[libc++] Add helper type non-propagating-cache
Differential Revision: https://reviews.llvm.org/D102121
NOKEYCHECK=True
GitOrigin-RevId: 3959c95deb115e0bc6122faaa244cfb2fc280b50
diff --git a/include/CMakeLists.txt b/include/CMakeLists.txt
index 007d3f6..fc095df 100644
--- a/include/CMakeLists.txt
+++ b/include/CMakeLists.txt
@@ -193,6 +193,7 @@
__ranges/empty.h
__ranges/enable_borrowed_range.h
__ranges/enable_view.h
+ __ranges/non_propagating_cache.h
__ranges/ref_view.h
__ranges/size.h
__ranges/subrange.h
diff --git a/include/__ranges/drop_view.h b/include/__ranges/drop_view.h
index 7b3b8da..099fd22 100644
--- a/include/__ranges/drop_view.h
+++ b/include/__ranges/drop_view.h
@@ -17,9 +17,11 @@
#include <__ranges/all.h>
#include <__ranges/concepts.h>
#include <__ranges/enable_borrowed_range.h>
+#include <__ranges/non_propagating_cache.h>
#include <__ranges/size.h>
#include <__ranges/view_interface.h>
-#include <optional>
+#include <__utility/move.h>
+#include <concepts>
#include <type_traits>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -36,21 +38,16 @@
namespace ranges {
template<view _View>
class drop_view
- : public view_interface<drop_view<_View>> {
-
+ : public view_interface<drop_view<_View>>
+ {
// We cache begin() whenever ranges::next is not guaranteed O(1) to provide an
// amortized O(1) begin() method. If this is an input_range, then we cannot cache
// begin because begin is not equality preserving.
// Note: drop_view<input-range>::begin() is still trivially amortized O(1) because
// one can't call begin() on it more than once.
static constexpr bool _UseCache = forward_range<_View> && !(random_access_range<_View> && sized_range<_View>);
- using _Cache = optional<iterator_t<_View>>;
- struct _Empty { };
-
- // For forward ranges use std::optional to cache the begin iterator.
- // No unique address + _Empty means we don't use any extra space when this
- // is not a forward iterator.
- [[no_unique_address]] conditional_t<_UseCache, _Cache, _Empty> __cached_begin_;
+ using _Cache = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
+ [[no_unique_address]] _Cache __cached_begin_ = _Cache();
range_difference_t<_View> __count_ = 0;
_View __base_ = _View();
@@ -59,48 +56,12 @@
_LIBCPP_HIDE_FROM_ABI
constexpr drop_view(_View __base, range_difference_t<_View> __count)
- : __cached_begin_()
- , __count_(__count)
+ : __count_(__count)
, __base_(_VSTD::move(__base))
{
_LIBCPP_ASSERT(__count_ >= 0, "count must be greater than or equal to zero.");
}
- _LIBCPP_HIDE_FROM_ABI
- constexpr drop_view(drop_view const& __other)
- : __cached_begin_() // Intentionally not propagating the cached begin iterator.
- , __count_(__other.__count_)
- , __base_(__other.__base_)
- { }
-
- _LIBCPP_HIDE_FROM_ABI
- constexpr drop_view(drop_view&& __other)
- : __cached_begin_() // Intentionally not propagating the cached begin iterator.
