gdb/common/gdb_optional.h

   1 /* An optional object.
   2
   3    Copyright (C) 2017 Free Software Foundation, Inc.
   4
   5    This file is part of GDB.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 #ifndef GDB_OPTIONAL_H
  21 #define GDB_OPTIONAL_H
  22
  23 namespace gdb
  24 {
  25
  26 /* This class attempts to be a compatible subset of std::optional,
  27    which is slated to be available in C++17.  This class optionally
  28    holds an object of some type -- by default it is constructed not
  29    holding an object, but later the object can be "emplaced".  This is
  30    similar to using std::unique_ptr, but in-object allocation is
  31    guaranteed.  */
  32 template<typename T>
  33 class optional
  34 {
  35 public:
  36
  37   constexpr optional ()
  38     : m_dummy (),
  39       m_instantiated (false)
  40   {
  41   }
  42
  43   ~optional ()
  44   {
  45     if (m_instantiated)
  46       destroy ();
  47   }
  48
  49   /* These aren't deleted in std::optional, but it was simpler to
  50      delete them here, because currently the users of this class don't
  51      need them, and making them depend on the definition of T is
  52      somewhat complicated.  */
  53   optional (const optional &other) = delete;
  54   optional<T> &operator= (const optional &other) = delete;
  55
  56   template<typename... Args>
  57   void emplace (Args &&... args)
  58   {
  59     if (m_instantiated)
  60       destroy ();
  61     new (&m_item) T (std::forward<Args>(args)...);
  62     m_instantiated = true;
  63   }
  64
  65   /* Observers.  */
  66   constexpr const T *operator-> () const
  67   { return std::addressof (this->get ()); }
  68
  69   T *operator-> ()
  70   { return std::addressof (this->get ()); }
  71
  72   constexpr const T &operator* () const &
  73   { return this->get (); }
  74
  75   T &operator* () &
  76   { return this->get (); }
  77
  78   T &&operator* () &&
  79   { return std::move (this->get ()); }
  80
  81   constexpr const T &&operator* () const &&
  82   { return std::move (this->get ()); }
  83
  84   constexpr explicit operator bool () const noexcept
  85   { return m_instantiated; }
  86
  87   constexpr bool has_value () const noexcept
  88   { return m_instantiated; }
  89
  90 private:
  91
  92   /* Destroy the object.  */
  93   void destroy ()
  94   {
  95     gdb_assert (m_instantiated);
  96     m_instantiated = false;
  97     m_item.~T ();
  98   }
  99
 100   /* The get operations have m_instantiated as a precondition.  */
 101   T &get () noexcept { return m_item; }
 102   constexpr const T &get () const noexcept { return m_item; }
 103
 104   /* The object.  */
 105   union
 106   {
 107     struct { } m_dummy;
 108     T m_item;
 109   };
 110
 111   /* True if the object was ever emplaced.  */
 112   bool m_instantiated;
 113 };
 114
 115 }
 116
 117 #endif /* GDB_OPTIONAL_H */