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

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,174 @@
+// -*- 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
+//
+//===----------------------------------------------------------------------===//
+
+/*
+ * __upper_bounded_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 stores the absolute maximum amount of elements the container can have, known at compile-time.
+ * As of writing, the only standard library containers which have this property are inplace_vector and optional.
+ */
+
+#ifndef _LIBCPP___ITERATOR_UPPER_BOUNDED_ITERATOR_H
+#define _LIBCPP___ITERATOR_UPPER_BOUNDED_ITERATOR_H
+
+#include <__compare/ordering.h>
+#include <__compare/three_way_comparable.h>
+#include <__config>
+#include <__cstddef/size_t.h>
+#include <__iterator/incrementable_traits.h>
+#include <__iterator/iterator_traits.h>
+#include <__memory/pointer_traits.h>
+#include <__type_traits/is_constructible.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
+
+template <class _Iter, class _Container, std::size_t _Max_Elements>
+class __upper_bounded_iterator {
+private:
+  _Iter __iter_;
+
+  friend _Container;
+
+public:
+  using iterator_category = iterator_traits<_Iter>::iterator_category;
+  using iterator_concept  = _Iter::iterator_concept;
+  using value_type        = iter_value_t<_Iter>;
+  using difference_type   = iter_difference_t<_Iter>;
+  using reference         = iter_reference_t<_Iter>;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __upper_bounded_iterator()
+    requires is_default_constructible_v<_Iter>
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __upper_bounded_iterator(_Iter __iter) : __iter_(std::move(__iter)) {}
----------------
ldionne wrote:

Can this really be a public constructor? I suspect this would have to be private, or at least contain a tag with a reserved name.

Otherwise, `std::optional<T>::iterator` is going to be constructible from something that is really just an implementation detail of the library, and that's a constructor that users of the library could actually access. For example,

```
std::vector<int> v = {1,2,3};
auto begin = v.begin(); // that's a std::__wrap_iter<int*> under the hood
std::optional<int>::iterator it(begin); // this compiles because of your public constructor from `__wrap_iter` to `optional::iterator`, but arguably should be an error.
```

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