- , __count_(_VSTD::move(__other.__count_))
- , __base_(_VSTD::move(__other.__base_))
- { }
-
- _LIBCPP_HIDE_FROM_ABI
- constexpr drop_view& operator=(drop_view const& __other) {
- if constexpr (_UseCache) {
- __cached_begin_.reset();
- }
- __base_ = __other.__base_;
- __count_ = __other.__count_;
- return *this;
- }
-
- _LIBCPP_HIDE_FROM_ABI
- constexpr drop_view& operator=(drop_view&& __other) {
- if constexpr (_UseCache) {
- __cached_begin_.reset();
- __other.__cached_begin_.reset();
- }
- __base_ = _VSTD::move(__other.__base_);
- __count_ = _VSTD::move(__other.__count_);
- return *this;
- }
-
_LIBCPP_HIDE_FROM_ABI constexpr _View base() const& requires copy_constructible<_View> { return __base_; }
_LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return _VSTD::move(__base_); }
@@ -110,12 +71,12 @@
random_access_range<const _View> && sized_range<const _View>))
{
if constexpr (_UseCache)
- if (__cached_begin_)
+ if (__cached_begin_.__has_value())
return *__cached_begin_;
auto __tmp = ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
if constexpr (_UseCache)
- __cached_begin_ = __tmp;
+ __cached_begin_.__set(__tmp);
return __tmp;
}
diff --git a/include/__ranges/non_propagating_cache.h b/include/__ranges/non_propagating_cache.h
new file mode 100644
index 0000000..878f707
--- /dev/null
+++ b/include/__ranges/non_propagating_cache.h
@@ -0,0 +1,99 @@
+// -*- 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___RANGES_NON_PROPAGATING_CACHE_H
+#define _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
+
+#include <__config>
+#include <__iterator/concepts.h> // indirectly_readable
+#include <__iterator/iterator_traits.h> // iter_reference_t
+#include <__memory/addressof.h>
+#include <concepts> // constructible_from
+#include <optional>
+#include <type_traits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// clang-format off
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+
+namespace ranges {
+ // __non_propagating_cache is a helper type that allows storing an optional value in it,
+ // but which does not copy the source's value when it is copy constructed/assigned to,
+ // and which resets the source's value when it is moved-from.
+ //
+ // This type is used as an implementation detail of some views that need to cache the
+ // result of `begin()` in order to provide an amortized O(1) begin() method. Typically,
+ // we don't want to propagate the value of the cache upon copy because the cached iterator
+ // may refer to internal details of the source view.
+ template<class _Tp>
+ requires is_object_v<_Tp>
+ class _LIBCPP_TEMPLATE_VIS __non_propagating_cache {
+ optional<_Tp> __value_ = nullopt;
+
+ public:
+ _LIBCPP_HIDE_FROM_ABI __non_propagating_cache() = default;
+
+ _LIBCPP_HIDE_FROM_ABI
+ constexpr __non_propagating_cache(__non_propagating_cache const&) noexcept
+ : __value_(nullopt)
+ { }
+
+ _LIBCPP_HIDE_FROM_ABI
+ constexpr __non_propagating_cache(__non_propagating_cache&& __other) noexcept
+ : __value_(nullopt)
+ {
+ __other.__value_.reset();
+ }
+
+ _LIBCPP_HIDE_FROM_ABI
+ constexpr __non_propagating_cache& operator=(__non_propagating_cache const& __other) noexcept {
+ if (this != _VSTD::addressof(__other)) {
+ __value_.reset();
+ }
+ return *this;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI
+ constexpr __non_propagating_cache& operator=(__non_propagating_cache&& __other) noexcept {
+ __value_.reset();
+ __other.__value_.reset();
+ return *this;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI
+ constexpr _Tp& operator*() { return *__value_; }
+ _LIBCPP_HIDE_FROM_ABI
+ constexpr _Tp const& operator*() const { return *__value_; }
+
+ _LIBCPP_HIDE_FROM_ABI
+ constexpr bool __has_value() const { return __value_.has_value(); }
+ _LIBCPP_HIDE_FROM_ABI
+ constexpr void __set(_Tp const& __value) { __value_.emplace(__value); }
+ _LIBCPP_HIDE_FROM_ABI
+ constexpr void __set(_Tp&& __value) { __value_.emplace(_VSTD::move(__value)); }
+ };
+
+ struct __empty_cache { };
+} // namespace ranges
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
diff --git a/include/module.modulemap b/include/module.modulemap
index 6931698..4d2101e 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -613,6 +613,7 @@
module empty_view { private header "__ranges/empty_view.h" }
module enable_borrowed_range { private header "__ranges/enable_borrowed_range.h" }
module enable_view { private header "__ranges/enable_view.h" }
+ module non_propagating_cache { private header "__ranges/non_propagating_cache.h" }
module ref_view { private header "__ranges/ref_view.h" }
module size { private header "__ranges/size.h" }
module subrange { private header "__ranges/subrange.h" }