libstdc++-v3/libsupc++/dyncast.cc

   1 // Copyright (C) 1994-2018 Free Software Foundation, Inc.
   2 //
   3 // This file is part of GCC.
   4 //
   5 // GCC 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, or (at your option)
   8 // any later version.
   9
  10 // GCC 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 // Under Section 7 of GPL version 3, you are granted additional
  16 // permissions described in the GCC Runtime Library Exception, version
  17 // 3.1, as published by the Free Software Foundation.
  18
  19 // You should have received a copy of the GNU General Public License and
  20 // a copy of the GCC Runtime Library Exception along with this program;
  21 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  22 // <http://www.gnu.org/licenses/>.
  23
  24 #include "tinfo.h"
  25
  26 namespace __cxxabiv1 {
  27
  28
  29 // this is the external interface to the dynamic cast machinery
  30 /* sub: source address to be adjusted; nonnull, and since the
  31  *      source object is polymorphic, *(void**)sub is a virtual pointer.
  32  * src: static type of the source object.
  33  * dst: destination type (the "T" in "dynamic_cast<T>(v)").
  34  * src2dst_offset: a static hint about the location of the
  35  *    source subobject with respect to the complete object;
  36  *    special negative values are:
  37  *       -1: no hint
  38  *       -2: src is not a public base of dst
  39  *       -3: src is a multiple public base type but never a
  40  *           virtual base type
  41  *    otherwise, the src type is a unique public nonvirtual
  42  *    base type of dst at offset src2dst_offset from the
  43  *    origin of dst.  */
  44 extern "C" void *
  45 __dynamic_cast (const void *src_ptr,    // object started from
  46                 const __class_type_info *src_type, // type of the starting object
  47                 const __class_type_info *dst_type, // desired target type
  48                 ptrdiff_t src2dst) // how src and dst are related
  49   {
  50   const void *vtable = *static_cast <const void *const *> (src_ptr);
  51   const vtable_prefix *prefix =
  52       adjust_pointer <vtable_prefix> (vtable,
  53                                       -offsetof (vtable_prefix, origin));
  54   const void *whole_ptr =
  55       adjust_pointer <void> (src_ptr, prefix->whole_object);
  56   const __class_type_info *whole_type = prefix->whole_type;
  57   __class_type_info::__dyncast_result result;
  58
  59   // If the whole object vptr doesn't refer to the whole object type, we're
  60   // in the middle of constructing a primary base, and src is a separate
  61   // base.  This has undefined behavior and we can't find anything outside
  62   // of the base we're actually constructing, so fail now rather than
  63   // segfault later trying to use a vbase offset that doesn't exist.
  64   const void *whole_vtable = *static_cast <const void *const *> (whole_ptr);
  65   const vtable_prefix *whole_prefix =
  66     adjust_pointer <vtable_prefix> (whole_vtable,
  67                                     -offsetof (vtable_prefix, origin));
  68   if (whole_prefix->whole_type != whole_type)
  69     return NULL;
  70
  71   whole_type->__do_dyncast (src2dst, __class_type_info::__contained_public,
  72                             dst_type, whole_ptr, src_type, src_ptr, result);
  73   if (!result.dst_ptr)
  74     return NULL;
  75   if (contained_public_p (result.dst2src))
  76     // Src is known to be a public base of dst.
  77     return const_cast <void *> (result.dst_ptr);
  78   if (contained_public_p (__class_type_info::__sub_kind (result.whole2src & result.whole2dst)))
  79     // Both src and dst are known to be public bases of whole. Found a valid
  80     // cross cast.
  81     return const_cast <void *> (result.dst_ptr);
  82   if (contained_nonvirtual_p (result.whole2src))
  83     // Src is known to be a non-public nonvirtual base of whole, and not a
  84     // base of dst. Found an invalid cross cast, which cannot also be a down
  85     // cast
  86     return NULL;
  87   if (result.dst2src == __class_type_info::__unknown)
  88     result.dst2src = dst_type->__find_public_src (src2dst, result.dst_ptr,
  89                                                   src_type, src_ptr);
  90   if (contained_public_p (result.dst2src))
  91     // Found a valid down cast
  92     return const_cast <void *> (result.dst_ptr);
  93   // Must be an invalid down cast, or the cross cast wasn't bettered
  94   return NULL;
  95 }
  96
  97 }