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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
#include "common/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 { 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 = false;
};
}
#endif /* GDB_OPTIONAL_H */
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