you'll
doubtlessly end up conflicting with a definition by another
collegue.

The second, of course, is the problem you are noticing. It's of
absolutely no use to try to overload on ofstream as opposed to
ostream -- for all practical purposes, it's best to consider
that an operator<< on ofstream will never be called.

The fundamental problem here is in your statement of the
problem. cout may be a file, and on some systems, an ofstream
that you create may end up going to a terminal, so your basic
criteria for the choice is incoherent. What you probably want
to do is to define manipulators to specify the separators,
setting them in stream specific variables allocated by means of
ios::xalloc, and reading them in the << operators.

(FWIW: to achieve what you are requesting, the only solution is
to compare &o == &cout in the << operators, and set the
separator according to that. But as I said, I'm pretty sure
that this is not the right solution.)

--
James Kanze kanze.james@neuf.fr
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