dyncast.cc revision 1.1.1.12 
1// Copyright (C) 1994-2022 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
26namespace __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.  */
44extern "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  whole_type->__do_dyncast (src2dst, __class_type_info::__contained_public,
75                            dst_type, whole_ptr, src_type, src_ptr, result);
76  if (!result.dst_ptr)
77    return NULL;
78  if (contained_public_p (result.dst2src))
79    // Src is known to be a public base of dst.
80    return const_cast <void *> (result.dst_ptr);
81  if (contained_public_p (__class_type_info::__sub_kind
82			  (result.whole2src & result.whole2dst)))
83    // Both src and dst are known to be public bases of whole. Found a valid
84    // cross cast.
85    return const_cast <void *> (result.dst_ptr);
86  if (contained_nonvirtual_p (result.whole2src))
87    // Src is known to be a non-public nonvirtual base of whole, and not a
88    // base of dst. Found an invalid cross cast, which cannot also be a down
89    // cast
90    return NULL;
91  if (result.dst2src == __class_type_info::__unknown)
92    result.dst2src = dst_type->__find_public_src (src2dst, result.dst_ptr,
93                                                  src_type, src_ptr);
94  if (contained_public_p (result.dst2src))
95    // Found a valid down cast
96    return const_cast <void *> (result.dst_ptr);
97  // Must be an invalid down cast, or the cross cast wasn't bettered
98  return NULL;
99}
100
101}
102