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

// Copyright (C) 1994-2021 Free Software Foundation, Inc.
//
// This file is part of GCC.
//
// GCC is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 3, or (at your option)
// any later version.

// GCC is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

#include "tinfo.h"

namespace __cxxabiv1 {


// this is the external interface to the dynamic cast machinery
/* sub: source address to be adjusted; nonnull, and since the
 *      source object is polymorphic, *(void**)sub is a virtual pointer.
 * src: static type of the source object.
 * dst: destination type (the "T" in "dynamic_cast<T>(v)").
 * src2dst_offset: a static hint about the location of the
 *    source subobject with respect to the complete object;
 *    special negative values are:
 *       -1: no hint
 *       -2: src is not a public base of dst
 *       -3: src is a multiple public base type but never a
 *           virtual base type
 *    otherwise, the src type is a unique public nonvirtual
 *    base type of dst at offset src2dst_offset from the
 *    origin of dst.  */
extern "C" void *
__dynamic_cast (const void *src_ptr,    // object started from
                const __class_type_info *src_type, // type of the starting object
                const __class_type_info *dst_type, // desired target type
                ptrdiff_t src2dst) // how src and dst are related
  {
  if (__builtin_expect(!src_ptr, 0))
    return NULL; // Handle precondition violations gracefully.

  const void *vtable = *static_cast <const void *const *> (src_ptr);
  const vtable_prefix *prefix =
    (adjust_pointer <vtable_prefix>
     (vtable,  -ptrdiff_t (offsetof (vtable_prefix, origin))));
  const void *whole_ptr =
      adjust_pointer <void> (src_ptr, prefix->whole_object);
  const __class_type_info *whole_type = prefix->whole_type;
  __class_type_info::__dyncast_result result;

  // If the whole object vptr doesn't refer to the whole object type, we're
  // in the middle of constructing a primary base, and src is a separate
  // base.  This has undefined behavior and we can't find anything outside
  // of the base we're actually constructing, so fail now rather than
  // segfault later trying to use a vbase offset that doesn't exist.
  const void *whole_vtable = *static_cast <const void *const *> (whole_ptr);
  const vtable_prefix *whole_prefix =
    (adjust_pointer <vtable_prefix>
     (whole_vtable, -ptrdiff_t (offsetof (vtable_prefix, origin))));
  if (whole_prefix->whole_type != whole_type)
    return NULL;

  whole_type->__do_dyncast (src2dst, __class_type_info::__contained_public,
                            dst_type, whole_ptr, src_type, src_ptr, result);
  if (!result.dst_ptr)
    return NULL;
  if (contained_public_p (result.dst2src))
    // Src is known to be a public base of dst.
    return const_cast <void *> (result.dst_ptr);
  if (contained_public_p (__class_type_info::__sub_kind
			  (result.whole2src & result.whole2dst)))
    // Both src and dst are known to be public bases of whole. Found a valid
    // cross cast.
    return const_cast <void *> (result.dst_ptr);
  if (contained_nonvirtual_p (result.whole2src))
    // Src is known to be a non-public nonvirtual base of whole, and not a
    // base of dst. Found an invalid cross cast, which cannot also be a down
    // cast
    return NULL;
  if (result.dst2src == __class_type_info::__unknown)
    result.dst2src = dst_type->__find_public_src (src2dst, result.dst_ptr,
                                                  src_type, src_ptr);
  if (contained_public_p (result.dst2src))
    // Found a valid down cast
    return const_cast <void *> (result.dst_ptr);
  // Must be an invalid down cast, or the cross cast wasn't bettered
  return NULL;
}

}