From dcd7c6337c13a8e4d0742765f335da0373ec2c86 Mon Sep 17 00:00:00 2001
From: Max Kellermann <mk@cm4all.com>
Date: Wed, 2 Aug 2023 23:25:50 +0200
Subject: [PATCH] util/Intrusive{List,HashSet}: reimplement hook mode detection
with implicit cast
---
src/util/IntrusiveHashSet.hxx | 31 ++++++++++++++-----------------
src/util/IntrusiveList.hxx | 33 ++++++++++++++-------------------
2 files changed, 28 insertions(+), 36 deletions(-)
diff --git a/src/util/IntrusiveHashSet.hxx b/src/util/IntrusiveHashSet.hxx
index fb949d642..4f4d2f5fb 100644
--- a/src/util/IntrusiveHashSet.hxx
+++ b/src/util/IntrusiveHashSet.hxx
@@ -25,29 +25,26 @@ struct IntrusiveHashSetHook {
}
};
-/**
- * Detect the hook type.
- */
-template<typename U>
-struct IntrusiveHashSetHookDetection {
- /* TODO can this be simplified somehow, without checking for
- all possible enum values? */
- using type = std::conditional_t<std::is_base_of_v<IntrusiveHashSetHook<IntrusiveHookMode::NORMAL>, U>,
- IntrusiveHashSetHook<IntrusiveHookMode::NORMAL>,
- std::conditional_t<std::is_base_of_v<IntrusiveHashSetHook<IntrusiveHookMode::TRACK>, U>,
- IntrusiveHashSetHook<IntrusiveHookMode::TRACK>,
- std::conditional_t<std::is_base_of_v<IntrusiveHashSetHook<IntrusiveHookMode::AUTO_UNLINK>, U>,
- IntrusiveHashSetHook<IntrusiveHookMode::AUTO_UNLINK>,
- void>>>;
-};
-
/**
* For classes which embed #IntrusiveHashSetHook as base class.
*/
template<typename T>
struct IntrusiveHashSetBaseHookTraits {
+ /* a never-called helper function which is used by _Cast() */
+ template<IntrusiveHookMode mode>
+ static constexpr IntrusiveHashSetHook<mode> _Identity(const IntrusiveHashSetHook<mode> &) noexcept;
+
+ /* another never-called helper function which "calls"
+ _Identity(), implicitly casting the item to the
+ IntrusiveHashSetHook specialization; we use this to detect
+ which IntrusiveHashSetHook specialization is used */
template<typename U>
- using Hook = typename IntrusiveHashSetHookDetection<U>::type;
+ static constexpr auto _Cast(const U &u) noexcept {
+ return decltype(_Identity(u))();
+ }
+
+ template<typename U>
+ using Hook = decltype(_Cast(std::declval<U>()));
static constexpr T *Cast(Hook<T> *node) noexcept {
return static_cast<T *>(node);
diff --git a/src/util/IntrusiveList.hxx b/src/util/IntrusiveList.hxx
index 27e5975d9..64ba6aaaf 100644
--- a/src/util/IntrusiveList.hxx
+++ b/src/util/IntrusiveList.hxx
@@ -77,31 +77,26 @@ using SafeLinkIntrusiveListHook =
using AutoUnlinkIntrusiveListHook =
IntrusiveListHook<IntrusiveHookMode::AUTO_UNLINK>;
-/**
- * Detect the hook type which is embedded in the given type as a base
- * class. This is a template to postpone the type checks, to allow
- * forward-declared types.
- */
-template<typename U>
-struct IntrusiveListHookDetection {
- /* TODO can this be simplified somehow, without checking for
- all possible enum values? */
- using type = std::conditional_t<std::is_base_of_v<IntrusiveListHook<IntrusiveHookMode::NORMAL>, U>,
- IntrusiveListHook<IntrusiveHookMode::NORMAL>,
- std::conditional_t<std::is_base_of_v<IntrusiveListHook<IntrusiveHookMode::TRACK>, U>,
- IntrusiveListHook<IntrusiveHookMode::TRACK>,
- std::conditional_t<std::is_base_of_v<IntrusiveListHook<IntrusiveHookMode::AUTO_UNLINK>, U>,
- IntrusiveListHook<IntrusiveHookMode::AUTO_UNLINK>,
- void>>>;
-};
-
/**
* For classes which embed #IntrusiveListHook as base class.
*/
template<typename T>
struct IntrusiveListBaseHookTraits {
+ /* a never-called helper function which is used by _Cast() */
+ template<IntrusiveHookMode mode>
+ static constexpr IntrusiveListHook<mode> _Identity(const IntrusiveListHook<mode> &) noexcept;
+
+ /* another never-called helper function which "calls"
+ _Identity(), implicitly casting the item to the
+ IntrusiveListHook specialization; we use this to detect
+ which IntrusiveListHook specialization is used */
template<typename U>
- using Hook = typename IntrusiveListHookDetection<U>::type;
+ static constexpr auto _Cast(const U &u) noexcept {
+ return decltype(_Identity(u))();
+ }
+
+ template<typename U>
+ using Hook = decltype(_Cast(std::declval<U>()));
static constexpr T *Cast(IntrusiveListNode *node) noexcept {
auto *hook = &Hook<T>::Cast(*node);