gdbsupport/scope-exit.h

   1 /* Copyright (C) 2019-2020 Free Software Foundation, Inc.
   2
   3    This file is part of GDB.
   4
   5    This program is free software; you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 #ifndef COMMON_SCOPE_EXIT_H
  19 #define COMMON_SCOPE_EXIT_H
  20
  21 #include <functional>
  22 #include <type_traits>
  23 #include "gdbsupport/preprocessor.h"
  24
  25 /* scope_exit is a general-purpose scope guard that calls its exit
  26    function at the end of the current scope.  A scope_exit may be
  27    canceled by calling the "release" method.  The API is modeled on
  28    P0052R5 - Generic Scope Guard and RAII Wrapper for the Standard
  29    Library, which is itself based on Andrej Alexandrescu's
  30    ScopeGuard/SCOPE_EXIT.
  31
  32    There are two forms available:
  33
  34    - The "make_scope_exit" form allows canceling the scope guard.  Use
  35      it like this:
  36
  37      auto cleanup = make_scope_exit ( <function, function object, lambda> );
  38      ...
  39      cleanup.release (); // cancel
  40
  41    - If you don't need to cancel the guard, you can use the SCOPE_EXIT
  42      macro, like this:
  43
  44      SCOPE_EXIT
  45        {
  46          // any code you like here.
  47        }
  48
  49    See also forward_scope_exit.
  50 */
  51
  52 /* CRTP base class for cancelable scope_exit-like classes.  Implements
  53    the common call-custom-function-from-dtor functionality.  Classes
  54    that inherit this implement the on_exit() method, which is called
  55    from scope_exit_base's dtor.  */
  56
  57 template <typename CRTP>
  58 class scope_exit_base
  59 {
  60 public:
  61   scope_exit_base () = default;
  62
  63   ~scope_exit_base ()
  64   {
  65     if (!m_released)
  66       {
  67         auto *self = static_cast<CRTP *> (this);
  68         self->on_exit ();
  69       }
  70   }
  71
  72   /* This is needed for make_scope_exit because copy elision isn't
  73      guaranteed until C++17.  An optimizing compiler will usually skip
  74      calling this, but it must exist.  */
  75   scope_exit_base (const scope_exit_base &other)
  76     : m_released (other.m_released)
  77   {
  78     other.m_released = true;
  79   }
  80
  81   void operator= (const scope_exit_base &) = delete;
  82
  83   /* If this is called, then the wrapped function will not be called
  84      on destruction.  */
  85   void release () noexcept
  86   {
  87     m_released = true;
  88   }
  89
  90 private:
  91
  92   /* True if released.  Mutable because of the copy ctor hack
  93      above.  */
  94   mutable bool m_released = false;
  95 };
  96
  97 /* The scope_exit class.  */
  98
  99 template<typename EF>
 100 class scope_exit : public scope_exit_base<scope_exit<EF>>
 101 {
 102   /* For access to on_exit().  */
 103   friend scope_exit_base<scope_exit<EF>>;
 104
 105 public:
 106
 107   template<typename EFP,
 108            typename = gdb::Requires<std::is_constructible<EF, EFP>>>
 109   scope_exit (EFP &&f)
 110     try : m_exit_function ((!std::is_lvalue_reference<EFP>::value
 111                             && std::is_nothrow_constructible<EF, EFP>::value)
 112                            ? std::move (f)
 113                            : f)
 114   {
 115   }
 116   catch (...)
 117     {
 118       /* "If the initialization of exit_function throws an exception,
 119          calls f()."  */
 120       f ();
 121     }
 122
 123   template<typename EFP,
 124            typename = gdb::Requires<std::is_constructible<EF, EFP>>>
 125   scope_exit (scope_exit &&rhs)
 126     noexcept (std::is_nothrow_move_constructible<EF>::value
 127               || std::is_nothrow_copy_constructible<EF>::value)
 128     : m_exit_function (std::is_nothrow_constructible<EFP>::value
 129                        ? std::move (rhs)
 130                        : rhs)
 131   {
 132     rhs.release ();
 133   }
 134
 135   /* This is needed for make_scope_exit because copy elision isn't
 136      guaranteed until C++17.  An optimizing compiler will usually skip
 137      calling this, but it must exist.  */
 138   scope_exit (const scope_exit &other)
 139     : scope_exit_base<scope_exit<EF>> (other),
 140       m_exit_function (other.m_exit_function)
 141   {
 142   }
 143
 144   void operator= (const scope_exit &) = delete;
 145   void operator= (scope_exit &&) = delete;
 146
 147 private:
 148   void on_exit ()
 149   {
 150     m_exit_function ();
 151   }
 152
 153   /* The function to call on scope exit.  */
 154   EF m_exit_function;
 155 };
 156
 157 template <typename EF>
 158 scope_exit<typename std::decay<EF>::type>
 159 make_scope_exit (EF &&f)
 160 {
 161   return scope_exit<typename std::decay<EF>::type> (std::forward<EF> (f));
 162 }
 163
 164 namespace detail
 165 {
 166
 167 enum class scope_exit_lhs {};
 168
 169 template<typename EF>
 170 scope_exit<typename std::decay<EF>::type>
 171 operator+ (scope_exit_lhs, EF &&rhs)
 172 {
 173   return scope_exit<typename std::decay<EF>::type> (std::forward<EF> (rhs));
 174 }
 175
 176 }
 177
 178 /* Register a block of code to run on scope exit.  Note that the local
 179    context is captured by reference, which means you should be careful
 180    to avoid inadvertently changing a captured local's value before the
 181    scope exit runs.  */
 182
 183 #define SCOPE_EXIT \
 184   auto CONCAT(scope_exit_, __LINE__) = ::detail::scope_exit_lhs () + [&] ()
 185
 186 #endif /* COMMON_SCOPE_EXIT_H */