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

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,204 @@
+// -*- 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 an underlying iterator.
+// It stores the underlying container type to prevent mixing iterators, and allow algorithms
+// to optimize based on the underlying container type.
+// It also encodes the container's (known at compile-time) maximum amount of elements as part of the type.
+// As of writing, the only standard library containers which have this property are inplace_vector and optional.
+
+template <class _Iter, class _Container, std::size_t _ContainerMaxElements>
+class __capacity_aware_iterator {
+private:
+  _Iter __iter_;
+
+  friend _Container;
+
+  _LIBCPP_HIDE_FROM_ABI static constexpr auto __get_iter_concept() {
+    if constexpr (contiguous_iterator<_Iter>) {
+      return contiguous_iterator_tag{};
+    } else if constexpr (random_access_iterator<_Iter>) {
+      return random_access_iterator_tag{};
+    } else if constexpr (bidirectional_iterator<_Iter>) {
+      return bidirectional_iterator_tag{};
+    } else if constexpr (forward_iterator<_Iter>) {
+      return forward_iterator_tag{};
+    } else {
+      return input_iterator_tag{};
+    }
+  }
+
+public:
+  using iterator_category = iterator_traits<_Iter>::iterator_category;
+  using iterator_concept  = decltype(__get_iter_concept());
+  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;
+
+  template <typename _Iter2>
+    requires is_convertible_v<_Iter2, _Iter>
+  _LIBCPP_HIDE_FROM_ABI constexpr __capacity_aware_iterator(
+      const __capacity_aware_iterator<_Iter2, _Container, _ContainerMaxElements>& __y)
+      : __iter_(__y.base()) {}
+
+private:
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __capacity_aware_iterator(_Iter __iter) : __iter_(std::move(__iter)) {}
+
+  template <typename _Tp, class>
+  friend struct __optional_iterator;
+
+  template <class _It, class _Container2, size_t _ContainerMaxElems2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr auto __make_capacity_aware_iterator(_It __iter);
+
+public:
+  _LIBCPP_HIDE_FROM_ABI constexpr _Iter base() const noexcept { return __iter_; }
----------------
ldionne wrote:

We shouldn't have a public `base()` member at all, since that's not part of the normal iterator API. I think there's a few places where we provide one elsewhere in the library, but we shouldn't.

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