libstdc++-v3/libsupc++/nested_exception.h

   1 // Nested Exception support header (nested_exception class) for -*- C++ -*-
   2
   3 // Copyright (C) 2009-2023 Free Software Foundation, Inc.
   4 //
   5 // This file is part of the GNU ISO C++ Library.  This library is free
   6 // software; you can redistribute it and/or modify it under the
   7 // terms of the GNU General Public License as published by the
   8 // Free Software Foundation; either version 3, or (at your option)
   9 // any later version.
  10
  11 // This library is distributed in the hope that it will be useful,
  12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 // GNU General Public License for more details.
  15
  16 // Under Section 7 of GPL version 3, you are granted additional
  17 // permissions described in the GCC Runtime Library Exception, version
  18 // 3.1, as published by the Free Software Foundation.
  19
  20 // You should have received a copy of the GNU General Public License and
  21 // a copy of the GCC Runtime Library Exception along with this program;
  22 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  23 // <http://www.gnu.org/licenses/>.
  24
  25 /** @file bits/nested_exception.h
  26  *  This is an internal header file, included by other library headers.
  27  *  Do not attempt to use it directly. @headername{exception}
  28  */
  29
  30 #ifndef _GLIBCXX_NESTED_EXCEPTION_H
  31 #define _GLIBCXX_NESTED_EXCEPTION_H 1
  32
  33 #if __cplusplus < 201103L
  34 # include <bits/c++0x_warning.h>
  35 #else
  36
  37 #include <bits/move.h>
  38 #include <bits/exception_ptr.h>
  39
  40 extern "C++" {
  41
  42 namespace std _GLIBCXX_VISIBILITY(default)
  43 {
  44   /**
  45    * @addtogroup exceptions
  46    * @{
  47    */
  48
  49   /** Mixin class that stores the current exception.
  50    *
  51    * This type can be used via `std::throw_with_nested` to store
  52    * the current exception nested within another exception.
  53    *
  54    * @headerfile exception
  55    * @since C++11
  56    * @see std::throw_with_nested
  57    * @ingroup exceptions
  58    */
  59   class nested_exception
  60   {
  61     exception_ptr _M_ptr;
  62
  63   public:
  64     /// The default constructor stores the current exception (if any).
  65     nested_exception() noexcept : _M_ptr(current_exception()) { }
  66
  67     nested_exception(const nested_exception&) noexcept = default;
  68
  69     nested_exception& operator=(const nested_exception&) noexcept = default;
  70
  71     virtual ~nested_exception() noexcept;
  72
  73     /// Rethrow the stored exception, or terminate if none was stored.
  74     [[noreturn]]
  75     void
  76     rethrow_nested() const
  77     {
  78       if (_M_ptr)
  79         rethrow_exception(_M_ptr);
  80       std::terminate();
  81     }
  82
  83     /// Access the stored exception.
  84     exception_ptr
  85     nested_ptr() const noexcept
  86     { return _M_ptr; }
  87   };
  88
  89   /// @cond undocumented
  90
  91   template<typename _Except>
  92     struct _Nested_exception : public _Except, public nested_exception
  93     {
  94       explicit _Nested_exception(const _Except& __ex)
  95       : _Except(__ex)
  96       { }
  97
  98       explicit _Nested_exception(_Except&& __ex)
  99       : _Except(static_cast<_Except&&>(__ex))
 100       { }
 101     };
 102
 103 #if __cplusplus < 201703L || ! defined __cpp_if_constexpr
 104   // [except.nested]/8
 105   // Throw an exception of unspecified type that is publicly derived from
 106   // both remove_reference_t<_Tp> and nested_exception.
 107   template<typename _Tp>
 108     [[noreturn]]
 109     inline void
 110     __throw_with_nested_impl(_Tp&& __t, true_type)
 111     {
 112       throw _Nested_exception<__remove_cvref_t<_Tp>>{std::forward<_Tp>(__t)};
 113     }
 114
 115   template<typename _Tp>
 116     [[noreturn]]
 117     inline void
 118     __throw_with_nested_impl(_Tp&& __t, false_type)
 119     { throw std::forward<_Tp>(__t); }
 120 #endif
 121
 122   /// @endcond
 123
 124   /** Throw an exception that also stores the currently active exception.
 125    *
 126    * If `_Tp` is derived from `std::nested_exception` or is not usable
 127    * as a base-class, throws a copy of `__t`.
 128    * Otherwise, throws an object of an implementation-defined type derived
 129    * from both `_Tp` and `std::nested_exception`, containing a copy of `__t`
 130    * and the result of `std::current_exception()`.
 131    *
 132    * In other words, throws the argument as a new exception that contains
 133    * the currently active exception nested within it. This is intended for
 134    * use in a catch handler to replace the caught exception with a different
 135    * type, while still preserving the original exception. When the new
 136    * exception is caught, the nested exception can be rethrown by using
 137    * `std::rethrow_if_nested`.
