[libcxx-commits] [flang] [libc] [mlir] [clang-tools-extra] [lld] [openmp] [llvm] [libcxx] [clang] [compiler-rt] [lldb] [libc++][memory] P1132R8: `out_ptr` - a scalable output pointer abstraction (PR #73618)

[Hristo Hristov via libcxx-commits]

================
@@ -0,0 +1,102 @@
+// -*- 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___INOUT_PTR_H
+#define _LIBCPP___INOUT_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_same.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 _Smart, class _Pointer, class... _Args>
+class _LIBCPP_TEMPLATE_VIS inout_ptr_t {
+  static_assert(!__is_specialization_v<_Smart, shared_ptr>, "std::shared_ptr<> is not supported");
+
+public:
+  _LIBCPP_HIDE_FROM_ABI explicit inout_ptr_t(_Smart& __s, _Args... __args)
+      : __s_(__s), __a_(std::forward<_Args>(__args)...), __p_([&__s] {
+          if constexpr (is_pointer_v<_Smart>) {
+            return __s;
+          } else {
+            return __s.get();
+          }
+        }()) {
+    if constexpr (requires { __s.release(); }) {
+      __s.release();
+    } else {
+      __s = _Smart();
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI inout_ptr_t(const inout_ptr_t&) = delete;
+
+  _LIBCPP_HIDE_FROM_ABI ~inout_ptr_t() {
+    if constexpr (!is_pointer_v<_Smart>) {
+      if (!__p_) {
+        return;
+      }
+    }
+
+    using _SP = __pointer_of_or_t<_Smart, _Pointer>;
+    if constexpr (is_pointer_v<_Smart>) {
+      std::apply([&](auto&&... __args) { __s_ = _Smart(static_cast<_SP>(__p_), std::forward<_Args>(__args)...); },
+                 std::move(__a_));
+    } else 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(is_pointer_v<_Smart> || __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_;
+};
+
+template <class _Pointer = void, class _Smart, class... _Args>
+_LIBCPP_HIDE_FROM_ABI auto inout_ptr(_Smart& __s, _Args&&... __args) {
+  using _Ptr = conditional_t<is_void_v<_Pointer>, __pointer_of_t<_Smart>, _Pointer>;
+  return std::inout_ptr_t<_Smart, _Ptr, _Args&&...>(__s, std::forward<_Args>(__args)...);
----------------
H-G-Hristov wrote:

Maybe I don't understand but the following fails to compile: https://godbolt.org/z/cseMEfdj5

> opt/compiler-explorer/gcc-trunk-20240117/include/c++/14.0.1/bits/out_ptr.h:212:42: error: use of deleted function 'constexpr Foo& Foo::operator=(const Foo&)'
  212 |               _M_smart._M_t._M_deleter() = std::forward<_Del2>(_M_del);
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

```c++
#include <memory>
#include <tuple>

struct Foo {
    Foo() = default;
    Foo(Foo const&) = delete;  // not copyable
    Foo(Foo&&) = delete;       // not movable either!

    void operator()(int*) {};
};

int main() {
    std::unique_ptr<int, Foo> smart;
    Foo arg;
    // std::ignore = std::out_ptr(smart, arg);
    std::ignore = std::inout_ptr(smart, arg);
}
```

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