util/Intrusive{List,HashSet}: add "tag" for base hooks

src/util/IntrusiveHashSet.hxx

@@ -19,7 +19,11 @@ struct IntrusiveHashSetOptions {
 	bool zero_initialized = false;
 };
 
-template<IntrusiveHookMode mode=IntrusiveHookMode::NORMAL>
+/**
+ * @param Tag an arbitrary tag type to allow using multiple base hooks
+ */
+template<IntrusiveHookMode mode=IntrusiveHookMode::NORMAL,
+	 typename Tag=void>
 struct IntrusiveHashSetHook {
 	using SiblingsHook = IntrusiveListHook<mode>;
 
@@ -36,12 +40,14 @@ struct IntrusiveHashSetHook {
 
 /**
  * For classes which embed #IntrusiveHashSetHook as base class.
+ *
+ * @param Tag selector for which #IntrusiveHashSetHook to use
  */
-template<typename T>
+template<typename T, typename Tag=void>
 struct IntrusiveHashSetBaseHookTraits {
 	/* a never-called helper function which is used by _Cast() */
 	template<IntrusiveHookMode mode>
-	static constexpr IntrusiveHashSetHook<mode> _Identity(const IntrusiveHashSetHook<mode> &) noexcept;
+	static constexpr IntrusiveHashSetHook<mode, Tag> _Identity(const IntrusiveHashSetHook<mode, Tag> &) noexcept;
 
 	/* another never-called helper function which "calls"
 	   _Identity(), implicitly casting the item to the

src/util/IntrusiveList.hxx

@@ -35,9 +35,13 @@ struct IntrusiveListNode {
 	}
 };
 
-template<IntrusiveHookMode _mode=IntrusiveHookMode::NORMAL>
+/**
+ * @param Tag an arbitrary tag type to allow using multiple base hooks
+ */
+template<IntrusiveHookMode _mode=IntrusiveHookMode::NORMAL,
+	 typename Tag=void>
 class IntrusiveListHook {
-	template<typename T> friend struct IntrusiveListBaseHookTraits;
+	template<typename, typename> friend struct IntrusiveListBaseHookTraits;
 	template<auto member> friend struct IntrusiveListMemberHookTraits;
 	template<typename T, typename HookTraits, IntrusiveListOptions> friend class IntrusiveList;
 
@@ -91,12 +95,14 @@ using AutoUnlinkIntrusiveListHook =
 
 /**
  * For classes which embed #IntrusiveListHook as base class.
+ *
+ * @param Tag selector for which #IntrusiveHashSetHook to use
  */
-template<typename T>
+template<typename T, typename Tag=void>
 struct IntrusiveListBaseHookTraits {
 	/* a never-called helper function which is used by _Cast() */
 	template<IntrusiveHookMode mode>
-	static constexpr IntrusiveListHook<mode> _Identity(const IntrusiveListHook<mode> &) noexcept;
+	static constexpr IntrusiveListHook<mode, Tag> _Identity(const IntrusiveListHook<mode, Tag> &) noexcept;
 
 	/* another never-called helper function which "calls"
 	   _Identity(), implicitly casting the item to the

test/util/TestIntrusiveHashSet.cxx

@@ -149,3 +149,74 @@ TEST(IntrusiveHashSet, Multi)
 	ASSERT_EQ(set.remove_and_dispose_key(b, [](auto*){}), 0U);
 	ASSERT_EQ(set.find(b), set.end());
 }
+
+TEST(IntrusiveHashSet, Tag)
+{
+	struct A {};
+	struct B {};
+
+	struct TaggedItem final
+		: IntrusiveHashSetHook<IntrusiveHookMode::NORMAL, A>,
+		  IntrusiveHashSetHook<IntrusiveHookMode::NORMAL, B> {
+		int a, b;
+
+		TaggedItem(int _a, int _b) noexcept
+			:a(_a), b(_b) {}
+	};
+
+	struct GetA {
+		int operator()(const TaggedItem &item) const noexcept {
+			return item.a;
+		}
+	};
+
+	struct GetB {
+		int operator()(const TaggedItem &item) const noexcept {
+			return item.b;
+		}
+	};
+
+	TaggedItem one{1, 11}, two{2, 22};
+
+	IntrusiveHashSet<TaggedItem, 3,
+			 IntrusiveHashSetOperators<std::hash<int>,
+						   std::equal_to<int>,
+						   GetA>,
+			 IntrusiveHashSetBaseHookTraits<TaggedItem, A>> a;
+
+	IntrusiveHashSet<TaggedItem, 3,
+			 IntrusiveHashSetOperators<std::hash<int>,
+						   std::equal_to<int>,
+						   GetB>,
+			 IntrusiveHashSetBaseHookTraits<TaggedItem, B>> b;
+
+	EXPECT_TRUE(a.empty());
+	EXPECT_TRUE(b.empty());
+
+	a.insert(one);
+	a.insert(two);
+
+	EXPECT_FALSE(a.empty());
+	EXPECT_TRUE(b.empty());
+
+	b.insert(one);
+
+	EXPECT_FALSE(a.empty());
+	EXPECT_FALSE(b.empty());
+
+	a.clear();
+
+	EXPECT_TRUE(a.empty());
+	EXPECT_FALSE(b.empty());
+
+	a.insert(two);
+	a.insert(one);
+
+	EXPECT_FALSE(a.empty());
+	EXPECT_FALSE(b.empty());
+
+	b.erase(b.iterator_to(one));
+
+	EXPECT_FALSE(a.empty());
+	EXPECT_TRUE(b.empty());
+}

test/util/TestIntrusiveList.cxx

@@ -166,6 +166,51 @@ TEST(IntrusiveList, AutoUnlink)
 	ASSERT_TRUE(b.is_linked());
 }
 
+TEST(IntrusiveList, Tag)
+{
+	struct A {};
+	struct B {};
+
+	struct TaggedItem
+		: public IntrusiveListHook<IntrusiveHookMode::NORMAL, A>,
+		  public IntrusiveListHook<IntrusiveHookMode::NORMAL, B> {};
+
+	TaggedItem one, two;
+
+	IntrusiveList<TaggedItem, IntrusiveListBaseHookTraits<TaggedItem, A>> a;
+	IntrusiveList<TaggedItem, IntrusiveListBaseHookTraits<TaggedItem, B>> b;
+
+	EXPECT_TRUE(a.empty());
+	EXPECT_TRUE(b.empty());
+
+	a.push_back(one);
+	a.push_back(two);
+
+	EXPECT_FALSE(a.empty());
+	EXPECT_TRUE(b.empty());
+
+	b.push_back(one);
+
+	EXPECT_FALSE(a.empty());
+	EXPECT_FALSE(b.empty());
+
+	a.clear();
+
+	EXPECT_TRUE(a.empty());
+	EXPECT_FALSE(b.empty());
+
+	a.push_back(two);
+	a.push_back(one);
+
+	EXPECT_FALSE(a.empty());
+	EXPECT_FALSE(b.empty());
+
+	b.erase(b.iterator_to(one));
+
+	EXPECT_FALSE(a.empty());
+	EXPECT_TRUE(b.empty());
+}
+
 TEST(IntrusiveList, Merge)
 {
 	using Item = CharItem<IntrusiveHookMode::NORMAL>;