 138    *
 139    * This can be used at API boundaries, for example to catch a library's
 140    * internal exception type and rethrow it nested with a `std::runtime_error`,
 141    * or vice versa.
 142    *
 143    * @since C++11
 144    */
 145   template<typename _Tp>
 146     [[noreturn]]
 147     inline void
 148     throw_with_nested(_Tp&& __t)
 149     {
 150       using _Up = typename decay<_Tp>::type;
 151       using _CopyConstructible
 152         = __and_<is_copy_constructible<_Up>, is_move_constructible<_Up>>;
 153       static_assert(_CopyConstructible::value,
 154           "throw_with_nested argument must be CopyConstructible");
 155
 156 #if __cplusplus >= 201703L && __cpp_if_constexpr
 157       if constexpr (is_class_v<_Up>)
 158         if constexpr (!is_final_v<_Up>)
 159           if constexpr (!is_base_of_v<nested_exception, _Up>)
 160             throw _Nested_exception<_Up>{std::forward<_Tp>(__t)};
 161       throw std::forward<_Tp>(__t);
 162 #else
 163       using __nest = __and_<is_class<_Up>, __bool_constant<!__is_final(_Up)>,
 164                             __not_<is_base_of<nested_exception, _Up>>>;
 165       std::__throw_with_nested_impl(std::forward<_Tp>(__t), __nest{});
 166 #endif
 167     }
 168
 169 #if __cplusplus < 201703L || ! defined __cpp_if_constexpr
 170   /// @cond undocumented
 171
 172   // Attempt dynamic_cast to nested_exception and call rethrow_nested().
 173   template<typename _Ex>
 174     inline void
 175     __rethrow_if_nested_impl(const _Ex* __ptr, true_type)
 176     {
 177       if (auto __ne_ptr = dynamic_cast<const nested_exception*>(__ptr))
 178         __ne_ptr->rethrow_nested();
 179     }
 180
 181   // Otherwise, no effects.
 182   inline void
 183   __rethrow_if_nested_impl(const void*, false_type)
 184   { }
 185
 186   /// @endcond
 187 #endif
 188
 189   /** Rethrow a nested exception
 190    *
 191    * If `__ex` contains a `std::nested_exception` object, call its
 192    * `rethrow_nested()` member to rethrow the stored exception.
 193    *
 194    * After catching an exception thrown by a call to `std::throw_with_nested`
 195    * this function can be used to rethrow the exception that was active when
 196    * `std::throw_with_nested` was called.
 197    *
 198    * @since C++11
 199    */
 200   // _GLIBCXX_RESOLVE_LIB_DEFECTS
 201   // 2484. rethrow_if_nested() is doubly unimplementable
 202   // 2784. Resolution to LWG 2484 is missing "otherwise, no effects" and [...]
 203   template<typename _Ex>
 204 # if ! __cpp_rtti
 205     [[__gnu__::__always_inline__]]
 206 #endif
 207     inline void
 208     rethrow_if_nested(const _Ex& __ex)
 209     {
 210       const _Ex* __ptr = __builtin_addressof(__ex);
 211 #if __cplusplus < 201703L || ! defined __cpp_if_constexpr
 212 # if __cpp_rtti
 213       using __cast = __and_<is_polymorphic<_Ex>,
 214                             __or_<__not_<is_base_of<nested_exception, _Ex>>,
 215                                   is_convertible<_Ex*, nested_exception*>>>;
 216 # else
 217       using __cast = __and_<is_polymorphic<_Ex>,
 218                             is_base_of<nested_exception, _Ex>,
 219                             is_convertible<_Ex*, nested_exception*>>;
 220 # endif
 221       std::__rethrow_if_nested_impl(__ptr, __cast{});
 222 #else
 223       if constexpr (!is_polymorphic_v<_Ex>)
 224         return;
 225       else if constexpr (is_base_of_v<nested_exception, _Ex>
 226                          && !is_convertible_v<_Ex*, nested_exception*>)
 227         return; // nested_exception base class is inaccessible or ambiguous.
 228 # if ! __cpp_rtti
 229       else if constexpr (!is_base_of_v<nested_exception, _Ex>)
 230         return; // Cannot do polymorphic casts without RTTI.
 231 # endif
 232       else if (auto __ne_ptr = dynamic_cast<const nested_exception*>(__ptr))
 233         __ne_ptr->rethrow_nested();
 234 #endif
 235     }
 236
 237   /// @} group exceptions
 238 } // namespace std
 239
 240 } // extern "C++"
 241
 242 #endif // C++11
 243 #endif // _GLIBCXX_NESTED_EXCEPTION_H