xref: /openbmc/sdbusplus/include/sdbusplus/async/stdexec/__detail/__manual_lifetime.hpp (revision f083bc1a)

/*
 * Copyright (c) 2023 Maikel Nadolski
 * Copyright (c) 2023 NVIDIA Corporation
 *
 * Licensed under the Apache License Version 2.0 with LLVM Exceptions
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *   https://llvm.org/LICENSE.txt
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#pragma once

#include "../concepts.hpp"

#include <cstddef>
#include <memory>
#include <new>
#include <type_traits>

namespace stdexec
{

//! Holds storage for a `_Ty`, but allows clients to `__construct(...)`,
//! `__destry()`, and `__get()` the `_Ty` without regard for usual lifetime
//! rules.
template <class _Ty>
class __manual_lifetime
{
  public:
    //! Constructor does nothing: It's on you to call `__construct(...)` or
    //! `__construct_from(...)` if you want the `_Ty`'s lifetime to begin.
    constexpr __manual_lifetime() noexcept {}
    //! Destructor does nothing: It's on you to call `__destroy()` if you mean
    //! to.
    constexpr ~__manual_lifetime() {}

    __manual_lifetime(const __manual_lifetime&) = delete;
    auto operator=(const __manual_lifetime&) -> __manual_lifetime& = delete;

    __manual_lifetime(__manual_lifetime&&) = delete;
    auto operator=(__manual_lifetime&&) -> __manual_lifetime& = delete;

    //! Construct the `_Ty` in place.
    //! There are no safeties guarding against the case that there's already one
    //! there.
    template <class... _Args>
    auto __construct(_Args&&... __args) noexcept(
        stdexec::__nothrow_constructible_from<_Ty, _Args...>) -> _Ty&
    {
        // Use placement new instead of std::construct_at to support aggregate
        // initialization with brace elision.
        return *std::launder(::new (static_cast<void*>(__buffer_))
                                 _Ty{static_cast<_Args&&>(__args)...});
    }

    //! Construct the `_Ty` in place from the result of calling `func`.
    //! There are no safeties guarding against the case that there's already one
    //! there.
    template <class _Func, class... _Args>
    auto __construct_from(_Func&& func, _Args&&... __args) -> _Ty&
    {
        // Use placement new instead of std::construct_at in case the function
        // returns an immovable type.
        return *std::launder(::new (static_cast<void*>(__buffer_)) _Ty{
            (static_cast<_Func&&>(func))(static_cast<_Args&&>(__args)...)});
    }
    //! End the lifetime of the contained `_Ty`.
    //! Precondition: The lifetime has started.
    void __destroy() noexcept
    {
        std::destroy_at(&__get());
    }
    //! Get access to the `_Ty`.
    //! Precondition: The lifetime has started.
    auto __get() & noexcept -> _Ty&
    {
        return *reinterpret_cast<_Ty*>(__buffer_);
    }

    //! Get access to the `_Ty`.
    //! Precondition: The lifetime has started.
    auto __get() && noexcept -> _Ty&&
    {
        return static_cast<_Ty&&>(*reinterpret_cast<_Ty*>(__buffer_));
    }

    //! Get access to the `_Ty`.
    //! Precondition: The lifetime has started.
    auto __get() const& noexcept -> const _Ty&
    {
        return *reinterpret_cast<const _Ty*>(__buffer_);
    }

    //! Move semantics aren't supported.
    //! If you want to move the `_Ty`, use `std::move(ml.__get())`.
    auto __get() const&& noexcept -> const _Ty&& = delete;

  private:
    alignas(_Ty) unsigned char __buffer_[sizeof(_Ty)]{};
};
} // namespace stdexec