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- /* An optional object.
- Copyright (C) 2017-2022 Free Software Foundation, Inc.
- This file is part of GDB.
- This program is free software; you can redistribute it and/or modify
- it under the terms of the GNU General Public License as published by
- the Free Software Foundation; either version 3 of the License, or
- (at your option) any later version.
- This program is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- GNU General Public License for more details.
- You should have received a copy of the GNU General Public License
- along with this program. If not, see <http://www.gnu.org/licenses/>. */
- #ifndef COMMON_GDB_OPTIONAL_H
- #define COMMON_GDB_OPTIONAL_H
- #include "gdbsupport/traits.h"
- namespace gdb
- {
- struct in_place_t
- {
- explicit in_place_t () = default;
- };
- constexpr gdb::in_place_t in_place {};
- /* This class attempts to be a compatible subset of std::optional,
- which is slated to be available in C++17. This class optionally
- holds an object of some type -- by default it is constructed not
- holding an object, but later the object can be "emplaced". This is
- similar to using std::unique_ptr, but in-object allocation is
- guaranteed.
- Unlike std::optional, we currently only support copy/move
- construction/assignment of an optional<T> from either exactly
- optional<T> or T. I.e., we don't support copy/move
- construction/assignment from optional<U> or U, when U is a type
- convertible to T. Making that work depending on the definitions of
- T and U is somewhat complicated, and currently the users of this
- class don't need it. */
- template<typename T>
- class optional
- {
- public:
- constexpr optional ()
- : m_dummy ()
- {}
- template<typename... Args>
- constexpr optional (in_place_t, Args &&... args)
- : m_item (std::forward<Args> (args)...),
- m_instantiated (true)
- {}
- ~optional ()
- { this->reset (); }
- /* Copy and move constructors. */
- optional (const optional &other)
- {
- if (other.m_instantiated)
- this->emplace (other.get ());
- }
- optional (optional &&other)
- noexcept(std::is_nothrow_move_constructible<T> ())
- {
- if (other.m_instantiated)
- this->emplace (std::move (other.get ()));
- }
- constexpr optional (const T &other)
- : m_item (other),
- m_instantiated (true)
- {}
- constexpr optional (T &&other)
- noexcept (std::is_nothrow_move_constructible<T> ())
- : m_item (std::move (other)),
- m_instantiated (true)
- {}
- /* Assignment operators. */
- optional &
- operator= (const optional &other)
- {
- if (m_instantiated && other.m_instantiated)
- this->get () = other.get ();
- else
- {
- if (other.m_instantiated)
- this->emplace (other.get ());
- else
- this->reset ();
- }
- return *this;
- }
- optional &
- operator= (optional &&other)
- noexcept (And<std::is_nothrow_move_constructible<T>,
- std::is_nothrow_move_assignable<T>> ())
- {
- if (m_instantiated && other.m_instantiated)
- this->get () = std::move (other.get ());
- else
- {
- if (other.m_instantiated)
- this->emplace (std::move (other.get ()));
- else
- this->reset ();
- }
- return *this;
- }
- optional &
- operator= (const T &other)
- {
- if (m_instantiated)
- this->get () = other;
- else
- this->emplace (other);
- return *this;
- }
- optional &
- operator= (T &&other)
- noexcept (And<std::is_nothrow_move_constructible<T>,
- std::is_nothrow_move_assignable<T>> ())
- {
- if (m_instantiated)
- this->get () = std::move (other);
- else
- this->emplace (std::move (other));
- return *this;
- }
- template<typename... Args>
- T &emplace (Args &&... args)
- {
- this->reset ();
- new (&m_item) T (std::forward<Args>(args)...);
- m_instantiated = true;
- return this->get ();
- }
- /* Observers. */
- constexpr const T *operator-> () const
- { return std::addressof (this->get ()); }
- T *operator-> ()
- { return std::addressof (this->get ()); }
- constexpr const T &operator* () const &
- { return this->get (); }
- T &operator* () &
- { return this->get (); }
- T &&operator* () &&
- { return std::move (this->get ()); }
- constexpr const T &&operator* () const &&
- { return std::move (this->get ()); }
- constexpr explicit operator bool () const noexcept
- { return m_instantiated; }
- constexpr bool has_value () const noexcept
- { return m_instantiated; }
- /* 'reset' is a 'safe' operation with no precondition. */
- void reset () noexcept
- {
- if (m_instantiated)
- this->destroy ();
- }
- private:
- /* Destroy the object. */
- void destroy ()
- {
- gdb_assert (m_instantiated);
- m_instantiated = false;
- m_item.~T ();
- }
- /* The get operations have m_instantiated as a precondition. */
- T &get () noexcept
- {
- #if defined(_GLIBCXX_DEBUG)
- gdb_assert (this->has_value ());
- #endif
- return m_item;
- }
- constexpr const T &get () const noexcept
- {
- #if defined(_GLIBCXX_DEBUG)
- gdb_assert (this->has_value ());
- #endif
- return m_item;
- }
- /* The object. */
- union
- {
- struct { } m_dummy;
- T m_item;
- volatile char dont_use; /* Silences -Wmaybe-uninitialized warning, see
- PR gcc/80635. */
- };
- /* True if the object was ever emplaced. */
- bool m_instantiated = false;
- };
- }
- #endif /* COMMON_GDB_OPTIONAL_H */
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