util/IntrusiveList: add enum LinkMode

Compile-time code simplification.

src/util/IntrusiveHookMode.hxx

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2022 Max Kellermann <max.kellermann@gmail.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+/**
+ * Specifies the mode in which a hook for intrusive containers
+ * operates.  This is meant to be used as a template argument to the
+ * hook class (e.g. #IntrusiveListHook).
+ */
+enum class IntrusiveHookMode {
+	/**
+	 * No implicit initialization.
+	 */
+	NORMAL,
+
+	/**
+	 * Keep track of whether the item is currently linked, allows
+	 * using method is_linked().  This requires implicit
+	 * initialization and requires iterating all items when
+	 * deleting them which adds a considerable amount of overhead.
+	 */
+	TRACK,
+
+	/**
+	 * Automatically unlinks the item in the destructor.  This
+	 * implies #TRACK and adds code to the destructor.
+	 */
+	AUTO_UNLINK,
+};