Konstantin Baumann wrote:
>
> Why is the "value_type" of an "output_iterator" (and
"back_insert_iterator<>", ...)
> "void"?
>
> IMHO it makes also sense to define the "value_type" of an
> "output_iterator".
>
> Suppose you have the following sequence of sequences:
>
> (
>         (1),
>         (1, 3),
>         (6, 23, 42, 45),
>         (123),
>         (1, 2, 3, 4, 5, 6, 7),
>         ...
> )
>
> This sequence could be of type:
>         list<list<int> >
> or
>         deque<vector<int> >
> or ...
>
> A generic function getting iterators to this sequence as
> input-parameters could look like:
>
> template<class InputIterator1>
> void in_func(InputIterator1 first, InputIterator1 last) {
>
>   // getting the type of the inner sequence-container
>   typedef typename iterator_traits<InputIterator>::value_type
>           SequenceType2;
>
>   // getting the type of the const_iterator of the inner
>   // sequence-container
>   typedef typename SequenceType2::const_iterator
>           InputIterator2;
>
>   for( ; first != last; ++first) {
>
>     for(InputIterator2 inner = (*first).begin();
>         inner != (*first).end();
>         ++inner) {
>
>       doSomething(*inner);
>     }
>   }
> }
>
> But you can't write a function returning such a sequence if you only
> know the type of the output-iterator of the outer-sequence:
>
> template<class OutputIterator>
> OutputIterator out_func(OutputIterator out) {
>
>   // getting the type of the inner sequence-container
>   typedef typename iterator_traits<OutputIterator>::value_type
>           SequenceType2; // ERROR: getting "void"!!!
>
>   while(...) {
>
>     list<int> seq;
>
>     while(...) {
>
>       seq.push_back(...);
>     }
>
>     *(out++) = SequenceType2(seq.begin(), seq.end()); // FAILING!!!
>   }
>
>   return(out);
> }

In e-mail that I exchanged with Konstantin I learned that he was
instantiating this template for output_iterator, and that

> I'm using SGI's STL-implementation shipping with g++-2.8.1.
>
> In the header-file "stl_iterator.h" is the following:
>
> [...]
>
> struct output_iterator {
>   typedef output_iterator_tag iterator_category;
>   typedef void                value_type; // <<<<<<<<<
>   typedef void                difference_type;
>   typedef void                pointer;
>   typedef void                reference;
> };

I believe that this is an invalid definition, at least according to CD2:

Section 24.3.1:

1 ... it is required that if Iterator is the type of an iterator, the
types
        ...
        iterator_traits<Iterator>::value_type
        ...

be defineded as the iterator's ...., value type, ...


2 ...
        template<class Iterator> struct iterator_traits{
                ...
                typedef Iterator::value_type value_type;
                ...

I couldn't find a more direct statement, and there's nothing about it in
the iterator requirements, but the combination of these two statements
seem to me to require that Iterator::value_type must exist for all
iterators, and must be a typedef for the value type of the iterator. An
iterator with a value type of 'void' seems pointless. Am I missing
something? In particular, I'm suprised that output_iterator isn't
templated by its value_type.

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Why is the "value_type" of an "output_iterator" (and "back_insert_iterator<>", ...)
"void"?

IMHO it makes also sense to define the "value_type" of an
"output_iterator".

Suppose you have the following sequence of sequences:

(
 (1),
 (1, 3),
 (6, 23, 42, 45),
 (123),
 (1, 2, 3, 4, 5, 6, 7),
 ...
)

This sequence could be of type:
 list<list<int> >
or
 deque<vector<int> >
or ...

A generic function getting iterators to this sequence as
input-parameters could look like:

template<class InputIterator1>
void in_func(InputIterator1 first, InputIterator1 last) {

  // getting the type of the inner sequence-container
  typedef typename iterator_traits<InputIterator>::value_type
          SequenceType2;

  // getting the type of the const_iterator of the inner
  // sequence-container
  typedef typename SequenceType2::const_iterator
          InputIterator2;

  for( ; first != last; ++first) {

    for(InputIterator2 inner = (*first).begin();
        inner != (*first).end();
        ++inner) {

      doSomething(*inner);
    }
  }
}

But you can't write a function returning such a sequence if you only
know the type of the output-iterator of the outer-sequence:

template<class OutputIterator>
OutputIterator out_func(OutputIterator out) {

  // getting the type of the inner sequence-container
  typedef typename iterator_traits<OutputIterator>::value_type
          SequenceType2; // ERROR: getting "void"!!!

  while(...) {

    list<int> seq;

    while(...) {

      seq.push_back(...);
    }

    *(out++) = SequenceType2(seq.begin(), seq.end()); // FAILING!!!
  }

  return(out);
}

--
Konstantin Baumann                   Westfaelische Wilhelms-Universitaet
Institut fuer Informatik (Zi. 603),  Einsteinstr. 62,   D-48149 Muenster
mailto:kostab@math.uni-muenster.de                  Tel:+49-251-83-32701
http://wwwmath.uni-muenster.de/cs/u/kostab/         Fax:+49-251-83-33755

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