[libcxx-commits] [libcxx] [libc++] Implement the `indirect` half of P3019R14: Vocabulary Types for Composite Class Design (PR #166717)
Nikolas Klauser via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Nov 7 05:49:07 PST 2025
================
@@ -0,0 +1,340 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___MEMORY_INDIRECT_H
+#define _LIBCPP___MEMORY_INDIRECT_H
+
+#include <__config>
+
+#include <__assert>
+#include <__compare/strong_order.h>
+#include <__compare/synth_three_way.h>
+#include <__functional/hash.h>
+#include <__fwd/memory_resource.h>
+#include <__memory/addressof.h>
+#include <__memory/allocation_guard.h>
+#include <__memory/allocator_arg_t.h>
+#include <__memory/allocator_traits.h>
+#include <__memory/swap_allocator.h>
+#include <__type_traits/is_array.h>
+#include <__type_traits/is_assignable.h>
+#include <__type_traits/is_constructible.h>
+#include <__type_traits/is_object.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/remove_cv.h>
+#include <__type_traits/remove_cvref.h>
+#include <__utility/exchange.h>
+#include <__utility/forward.h>
+#include <__utility/in_place.h>
+#include <__utility/move.h>
+#include <__utility/swap.h>
+#include <initializer_list>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+# pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 26
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Tp, class _Allocator = allocator<_Tp>>
+class _LIBCPP_NO_SPECIALIZATIONS indirect {
+public:
+ using value_type = _Tp;
+ using allocator_type = _Allocator;
+ using pointer = allocator_traits<_Allocator>::pointer;
+ using const_pointer = allocator_traits<_Allocator>::const_pointer;
+
+ static_assert(__check_valid_allocator<allocator_type>::value);
+ static_assert(is_same_v<typename allocator_type::value_type, value_type>);
+ static_assert(is_object_v<value_type>);
+ static_assert(!is_array_v<value_type>);
+ static_assert(!is_same_v<value_type, in_place_t>);
+ static_assert(!__is_inplace_type<value_type>::value);
+ static_assert(std::is_same_v<value_type, remove_cv_t<value_type>>,
+ "value_type must not be const or volatile qualified");
+
+ // [indirect.ctor], constructors
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit indirect()
+ requires is_default_constructible_v<_Allocator>
+ : __p_(__allocate_owned_object(__alloc_)) {}
+
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit indirect(allocator_arg_t, const _Allocator& __a)
+ : __alloc_(__a), __p_(__allocate_owned_object(__alloc_)) {}
+
+ _LIBCPP_HIDE_FROM_ABI constexpr indirect(const indirect& __other)
+ : __alloc_(allocator_traits<_Allocator>::select_on_container_copy_construction(__other.__alloc_)),
+ __p_(__other.valueless_after_move() ? nullptr : __allocate_owned_object(__alloc_, *__other)) {}
+
+ _LIBCPP_HIDE_FROM_ABI constexpr indirect(allocator_arg_t, const _Allocator& __a, const indirect& __other)
+ : __alloc_(__a), __p_(__other.valueless_after_move() ? nullptr : __allocate_owned_object(__alloc_, *__other)) {}
+
+ _LIBCPP_HIDE_FROM_ABI constexpr indirect(indirect&& __other) noexcept
+ : __alloc_(std::move(__other.__alloc_)), __p_(std::exchange(__other.__p_, nullptr)) {}
+
+ _LIBCPP_HIDE_FROM_ABI constexpr indirect(allocator_arg_t, const _Allocator& __a, indirect&& __other) noexcept
+ requires allocator_traits<_Allocator>::is_always_equal::value
+ : __alloc_(__a), __p_(std::exchange(__other.__p_, nullptr)) {}
+
+ _LIBCPP_HIDE_FROM_ABI constexpr indirect(allocator_arg_t, const _Allocator& __a, indirect&& __other) : __alloc_(__a) {
+ if (__other.valueless_after_move()) {
+ __p_ = nullptr;
+ } else if (__alloc_ == __other.__alloc_) {
+ __p_ = std::exchange(__other.__p_, nullptr);
+ } else {
+ __p_ = __allocate_owned_object(__alloc_, *std::move(__other));
+ __other.__destroy_owned_object();
+ __other.__p_ = nullptr;
+ }
+ }
+
+ template <class _Up = _Tp>
+ requires(!is_same_v<remove_cvref_t<_Up>, indirect> && !is_same_v<remove_cvref_t<_Up>, in_place_t> &&
+ is_constructible_v<_Tp, _Up> && is_default_constructible_v<_Allocator>)
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit indirect(_Up&& __u)
+ : __p_(__allocate_owned_object(__alloc_, std::forward<_Up>(__u))) {}
+
+ template <class _Up = _Tp>
+ requires(!is_same_v<remove_cvref_t<_Up>, indirect> && !is_same_v<remove_cvref_t<_Up>, in_place_t> &&
+ is_constructible_v<_Tp, _Up>)
