220 lines
5.1 KiB
C++
220 lines
5.1 KiB
C++
/* An optional object.
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Copyright (C) 2017-2019 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef COMMON_GDB_OPTIONAL_H
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#define COMMON_GDB_OPTIONAL_H
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#include "common/traits.h"
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namespace gdb
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{
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struct in_place_t
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{
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explicit in_place_t () = default;
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};
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constexpr gdb::in_place_t in_place {};
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/* This class attempts to be a compatible subset of std::optional,
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which is slated to be available in C++17. This class optionally
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holds an object of some type -- by default it is constructed not
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holding an object, but later the object can be "emplaced". This is
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similar to using std::unique_ptr, but in-object allocation is
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guaranteed.
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Unlike std::optional, we currently only support copy/move
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construction/assignment of an optional<T> from either exactly
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optional<T> or T. I.e., we don't support copy/move
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construction/assignment from optional<U> or U, when U is a type
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convertible to T. Making that work depending on the definitions of
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T and U is somewhat complicated, and currently the users of this
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class don't need it. */
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template<typename T>
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class optional
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{
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public:
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constexpr optional ()
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: m_dummy ()
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{}
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template<typename... Args>
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constexpr optional (in_place_t, Args &&... args)
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: m_item (std::forward<Args> (args)...),
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m_instantiated (true)
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{}
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~optional ()
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{ this->reset (); }
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/* Copy and move constructors. */
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optional (const optional &other)
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{
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if (other.m_instantiated)
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this->emplace (other.get ());
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}
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optional (optional &&other)
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noexcept(std::is_nothrow_move_constructible<T> ())
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{
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if (other.m_instantiated)
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this->emplace (std::move (other.get ()));
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}
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constexpr optional (const T &other)
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: m_item (other),
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m_instantiated (true)
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{}
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constexpr optional (T &&other)
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noexcept (std::is_nothrow_move_constructible<T> ())
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: m_item (std::move (other)),
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m_instantiated (true)
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{}
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/* Assignment operators. */
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optional &
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operator= (const optional &other)
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{
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if (m_instantiated && other.m_instantiated)
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this->get () = other.get ();
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else
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{
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if (other.m_instantiated)
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this->emplace (other.get ());
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else
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this->reset ();
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}
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return *this;
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}
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optional &
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operator= (optional &&other)
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noexcept (And<std::is_nothrow_move_constructible<T>,
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std::is_nothrow_move_assignable<T>> ())
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{
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if (m_instantiated && other.m_instantiated)
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this->get () = std::move (other.get ());
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else
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{
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if (other.m_instantiated)
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this->emplace (std::move (other.get ()));
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else
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this->reset ();
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}
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return *this;
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}
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optional &
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operator= (const T &other)
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{
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if (m_instantiated)
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this->get () = other;
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else
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this->emplace (other);
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return *this;
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}
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optional &
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operator= (T &&other)
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noexcept (And<std::is_nothrow_move_constructible<T>,
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std::is_nothrow_move_assignable<T>> ())
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{
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if (m_instantiated)
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this->get () = std::move (other);
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else
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this->emplace (std::move (other));
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return *this;
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}
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template<typename... Args>
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T &emplace (Args &&... args)
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{
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this->reset ();
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new (&m_item) T (std::forward<Args>(args)...);
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m_instantiated = true;
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return this->get ();
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}
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/* Observers. */
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constexpr const T *operator-> () const
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{ return std::addressof (this->get ()); }
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T *operator-> ()
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{ return std::addressof (this->get ()); }
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constexpr const T &operator* () const &
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{ return this->get (); }
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T &operator* () &
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{ return this->get (); }
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T &&operator* () &&
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{ return std::move (this->get ()); }
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constexpr const T &&operator* () const &&
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{ return std::move (this->get ()); }
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constexpr explicit operator bool () const noexcept
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{ return m_instantiated; }
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constexpr bool has_value () const noexcept
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{ return m_instantiated; }
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/* 'reset' is a 'safe' operation with no precondition. */
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void reset () noexcept
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{
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if (m_instantiated)
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this->destroy ();
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}
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private:
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/* Destroy the object. */
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void destroy ()
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{
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gdb_assert (m_instantiated);
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m_instantiated = false;
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m_item.~T ();
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}
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/* The get operations have m_instantiated as a precondition. */
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T &get () noexcept { return m_item; }
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constexpr const T &get () const noexcept { return m_item; }
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/* The object. */
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union
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{
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struct { } m_dummy;
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T m_item;
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};
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/* True if the object was ever emplaced. */
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bool m_instantiated = false;
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};
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}
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#endif /* COMMON_GDB_OPTIONAL_H */
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