iterator: add counting_output_iterator

iterator.hpp

@@ -606,6 +606,45 @@ namespace cruft {
             std::forward<Args> (src)...
         );
     }
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// Counts the number of times the iterator is assigned to.
+    struct counting_output_iterator {
+        // An internal proxy value which is returned when the iterator is
+        // dereferenced. It increments the assignment value in the host
+        // iterator.
+        //
+        // The internals of this object are not a stable interface.
+        struct assignable {
+            assignable (std::size_t &_count)
+                : m_count (_count)
+            { ; }
+
+            template <typename Arg>
+            void
+            operator= (Arg&&)
+            {
+                ++m_count;
+            }
+
+        private:
+            std::size_t &m_count;
+        };
+
+        using value_type = assignable;
+        using iterator_category = std::output_iterator_tag;
+        using reference = assignable&;
+
+        counting_output_iterator& operator++ () { return *this; }
+        assignable operator* () { return assignable (m_count); }
+
+        /// Returns the number of times the iterator has been assigned to.
+        auto count (void) const { return m_count; }
+
+    private:
+        std::size_t m_count = 0;
+    };
 };
 
 #endif

test/iterator.cpp

@@ -6,6 +6,19 @@
 #include <array>
 
 
+///////////////////////////////////////////////////////////////////////////////
+void
+test_counting_output_iterator (cruft::TAP::logger &tap)
+{
+    cruft::counting_output_iterator out;
+    *out = 0; ++out;
+    *out = 1; ++out;
+    *out = 2; ++out;
+
+    tap.expect_eq (out.count (), 3u, "counting_output_iterator simple manual assignment");
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 int
 main (int, char**)
@@ -49,5 +62,7 @@ main (int, char**)
         tap.expect_eq (src, dst, "copy using structured bindings");
     }
 
+    test_counting_output_iterator (tap);
+
     return tap.status ();
 }