220 15807 <1499974.zpI02sfgj3@basis> article
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From: Taahir Ahmed <ahmed.taahir@gmail.com>
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Subject: Re: Output-Restricted Variants in the <algorithm> Header
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> ... why not define an output-restricted iterator category ...?

For three main reasons:

  1) It does not fit with current practice.

     Everywhere else in the STL, wherever a range is called for, it is
     represented in half-open form using a source iterator and limit
     iterator.

     In addition, partial_sort_copy is already output-restricted, and is
     implemented in this way.

  2) It does not fit with future practice.

     Explicit specification of the output range as a half-open range
     pair means that the algorithm can be used as the "4-legged" form
     mentioned in n4128 Section (the ranges proposal), Section 3.3.8
     [1].

     Thus, there would be a straightforward interoperation with ranges,
     when they are introduced.

  3) It seems Byzantine.

     To force iterators to do a range's job requires that the iterator
     be stateful, and seems to be a lot of effort to avoid defining some
     small functions.

     Many of the functions I propose will be nearly textually identical
     to their non-restricted counterparts.  For example, take
     restricted_copy and copy.  They can share a common implementation:

     ----%<-------------------------------------------------------------

     template<class ItA, class ItD>
     std::tuple<ItA, ItD> copy_base(
         ItA a_src, ItA a_lim,
         ItD d_src, ItD d_lim,
         bool restrict_output
     )
     {
         while(a_src != a_lim && (!restrict_output || (d_src != d_lim)))
         {
             *d_src = *a_src;
             ++d_src;
             ++a_src;
         }

         return make_tuple(a_src, d_src);
     }
     
     template<class ItA, class ItD>
     std::tuple<ItA, ItD> restricted_copy(
         ItA a_src, ItA a_lim,
         ItD d_src, ItD d_lim
     )
     {
         auto lims = copy_base(a_src, a_lim, d_src, d_lim, true);
         return std::get<1>(lims);
     }

     template<class ItA, class ItD>
     ItD copy(
         ItA a_src, ItA a_lim,
         ItD d_src
     )
     {
         // Here, the fourth argument to copy_base could be anything.
         auto lims = copy_base(a_src, a_lim, d_src, d_src, false);
         return std::get<0>(lims);
     }

     ----%<-------------------------------------------------------------

     Implemented like this, there isn't any repeated code, AND all the
     dirty tricks currently used by implementations of std::copy
     (std::memmove for appropriate iterator and data types) can placed
     into copy_base.


Taahir

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4128.html#additional-overloads-of-the-algorithms

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