Add QtPrivate::ArrowProxy

... use it in QKeyValueIterator.

This is somewhat simpler than the old QKeyValueIterator::pointer.

I don't think the pointer::operator*() could have ever worked, because
it returns value_type&, but is const.

That leaves the defaulted copy and move SMFs as a difference (ArrowProxy
is using aggregate initialization, so doesn't need any), and the fact
that operator->() is const.

Change-Id: I80b9c5f696de6ae30f3939166b205b9213eac57e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qcontainertools_impl.h

@@ -254,6 +254,13 @@ void q_relocate_overlap_n(T *first, N n, T *d_first)
     }
 }
 
+template <typename T>
+struct ArrowProxy
+{
+    T t;
+    T *operator->() noexcept { return &t; }
+};
+
 template <typename Iterator>
 using IfIsInputIterator = typename std::enable_if<
     std::is_convertible<typename std::iterator_traits<Iterator>::iterator_category, std::input_iterator_tag>::value,

src/corelib/tools/qiterator.h

@@ -5,6 +5,7 @@
 #define QITERATOR_H
 
 #include <QtCore/qglobal.h>
+#include <QtCore/qcontainertools_impl.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -253,26 +254,10 @@ public:
         return std::pair<Key, T>(i.key(), i.value());
     }
 
-    struct pointer
-    {
-        pointer(value_type &&r_) : r(std::move(r_)) { }
-
-        pointer() = default;
-        pointer(const pointer &other) = default;
-        pointer(pointer &&other) = default;
-        pointer &operator=(const pointer &other) = default;
-        pointer &operator=(pointer &&other) = default;
-
-        value_type &operator*() const { return r; }
-
-        value_type r;
-        const value_type *operator->() const {
-            return &r;
-        }
-    };
+    using pointer = QtPrivate::ArrowProxy<value_type>;
 
     pointer operator->() const {
-        return pointer(std::pair<Key, T>(i.key(), i.value()));
+        return pointer{std::pair<Key, T>(i.key(), i.value())};
     }
 
     friend bool operator==(QKeyValueIterator lhs, QKeyValueIterator rhs) noexcept { return lhs.i == rhs.i; }