From b99a20378dfcf8b94cbe228ef8a3775516dbb065 Mon Sep 17 00:00:00 2001
From: artemp <artem@mapnik.org>
Date: Tue, 27 Oct 2015 14:46:58 +0000
Subject: [PATCH] remove `swap` and implement `assign`

    https://github.com/mapnik/mapnik/issues/3103
    https://github.com/mapnik/mapnik/pull/3146
---
 include/mapnik/util/variant.hpp | 38 ++++++++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 8 deletions(-)

diff --git a/include/mapnik/util/variant.hpp b/include/mapnik/util/variant.hpp
index 100088724..01db3a807 100644
--- a/include/mapnik/util/variant.hpp
+++ b/include/mapnik/util/variant.hpp
@@ -240,6 +240,19 @@ struct variant_helper<T, Types...>
             variant_helper<Types...>::copy(old_id, old_value, new_value);
         }
     }
+    VARIANT_INLINE static void direct_swap(const std::size_t id, void * lhs, void * rhs)
+    {
+        using std::swap; //enable ADL
+        if (id == sizeof...(Types))
+        {
+            // both lhs and rhs hold T
+            swap(*reinterpret_cast<T*>(lhs), *reinterpret_cast<T*>(rhs));
+        }
+        else
+        {
+            variant_helper<Types...>::direct_swap(id, lhs, rhs);
+        }
+    }
 };
 
 template<> struct variant_helper<>
@@ -247,6 +260,7 @@ template<> struct variant_helper<>
     VARIANT_INLINE static void destroy(const std::size_t, void *) {}
     VARIANT_INLINE static void move(const std::size_t, void *, void *) {}
     VARIANT_INLINE static void copy(const std::size_t, const void *, void *) {}
+    VARIANT_INLINE static void direct_swap(const std::size_t, void *, void *) {}
 };
 
 namespace detail {
@@ -274,7 +288,7 @@ struct unwrapper<recursive_wrapper<T>>
     {
         return obj.get();
     }
-    
+
     auto operator() (recursive_wrapper<T> & obj) const
         -> typename recursive_wrapper<T>::type &
     {
@@ -595,16 +609,24 @@ public:
         helper_type::move(old.type_index, &old.data, &data);
     }
 
-    friend void swap(variant<Types...> & first, variant<Types...> & second)
+    void assign(variant<Types...> & rhs)
     {
-        using std::swap; //enable ADL
-        swap(first.type_index, second.type_index);
-        swap(first.data, second.data);
+        if (type_index == rhs.type_index)
+        {
+            helper_type::direct_swap(rhs.type_index, &rhs.data, &data);
+        }
+        else
+        {
+            helper_type::destroy(type_index, &data);
+            type_index = detail::invalid_value;
+            helper_type::copy(rhs.type_index, &rhs.data, &data);
+            type_index = rhs.type_index;
+        }
     }
 
     VARIANT_INLINE variant<Types...>& operator=(variant<Types...> other)
     {
-        swap(*this, other);
+        assign(other);
         return *this;
     }
 
@@ -614,7 +636,7 @@ public:
     VARIANT_INLINE variant<Types...>& operator=(T && rhs) noexcept
     {
         variant<Types...> temp(std::forward<T>(rhs));
-        swap(*this, temp);
+        assign(temp);
         return *this;
     }
 
@@ -623,7 +645,7 @@ public:
     VARIANT_INLINE variant<Types...>& operator=(T const& rhs)
     {
         variant<Types...> temp(rhs);
-        swap(*this, temp);
+        assign(temp);
         return *this;
     }