[flang] [clang] [libc] [llvm] [mlir] [libcxx] [libc++][memory] P1132R8: out_ptr - a scalable output pointer abstraction (PR #73618)

[Louis Dionne via cfe-commits]

================
@@ -0,0 +1,97 @@
+// -*- 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___OUT_PTR_H
+#define _LIBCPP___OUT_PTR_H
+
+#include <__config>
+#include <__memory/addressof.h>
+#include <__memory/pointer_traits.h>
+#include <__memory/shared_ptr.h>
+#include <__memory/unique_ptr.h>
+#include <__type_traits/is_specialization.h>
+#include <__type_traits/is_void.h>
+#include <tuple>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+template <class _Tp>
+concept __resettable_adapted_ptr = requires(_Tp __ptr) { __ptr().reset(); };
+
+template <class _Smart, class _Pointer, class... _Args>
+class _LIBCPP_TEMPLATE_VIS out_ptr_t {
+  static_assert(!__is_specialization_v<_Smart, shared_ptr> || sizeof...(_Args) > 0,
+                "Specialization of std::shared_ptr<> requires a deleter.");
+
+public:
+  _LIBCPP_HIDE_FROM_ABI explicit out_ptr_t(_Smart& __s, _Args... __args)
+      : __s_(__s), __a_(std::forward<_Args>(__args)...), __p_() {
+    using _Ptr = decltype(__s);
+    if constexpr (__resettable_smart_pointer<_Ptr>) {
+      __s_.reset();
+    } else if constexpr (is_constructible_v<_Smart>) {
+      __s_ = _Smart();
+    } else {
+      static_assert(__resettable_smart_pointer<_Ptr> || is_constructible_v<_Smart>);
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI out_ptr_t(const out_ptr_t&) = delete;
+
+  _LIBCPP_HIDE_FROM_ABI ~out_ptr_t() {
+    if (!__p_) {
+      return;
+    }
+
+    using _SP = __pointer_of_or_t<_Smart, _Pointer>;
+    if constexpr (__resettable_smart_pointer_with_args<_Smart, _Pointer, _Args...>) {
+      std::apply([&](auto&&... __args) { __s_.reset(static_cast<_SP>(__p_), std::forward<_Args>(__args)...); },
+                 std::move(__a_));
+    } else if constexpr (is_constructible_v<_Smart, _SP, _Args...>) {
+      std::apply([&](auto&&... __args) { __s_ = _Smart(static_cast<_SP>(__p_), std::forward<_Args>(__args)...); },
+                 std::move(__a_));
+    } else {
+      static_assert(__resettable_smart_pointer_with_args<_Smart, _Pointer, _Args...> ||
+                    is_constructible_v<_Smart, _SP, _Args...>);
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI operator _Pointer*() const noexcept { return std::addressof(const_cast<_Pointer&>(__p_)); }
+
+  _LIBCPP_HIDE_FROM_ABI operator void**() const noexcept
+    requires(!is_same_v<_Pointer, void*>)
+  {
+    static_assert(is_pointer_v<_Pointer>);
+
+    return reinterpret_cast<void**>(static_cast<_Pointer*>(*this));
+  }
+
+private:
+  _Smart& __s_;
+  tuple<_Args...> __a_;
+  _Pointer __p_ = _Pointer();
+};
+
+template <class _Pointer = void, class _Smart, class... _Args>
+_LIBCPP_HIDE_FROM_ABI auto out_ptr(_Smart& __s, _Args&&... __args) {
+  using _Ptr = conditional_t<is_void_v<_Pointer>, __pointer_of_t<_Smart>, _Pointer>;
----------------
ldionne wrote:

I think there's a pedantic bug here. If fetching `__pointer_of_t<_Smart>` is a hard error but we specify a `_Pointer` type explicitly, the current implementation is going to fail to compile because we always eagerly evaluate `__pointer_of_t<_Smart>`. But I think it should compile successfully per the standard.

Edit: I'm unable to find a way to trigger that, since the typical way would involve making a smart ptr that contains a hard-error when you instantiate it to get the nested `::pointer` type. But that strategy obviously doesn't work here. So maybe this isn't a problem after all.

https://github.com/llvm/llvm-project/pull/73618