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit indirect(allocator_arg_t, const _Allocator& __a, _Up&& __u)
+ : __alloc_(__a), __p_(__allocate_owned_object(__alloc_, std::forward<_Up>(__u))) {}
+
+ template <class... _Us>
+ requires(is_constructible_v<_Tp, _Us...> && is_default_constructible_v<_Allocator>)
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit indirect(in_place_t, _Us&&... __us)
+ : __p_(__allocate_owned_object(__alloc_, std::forward<_Us>(__us)...)) {}
+
+ template <class... _Us>
+ requires is_constructible_v<_Tp, _Us...>
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit indirect(allocator_arg_t, const _Allocator& __a, in_place_t, _Us&&... __us)
+ : __alloc_(__a), __p_(__allocate_owned_object(__alloc_, std::forward<_Us>(__us)...)) {}
+
+ template <class _In, class... _Us>
+ requires(is_constructible_v<_Tp, initializer_list<_In>&, _Us...> && is_default_constructible_v<_Allocator>)
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit indirect(in_place_t, initializer_list<_In> __ilist, _Us&&... __us)
+ : __p_(__allocate_owned_object(__alloc_, __ilist, std::forward<_Us>(__us)...)) {}
+
+ template <class _In, class... _Us>
+ requires is_constructible_v<_Tp, initializer_list<_In>&, _Us...>
+ _LIBCPP_HIDE_FROM_ABI constexpr explicit indirect(
+ allocator_arg_t, const _Allocator& __a, in_place_t, initializer_list<_In> __ilist, _Us&&... __us)
+ : __alloc_(__a), __p_(__allocate_owned_object(__alloc_, __ilist, std::forward<_Us>(__us)...)) {}
+
+ // [indirect.dtor], destructor
+ _LIBCPP_HIDE_FROM_ABI constexpr ~indirect() { __destroy_owned_object(); }
+
+ // [indirect.assign], assignment
+ _LIBCPP_HIDE_FROM_ABI constexpr indirect& operator=(const indirect& __other) {
+ if (std::addressof(__other) == this)
+ return *this;
+
+ static constexpr bool __propagate_allocator =
+ allocator_traits<_Allocator>::propagate_on_container_copy_assignment::value;
+ if (__other.valueless_after_move()) {
+ __destroy_owned_object();
+ __p_ = nullptr;
+ } else if (!valueless_after_move() && __alloc_ == __other.__alloc_) {
+ *__p_ = *__other;
+ } else {
+ pointer __new_p;
+ if constexpr (__propagate_allocator) {
+ // We need a mutable instance of the allocator, so make a copy.
+ _Allocator __alloc_copy = __other.__alloc_;
+ __new_p = __allocate_owned_object(__alloc_copy, *__other);
+ } else {
+ __new_p = __allocate_owned_object(__alloc_, *__other);
+ }
+ __destroy_owned_object();
+ __p_ = __new_p;
+ }
+
+ if constexpr (__propagate_allocator)
+ __alloc_ = __other.__alloc_;
+
+ return *this;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr indirect& operator=(indirect&& __other) noexcept(
+ allocator_traits<_Allocator>::propagate_on_container_move_assignment::value ||
+ allocator_traits<_Allocator>::is_always_equal::value) {
+ if (std::addressof(__other) == this)
+ return *this;
+
+ static constexpr bool __propagate_allocator =
+ allocator_traits<_Allocator>::propagate_on_container_move_assignment::value;
+
+ pointer __new_p;
+ if constexpr (__propagate_allocator || allocator_traits<_Allocator>::is_always_equal::value) {
+ __new_p = __other.__p_;
+ } else if (__other.valueless_after_move()) {
+ __new_p = nullptr;
+ } else if (__alloc_ == __other.__alloc_) {
+ __new_p = __other.__p_;
+ } else {
+ __new_p = __allocate_owned_object(__alloc_, *std::move(__other));
+ __other.__destroy_owned_object();
+ }
+ __other.__p_ = nullptr;
+ __destroy_owned_object();
+ __p_ = __new_p;
+
+ if constexpr (__propagate_allocator)
+ __alloc_ = __other.__alloc_;
+
+ return *this;
+ }
+
+ template <class _Up = _Tp>
+ requires(!is_same_v<remove_cvref_t<_Up>, indirect> && is_constructible_v<_Tp, _Up> && is_assignable_v<_Tp&, _Up>)
+ _LIBCPP_HIDE_FROM_ABI constexpr indirect& operator=(_Up&& __u) {
+ if (valueless_after_move())
+ __p_ = __allocate_owned_object(__alloc_, std::forward<_Up>(__u));
+ else
+ *__p_ = std::forward<_Up>(__u);
+ return *this;
+ }
+
+ // [indirect.obs], observers
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& operator*() const& noexcept {
+ _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+ !valueless_after_move(), "operator* called on a valueless std::indirect object");
+ return *__p_;
+ }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() & noexcept {
+ _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+ !valueless_after_move(), "operator* called on a valueless std::indirect object");
