[Boost-bugs] [Boost C++ Libraries] #12071: postfix_increment_proxy is not DefaultConstructible and doesn't have a nested public value_type

Subject: [Boost-bugs] [Boost C++ Libraries] #12071: postfix_increment_proxy is not DefaultConstructible and doesn't have a nested public value_type
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2016-03-16 15:36:58

#12071: postfix_increment_proxy is not DefaultConstructible and doesn't have a
nested public value_type
---------------------------------------------+-----------------------------
 Reporter:  Krzysztof Czaiński <1czajnik@…>  |      Owner:
     Type:  Bugs                             |  jeffrey.hellrung
Milestone:  To Be Determined                 |     Status:  new
  Version:  Boost 1.61.0                     |  Component:  iterator
 Keywords:  range-v3                         |   Severity:  Problem
---------------------------------------------+-----------------------------
 The fact that {{{postfix_increment_proxy}}} is not
 {{{DefaultConstructible}}} and doesn't have a nested public
 {{{value_type}}} makes iterators unusable with range-v3, which check the
 concepts.

 Example:
 {{{
 #include "boost/iterator/iterator_facade.hpp"
 #include "range/v3/utility/iterator.hpp"

 template <class V, class Category>
 class TestIter
     : public boost::iterator_facade<TestIter<V, Category>, V, Category, V>
 {
  public:
   using typename boost::iterator_facade<TestIter<V, Category>, V,
 Category,
                                         V>::difference_type;

   TestIter() : v_() {}

   explicit TestIter(V v) : v_(v) {}

  private:
   friend class boost::iterator_core_access;

   V dereference() const { return v_; }
   bool equal(const TestIter& other) const { return v_ == other.v_; }
   void increment() { ++v_; }
   void decrement() { --v_; }
   void advance(difference_type n) { v_ += n; }
   difference_type distance_to(const TestIter& other) const {
     return other.v_ - v_;
   }

   V v_;
 };

 using InIter = TestIter<int, std::input_iterator_tag>;

 static_assert(ranges::InputIterator<InIter>(), "");

 void f(InIter x) {
   static_assert(ranges::Readable<decltype(x++)>(), "");
   static_assert(ranges::DefaultConstructible<decltype(x++)>(), "");
 }
 }}}

 I think {{{static_assert(ranges::InputIterator<InIter>(), "")}}} fails,
 because the two static_asserts in {{{f()}}} fail.

 From what I've learned here:
 https://github.com/ericniebler/range-v3/issues/304

 it seems a good idea to fix {{{postfix_increment_proxy}}} to conform to
 these requirements. Ditto {{{writable_postfix_increment_proxy}}}.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/12071>
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