- BREAKING CHANGE: iterator_facade::pointer now corresponds to the actual result of iterator_facade::operator-> rather than Value*. This required an adjustment to a test.

- The logic for determining the result of iterator_facade::operator[] has been factored out into a separate detail header in preparation for its potential use in iterator_range to avoid iterator_range::operator[] from returning a reference to a temporary.

[SVN r80901]

include/boost/iterator/detail/operator_brackets_dispatch.hpp

@@ -0,0 +1,88 @@
+// (C) Copyright David Abrahams 2002.
+// (C) Copyright Jeremy Siek    2002.
+// (C) Copyright Thomas Witt    2002.
+// (C) Copyright Jeffrey Lee Hellrung, Jr. 2012.
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+#ifndef BOOST_OPERATOR_BRACKETS_DISPATCH_07102012JLH_HPP
+#define BOOST_OPERATOR_BRACKETS_DISPATCH_07102012JLH_HPP
+
+#include <boost/iterator/detail/facade_iterator_category.hpp>
+
+#include <boost/type_traits/is_pod.hpp>
+#include <boost/type_traits/remove_const.hpp>
+
+#include <boost/mpl/if.hpp>
+
+namespace boost { namespace detail {
+
+// operator[] must return a proxy in case iterator destruction invalidates
+// referents.
+// To see why, consider the following implementation of operator[]:
+//   reference operator[](difference_type n) const
+//   { return *(*this + n); }
+// The problem here is that operator[] would return a reference created from
+// a temporary iterator.
+
+template <class Value>
+struct operator_brackets_value
+{
+    typedef Value result_type;
+    template <class Iterator>
+    static result_type apply(Iterator const & i)
+    { return *i; }
+};
+
+template <class Iterator, class Reference>
+struct operator_brackets_const_proxy
+{
+    class result_type
+    {
+        Iterator const m_i;
+        explicit result_type(Iterator const & i) : m_i(i) { }
+        friend struct operator_brackets_const_proxy;
+        void operator=(result_type&);
+    public:
+        operator Reference() const { return *m_i; }
+    };
+    static result_type apply(Iterator const & i)
+    { return result_type(i); }
+};
+
+template <class Iterator, class Reference>
+struct operator_brackets_proxy
+{
+    class result_type
+    {
+        Iterator const m_i;
+        explicit result_type(Iterator const & i) : m_i(i) { }
+        friend struct operator_brackets_proxy;
+        void operator=(result_type&);
+    public:
+        operator Reference() const { return *m_i; }
+        operator_brackets_proxy const & operator=(
+          typename Iterator::value_type const & x) const
+        { *m_i = x; return *this; }
+    };
+    static result_type apply(Iterator const & i)
+    { return result_type(i); }
+};
+
+template <class Iterator, class ValueType, class Reference>
+struct operator_brackets_dispatch
+{
+    typedef typename mpl::if_c<
+        iterator_writability_disabled<ValueType,Reference>::value,
+        typename mpl::if_c<
+            boost::is_POD<ValueType>::value,
+            operator_brackets_value<typename boost::remove_const<ValueType>::type>,
+            operator_brackets_const_proxy<Iterator,Reference>
+        >::type,
+        operator_brackets_proxy<Iterator,Reference>
+    >::type type;
+};
+
+} } // namespace detail / namespace boost
+
+#endif // #ifndef BOOST_OPERATOR_BRACKETS_DISPATCH_07102012JLH_HPP

include/boost/iterator/iterator_facade.hpp

@@ -1,6 +1,7 @@
 // (C) Copyright David Abrahams 2002.
 // (C) Copyright Jeremy Siek    2002.
 // (C) Copyright Thomas Witt    2002.
+// (C) copyright Jeffrey Lee Hellrung, Jr. 2012.
 // Distributed under the Boost Software License, Version 1.0. (See
 // accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
@@ -13,6 +14,7 @@
 
 #include <boost/iterator/detail/facade_iterator_category.hpp>
 #include <boost/iterator/detail/enable_if.hpp>
+#include <boost/iterator/detail/operator_brackets_dispatch.hpp>
 
 #include <boost/static_assert.hpp>
 #include <boost/utility/addressof.hpp>
@@ -296,7 +298,7 @@ namespace boost
     // standard's requirements. If *i is not a reference type, we must still
     // produce an lvalue to which a pointer can be formed.  We do that by
     // returning a proxy object containing an instance of the reference object.
-    template <class Reference, class Pointer>
+    template <class Reference>
     struct operator_arrow_dispatch // proxy references
     {
         struct proxy
@@ -315,10 +317,10 @@ namespace boost
         }
     };
 
