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/* 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:
optional ()
: 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 */
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