[libcxx-commits] [libcxx] [libc++] Implement a type-safe iterator for optional (PR #154239)

[A. Jiang via libcxx-commits]

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+// -*- 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___CAPACITY_AWARE_ITERATOR_H
+#define _LIBCPP___CAPACITY_AWARE_ITERATOR_H
+
+#include <__assert>
+#include <__compare/ordering.h>
+#include <__compare/three_way_comparable.h>
+#include <__config>
+#include <__cstddef/size_t.h>
+#include <__iterator/concepts.h>
+#include <__iterator/incrementable_traits.h>
+#include <__iterator/iterator_traits.h>
+#include <__memory/pointer_traits.h>
+#include <__type_traits/is_constructible.h>
+#include <__type_traits/is_convertible.h>
+#include <__utility/move.h>
+
+#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
+
+// __capacity_aware_iterator is an iterator that wraps a contiguous iterator and encodes the maximum number of
+// elements that can appear in a range of such iterators. That maximum number of elements must be known at compile-time.
+//
+// It also embeds a tag type to prevent mixing iterators from e.g. different containers. This also allows for some
+// algorithms to detect this iterator and perform optimizations based on the added semantic information.
+//
+// As of writing, the only standard library containers which fulfill the requirements are inplace_vector and optional.
+
+template <class _Iter, class _Tag, std::size_t _RangeMaxElements>
+class __capacity_aware_iterator {
+private:
+  _Iter __iter_;
+
+  template <class, class, std::size_t>
+  friend class __capacity_aware_iterator;
+
+public:
+  static_assert(contiguous_iterator<_Iter>, "__capacity_aware_iterator can only be used with contiguous iterators");
+
+  using iterator_category = iterator_traits<_Iter>::iterator_category;
+  using iterator_concept  = contiguous_iterator_tag;
+  using difference_type   = iter_difference_t<_Iter>;
+  using pointer           = iterator_traits<_Iter>::pointer;
+  using reference         = iter_reference_t<_Iter>;
+  using value_type        = iter_value_t<_Iter>;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __capacity_aware_iterator()
+    requires is_default_constructible_v<_Iter>
+  = default;
+
+  friend struct pointer_traits<__capacity_aware_iterator<_Iter, _Tag, _RangeMaxElements>>;
+
+  template <typename _Iter2>
+    requires is_convertible_v<_Iter2, _Iter>
+  _LIBCPP_HIDE_FROM_ABI constexpr __capacity_aware_iterator(
+      const __capacity_aware_iterator<_Iter2, _Tag, _RangeMaxElements>& __y) noexcept
+      : __iter_(__y.__iter_) {}
+
+  template <class _It, class _Tag2, size_t _RangeMaxElems2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr auto __make_capacity_aware_iterator(_It __iter) noexcept;
+
+private:
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __capacity_aware_iterator(_Iter __iter) : __iter_(std::move(__iter)) {}
+
+public:
+  _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator*() const noexcept { return *__iter_; }
+  _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator->() const noexcept { return std::__to_address(__iter_); }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __capacity_aware_iterator& operator++() noexcept {
+    ++__iter_;
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __capacity_aware_iterator operator++(int) noexcept {
+    __capacity_aware_iterator __tmp(*this);
+    ++*this;
+    return __tmp;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __capacity_aware_iterator& operator--() noexcept {
+    --__iter_;
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __capacity_aware_iterator operator--(int) noexcept {
+    __capacity_aware_iterator __tmp(*this);
+    --*this;
+    return __tmp;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __capacity_aware_iterator& operator+=(difference_type __n) noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        static_cast<size_t>((__n >= 0 ? __n : -__n)) <= _RangeMaxElements,
+        "__capacity_aware_iterator::operator+=: Attempting to move iterator past its container's possible range");
+
+    __iter_ += __n;
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __capacity_aware_iterator& operator-=(difference_type __n) noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        static_cast<size_t>((__n >= 0 ? __n : -__n)) <= _RangeMaxElements,
+        "__capacity_aware_iterator::operator-=: Attempting to move iterator past its container's possible range");
+
+    __iter_ -= __n;
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator[](difference_type __n) const noexcept {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
+        static_cast<size_t>(__n >= 0 ? __n : -__n) < _RangeMaxElements,
+        "__capacity_aware_iterator::operator[]: Attempting to index iterator past its container's possible range");
+    return *(*this + __n);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool
+  operator==(const __capacity_aware_iterator& __x, const __capacity_aware_iterator& __y) noexcept {
+    return __x.__iter_ == __y.__iter_;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr auto
+  operator<=>(const __capacity_aware_iterator& __x, const __capacity_aware_iterator& __y) noexcept {
+    if constexpr (three_way_comparable_with<_Iter, _Iter, strong_ordering>) {
+      return __x.__iter_ <=> __y.__iter_;
+    } else {
+      if (__x.__iter_ < __y.__iter_) {
+        return strong_ordering::less;
+      } else if (__x.__iter_ == __y.__iter_) {
+        return strong_ordering::equal;
+      }
+      return strong_ordering::greater;
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr __capacity_aware_iterator
+  operator+(const __capacity_aware_iterator& __i, difference_type __n) noexcept {
+    auto __tmp = __i;
+    __tmp += __n;
+    return __tmp;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr __capacity_aware_iterator
+  operator+(difference_type __n, const __capacity_aware_iterator& __i) noexcept {
+    auto __tmp = __i;
+    __tmp += __n;
+    return __tmp;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr __capacity_aware_iterator
+  operator-(const __capacity_aware_iterator& __i, difference_type __n) noexcept {
+    auto __tmp = __i;
+    __tmp -= __n;
+    return __tmp;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type
+  operator-(const __capacity_aware_iterator& __x, const __capacity_aware_iterator& __y) noexcept {
+    return difference_type(__x.__iter_ - __y.__iter_);
+  }
+};
+
+template <class _It, class _Tag2, size_t _RangeMaxElems2>
+_LIBCPP_HIDE_FROM_ABI constexpr auto __make_capacity_aware_iterator(_It __iter) noexcept {
+  return __capacity_aware_iterator<_It, _Tag2, _RangeMaxElems2>(__iter);
+}
+
+template <class _Iter, class _Tag, std::size_t _RangeMaxElements>
+struct pointer_traits<__capacity_aware_iterator<_Iter, _Tag, _RangeMaxElements>> {
+  using pointer         = __capacity_aware_iterator<_Iter, _Tag, _RangeMaxElements>;
+  using element_type    = typename pointer_traits<_Iter>::element_type;
+  using difference_type = typename pointer_traits<_Iter>::difference_type;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR static element_type* to_address(pointer __it) _NOEXCEPT {
+    return std::__to_address(__it.__iter_);
+  }
+};
----------------
frederick-vs-ja wrote:

This will be necessary if we want to reject calling `operator->` on a past-of-end iterator, as `std::to_address` call still needs to be valid. I'm not sure whether we should want to such `operator->` call, though.

https://github.com/llvm/llvm-project/pull/154239