-    template <class T, class Pointer>
-    struct operator_arrow_dispatch<T&, Pointer> // "real" references
+    template <class T>
+    struct operator_arrow_dispatch<T&> // "real" references
     {
-        typedef Pointer result_type;
+        typedef T* result_type;
         static result_type apply(T& x)
         {
             return boost::addressof(x);
@@ -328,79 +330,12 @@ namespace boost
 # if BOOST_WORKAROUND(BOOST_MSVC, < 1300)
     // Deal with ETI
     template<>
-    struct operator_arrow_dispatch<int, int>
+    struct operator_arrow_dispatch<int>
     {
         typedef int result_type;
     };
 # endif
 
-    // A proxy return type for operator[], needed to deal with
-    // iterators that may invalidate referents upon destruction.
-    // Consider the temporary iterator in *(a + n)
-    template <class Iterator>
-    class operator_brackets_proxy
-    {
-        // Iterator is actually an iterator_facade, so we do not have to
-        // go through iterator_traits to access the traits.
-        typedef typename Iterator::reference  reference;
-        typedef typename Iterator::value_type value_type;
-
-     public:
-        operator_brackets_proxy(Iterator const& iter)
-          : m_iter(iter)
-        {}
-
-        operator reference() const
-        {
-            return *m_iter;
-        }
-
-        operator_brackets_proxy& operator=(value_type const& val)
-        {
-            *m_iter = val;
-            return *this;
-        }
-
-     private:
-        Iterator m_iter;
-    };
-
-    // A metafunction that determines whether operator[] must return a
-    // proxy, or whether it can simply return a copy of the value_type.
-    template <class ValueType, class Reference>
-    struct use_operator_brackets_proxy
-      : mpl::not_<
-            mpl::and_<
-                // Really we want an is_copy_constructible trait here,
-                // but is_POD will have to suffice in the meantime.
-                boost::is_POD<ValueType>
-              , iterator_writability_disabled<ValueType,Reference>
-            >
-        >
-    {};
-        
-    template <class Iterator, class Value, class Reference>
-    struct operator_brackets_result
-    {
-        typedef typename mpl::if_<
-            use_operator_brackets_proxy<Value,Reference>
-          , operator_brackets_proxy<Iterator>
-          , Value
-        >::type type;
-    };
-
-    template <class Iterator>
-    operator_brackets_proxy<Iterator> make_operator_brackets_result(Iterator const& iter, mpl::true_)
-    {
-        return operator_brackets_proxy<Iterator>(iter);
-    }
-
-    template <class Iterator>
-    typename Iterator::value_type make_operator_brackets_result(Iterator const& iter, mpl::false_)
-    {
-      return *iter;
-    }
-
     struct choose_difference_type
     {
         template <class I1, class I2>
@@ -616,9 +551,10 @@ namespace boost
       > associated_types;
 
       typedef boost::detail::operator_arrow_dispatch<
-          Reference
-        , typename associated_types::pointer
-      > operator_arrow_dispatch_;
+          Reference> operator_arrow_dispatch_;
+
+      typedef typename boost::detail::operator_brackets_dispatch<
+          Derived, Value, Reference>::type operator_brackets_dispatch_;
 
    protected:
       // For use by derived classes
@@ -644,15 +580,10 @@ namespace boost
           return operator_arrow_dispatch_::apply(*this->derived());
       }
 
-      typename boost::detail::operator_brackets_result<Derived,Value,reference>::type
+      typename operator_brackets_dispatch_::result_type
       operator[](difference_type n) const
       {
-          typedef boost::detail::use_operator_brackets_proxy<Value,Reference> use_proxy;
-          
-          return boost::detail::make_operator_brackets_result<Derived>(
-              this->derived() + n
-            , use_proxy()
-          );
+          return operator_brackets_dispatch_::apply(this->derived() + n);
       }
 
       Derived& operator++()

test/indirect_iter_member_types.cpp

@@ -82,7 +82,7 @@ int main()
     typedef boost::indirect_iterator<char**, int, std::random_access_iterator_tag, long&, short> Iter;
     STATIC_ASSERT_SAME(Iter::value_type, int);
     STATIC_ASSERT_SAME(Iter::reference, long&);  
-    STATIC_ASSERT_SAME(Iter::pointer, int*);  
+    STATIC_ASSERT_SAME(Iter::pointer, long*);  
     STATIC_ASSERT_SAME(Iter::difference_type, short);  
   }
   return 0;