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

   1 // Copyright (C) 1994-2023 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   if (__builtin_expect(!src_ptr, 0))
  51     return NULL; // Handle precondition violations gracefully.
  52
  53   const void *vtable = *static_cast <const void *const *> (src_ptr);
  54   const vtable_prefix *prefix =
  55     (adjust_pointer <vtable_prefix>
  56      (vtable,  -ptrdiff_t (offsetof (vtable_prefix, origin))));
  57   const void *whole_ptr =
  58       adjust_pointer <void> (src_ptr, prefix->whole_object);
  59   const __class_type_info *whole_type = prefix->whole_type;
  60   __class_type_info::__dyncast_result result;
  61
  62   // If the whole object vptr doesn't refer to the whole object type, we're
  63   // in the middle of constructing a primary base, and src is a separate
  64   // base.  This has undefined behavior and we can't find anything outside
  65   // of the base we're actually constructing, so fail now rather than
  66   // segfault later trying to use a vbase offset that doesn't exist.
  67   const void *whole_vtable = *static_cast <const void *const *> (whole_ptr);
  68   const vtable_prefix *whole_prefix =
  69     (adjust_pointer <vtable_prefix>
  70      (whole_vtable, -ptrdiff_t (offsetof (vtable_prefix, origin))));
  71   if (whole_prefix->whole_type != whole_type)
  72     return NULL;
  73
  74   // Avoid virtual function call in the simple success case.
  75   if (src2dst >= 0
  76       && src2dst == -prefix->whole_object
  77       && *whole_type == *dst_type)
  78     return const_cast <void *> (whole_ptr);
  79
  80   whole_type->__do_dyncast (src2dst, __class_type_info::__contained_public,
  81                             dst_type, whole_ptr, src_type, src_ptr, result);
  82   if (!result.dst_ptr)
  83     return NULL;
  84   if (contained_public_p (result.dst2src))
  85     // Src is known to be a public base of dst.
  86     return const_cast <void *> (result.dst_ptr);
  87   if (contained_public_p (__class_type_info::__sub_kind
  88                           (result.whole2src & result.whole2dst)))
  89     // Both src and dst are known to be public bases of whole. Found a valid
  90     // cross cast.
  91     return const_cast <void *> (result.dst_ptr);
  92   if (contained_nonvirtual_p (result.whole2src))
  93     // Src is known to be a non-public nonvirtual base of whole, and not a
  94     // base of dst. Found an invalid cross cast, which cannot also be a down
  95     // cast
  96     return NULL;
  97   if (result.dst2src == __class_type_info::__unknown)
  98     result.dst2src = dst_type->__find_public_src (src2dst, result.dst_ptr,
  99                                                   src_type, src_ptr);
 100   if (contained_public_p (result.dst2src))
 101     // Found a valid down cast
 102     return const_cast <void *> (result.dst_ptr);
 103   // Must be an invalid down cast, or the cross cast wasn't bettered
 104   return NULL;
 105 }
 106
 107 }