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

[Louis Dionne via cfe-commits]

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@@ -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() {
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ldionne wrote:

I think we need to do something about https://eel.is/c++draft/smartptr#out.ptr.t-7. Otherwise, when we use a `shared_ptr` here, if we fail to allocate the control block we'll be in trouble, since this is a destructor. So, instead what we could do is allocate a control block in the constructor, then use `shared_ptr<T>::__create_with_control_block` in the destructor. The simplest way to do this might be to create a specialization for `shared_ptr<T>`. Do you think that is overkill?

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