| /* An optional object. |
| |
| Copyright (C) 2017 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 GDB_OPTIONAL_H |
| #define GDB_OPTIONAL_H |
| |
| namespace gdb |
| { |
| |
| /* 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. */ |
| template<typename T> |
| class optional |
| { |
| public: |
| |
| constexpr optional () |
| : m_dummy (), |
| m_instantiated (false) |
| { |
| } |
| |
| ~optional () |
| { |
| if (m_instantiated) |
| destroy (); |
| } |
| |
| /* These aren't deleted in std::optional, but it was simpler to |
| delete them here, because currently the users of this class don't |
| need them, and making them depend on the definition of T is |
| somewhat complicated. */ |
| optional (const optional &other) = delete; |
| optional<T> &operator= (const optional &other) = delete; |
| |
| template<typename... Args> |
| void emplace (Args &&... args) |
| { |
| if (m_instantiated) |
| destroy (); |
| new (&m_item) T (std::forward<Args>(args)...); |
| m_instantiated = true; |
| } |
| |
| /* 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; } |
| |
| 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 { return m_item; } |
| constexpr const T &get () const noexcept { return m_item; } |
| |
| /* The object. */ |
| union |
| { |
| struct { } m_dummy; |
| T m_item; |
| }; |
| |
| /* True if the object was ever emplaced. */ |
| bool m_instantiated; |
| }; |
| |
| } |
| |
| #endif /* GDB_OPTIONAL_H */ |