Hi,

This question is regarding the C++ standard library open issue submitted
by Bjarne Stroustrup on missing binders for non-const sequence elements.

Refer

http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html

issue numbered 109.

The issue says

  There are no versions of binders that apply to non-const elements of a
  sequence. This makes examples like for_each() using bind2nd() on page
  521 of "The C++ Programming Language (3rd)" non-conforming. Suitable
  versions of the binders need to be added.

The proposed resolution seems to have some deficiencies.
The proposed solution adds application operators taking non-const
arguments to binder1st and binder2nd. The new application operator member
functions are also declared const like the earlier ones.

This alone is not adequate for the following reasons:

1. Since the application operator is declared const, the first argument
   (the bound argument held in the binder object) cannot be passed as a
   non-const reference. That is, the proposed solution only considers
   the second argument being passed as non-const reference.

2. Let us assume that we have to bind a functor F having

   void operator()(T1& t1, T2& t2);

   T1 and/or T2 could be in-out arguments. Passing values (x1, y1) might
   yield different values (x2, y2). Such a function may be bound with a
   T1 argument having value x1. The binder may be passed to an algorithm
   like:

   F f(...); // construct
   T1 t1 = x1;

   Vector<T2> v;
   // fill up v having values y1, y2, ..., yn.

   for_each(v.begin(), v.end(), bind1st(f, t1));

   Our intention is this:

   (x1, y1) yields a new x2.
   That x2 is applied next with y2 given by for_each, yielding x3, and
   so on.

   After the algorithm application we would like to have the value xn
   for t1.

   However, this is _not_ possible with the current specs of
   binders. The bind argument is copied and held private. Instead, it
   should have been held by reference in this case. The constructor
   needs to take a reference and hold it by reference.

I would like to have your comments on this.

I know that (to quote from the web page):

  A Proposed Resolution for an open issue is still not be construed as
  the view of LWG.  Comments on the current state of discussions are often
  given at the end of open issues in an italic font. Such comments are for
  information only and should not be given undue importance. They do not
  appear in the public version.

but I would like to know the current thoughts of the working group on
this issue.

Thanks!

Regards
Sankar

--
Sankaranarayanan K. V.                  Motorola India Electronics Ltd.
Senior Software Engineer                "The Senate," 33A Ulsoor Road
tel: +91-80-559-8615 ext 4047           Bangalore, Karnataka
fax: +91-80-559-8660                    India 560 042
mailto: sankar@miel.mot.com             http://www.mot.com/miel
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