+ return *__p_;
+ }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& operator*() const&& noexcept {
+ _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+ !valueless_after_move(), "operator* called on a valueless std::indirect object");
+ return std::move(*__p_);
+ }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator*() && noexcept {
+ _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+ !valueless_after_move(), "operator* called on a valueless std::indirect object");
+ return std::move(*__p_);
+ }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const_pointer operator->() const noexcept {
+ _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+ !valueless_after_move(), "operator-> called on a valueless std::indirect object");
+ return __p_;
+ }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr pointer operator->() noexcept {
+ _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+ !valueless_after_move(), "operator-> called on a valueless std::indirect object");
+ return __p_;
+ }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool valueless_after_move() const noexcept { return !__p_; }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr allocator_type get_allocator() const noexcept { return __alloc_; }
+
+ // [indirect.swap], swap
+ _LIBCPP_HIDE_FROM_ABI constexpr void
+ swap(indirect& __other) noexcept(allocator_traits<_Allocator>::propagate_on_container_swap::value ||
+ allocator_traits<_Allocator>::is_always_equal::value) {
+ _LIBCPP_ASSERT_COMPATIBLE_ALLOCATOR(
+ allocator_traits<_Allocator>::propagate_on_container_swap::value || get_allocator() == __other.get_allocator(),
+ "swapping std::indirect objects with different allocators");
+ std::swap(__p_, __other.__p_);
+ std::__swap_allocator(__alloc_, __other.__alloc_);
+ }
+
+ _LIBCPP_HIDE_FROM_ABI friend constexpr void
+ swap(indirect& __lhs, indirect& __rhs) noexcept(noexcept(__lhs.swap(__rhs))) {
+ __lhs.swap(__rhs);
+ }
+
+ // [indirect.relops], relational operators
+ template <class _Up, class _AA>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr bool
+ operator==(const indirect& __lhs, const indirect<_Up, _AA>& __rhs) noexcept(noexcept(*__lhs == *__rhs)) {
+ return (__lhs.valueless_after_move() == __rhs.valueless_after_move()) &&
+ (__lhs.valueless_after_move() || *__lhs == *__rhs);
+ }
+
+ template <class _Up, class _AA>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __synth_three_way_result<_Tp, _Up>
+ operator<=>(const indirect& __lhs, const indirect<_Up, _AA>& __rhs) {
+ if (__lhs.valueless_after_move() || __rhs.valueless_after_move())
+ return !__lhs.valueless_after_move() <=> !__rhs.valueless_after_move();
+ return std::__synth_three_way(*__lhs, *__rhs);
+ }
+
+ // [indirect.comp.with.t], comparison with T
+ template <class _Up>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr bool
+ operator==(const indirect& __lhs, const _Up& __rhs) noexcept(noexcept(*__lhs == __rhs)) {
+ return !__lhs.valueless_after_move() && *__lhs == __rhs;
+ }
+
+ template <class _Up>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __synth_three_way_result<_Tp, _Up>
+ operator<=>(const indirect& __lhs, const _Up& __rhs) {
+ return __lhs.valueless_after_move() ? strong_ordering::less : std::__synth_three_way(*__lhs, __rhs);
+ }
+
+private:
+ template <class... _Us>
+ _LIBCPP_HIDE_FROM_ABI static constexpr pointer __allocate_owned_object(_Allocator& __a, _Us&&... __us) {
+ __allocation_guard<_Allocator> __guard(__a, 1);
+ allocator_traits<_Allocator>::construct(__a, __guard.__get(), std::forward<_Us>(__us)...);
+ return __guard.__release_ptr();
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr void __destroy_owned_object() noexcept {
+ if (!valueless_after_move()) {
+ allocator_traits<_Allocator>::destroy(__alloc_, __p_);
+ allocator_traits<_Allocator>::deallocate(__alloc_, __p_, 1);
+ }
+ }
+
+ _LIBCPP_NO_UNIQUE_ADDRESS _Allocator __alloc_ = _Allocator();
+ pointer __p_;
----------------
philnik777 wrote:
The motivation is that we then have low bits we can use to store further information, e.g. for `optional`. Since the paper explicitly recommends `optional<indirect<T>>` we should really try to make it just one pointer in size.
https://github.com/llvm/llvm-project/pull/166717
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