1 /* Dynamic testing for abstract is-a relationships.
2 Copyright (C) 2012-2023 Free Software Foundation, Inc.
3 Contributed by Lawrence Crowl.
4
5 This file is part of GCC.
6
7 GCC is free software; you can redistribute it and/or modify it under
8 the terms of the GNU General Public License as published by the Free
9 Software Foundation; either version 3, or (at your option) any later
10 version.
11
12 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
13 WARRANTY; without even the implied warranty of MERCHANTABILITY or
14 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
15 for more details.
16
17 You should have received a copy of the GNU General Public License
18 along with GCC; see the file COPYING3. If not see
19 <http://www.gnu.org/licenses/>. */
20
21
22 /* This header generic type query and conversion functions.
23
24
25 USING THE GENERIC TYPE FACILITY
26
27
28 The user functions are:
29
30 bool is_a <TYPE> (pointer)
31
32 Tests whether the pointer actually points to a more derived TYPE.
33
34 Suppose you have a symtab_node *ptr, AKA symtab_node *ptr. You can test
35 whether it points to a 'derived' cgraph_node as follows.
36
37 if (is_a <cgraph_node *> (ptr))
38 ....
39
40
41 TYPE as_a <TYPE> (pointer)
42
43 Converts pointer to a TYPE.
44
45 You can just assume that it is such a node.
46
47 do_something_with (as_a <cgraph_node *> *ptr);
48
49 TYPE safe_as_a <TYPE> (pointer)
50
51 Like as_a <TYPE> (pointer), but where pointer could be NULL. This
52 adds a check against NULL where the regular is_a_helper hook for TYPE
53 assumes non-NULL.
54
55 do_something_with (safe_as_a <cgraph_node *> *ptr);
56
57 TYPE dyn_cast <TYPE> (pointer)
58
59 Converts pointer to TYPE if and only if "is_a <TYPE> pointer". Otherwise,
60 returns NULL. This function is essentially a checked down cast.
61
62 This functions reduce compile time and increase type safety when treating a
63 generic item as a more specific item.
64
65 You can test and obtain a pointer to the 'derived' type in one indivisible
66 operation.
67
68 if (cgraph_node *cptr = dyn_cast <cgraph_node *> (ptr))
69 ....
70
71 As an example, the code change is from
72
73 if (symtab_function_p (node))
74 {
75 struct cgraph_node *cnode = cgraph (node);
76 ....
77 }
78
79 to
80
81 if (cgraph_node *cnode = dyn_cast <cgraph_node *> (node))
82 {
83 ....
84 }
85
86 The necessary conditional test defines a variable that holds a known good
87 pointer to the specific item and avoids subsequent conversion calls and
88 the assertion checks that may come with them.
89
90 When, the property test is embedded within a larger condition, the
91 variable declaration gets pulled out of the condition. (This approach
92 leaves some room for using the variable inappropriately.)
93
94 if (symtab_variable_p (node) && varpool (node)->finalized)
95 varpool_analyze_node (varpool (node));
96
97 becomes
98
99 varpool_node *vnode = dyn_cast <varpool_node *> (node);
100 if (vnode && vnode->finalized)
101 varpool_analyze_node (vnode);
102
103 Note that we have converted two sets of assertions in the calls to varpool
104 into safe and efficient use of a variable.
105
106 TYPE safe_dyn_cast <TYPE> (pointer)
107
108 Like dyn_cast <TYPE> (pointer), except that it accepts null pointers
109 and returns null results for them.
110
111
112 If you use these functions and get a 'inline function not defined' or a
113 'missing symbol' error message for 'is_a_helper<....>::test', it means that
114 the connection between the types has not been made. See below.
115
116
117 EXTENDING THE GENERIC TYPE FACILITY
118
119 Method 1
120 --------
121
122 If DERIVED is derived from BASE, and if BASE contains enough information
123 to determine whether an object is actually an instance of DERIVED,
124 then you can make the above routines work for DERIVED by defining
125 a specialization of is_a_helper such as:
126
127 template<>
128 struct is_a_helper<DERIVED *> : static_is_a_helper<DERIVED *>
129 {
130 static inline bool test (const BASE *p) { return ...; }
131 };
132
133 This test function should return true if P is an instanced of DERIVED.
134 This on its own is enough; the comments below for method 2 do not apply.
135
136 Method 2
137 --------
138
139 Alternatively, if two types are connected in ways other than C++
140 inheritance, each connection between them must be made by defining a
141 specialization of the template member function 'test' of the template
142 class 'is_a_helper'. For example,
143
144 template <>
145 template <>
146 inline bool
147 is_a_helper <cgraph_node *>::test (symtab_node *p)
148 {
149 return p->type == SYMTAB_FUNCTION;
150 }
151
152 If a simple reinterpret_cast between the pointer types is incorrect, then you
153 must also specialize the template member function 'cast'. Failure to do so
154 when needed may result in a crash. For example,
155
156 template <>
157 template <>
158 inline bool
159 is_a_helper <cgraph_node *>::cast (symtab_node *p)
160 {
161 return &p->x_function;
162 }
163
164 */
165
166 #ifndef GCC_IS_A_H
167 #define GCC_IS_A_H
168
169 /* A base class that specializations of is_a_helper can use if casting
170 U * to T is simply a reinterpret_cast. */
171
172 template <typename T>
173 struct reinterpret_is_a_helper
174 {
175 template <typename U>
176 static inline T cast (U *p) { return reinterpret_cast <T> (p); }
177 };
178
179 /* A base class that specializations of is_a_helper can use if casting
180 U * to T is simply a static_cast. This is more type-safe than
181 reinterpret_is_a_helper. */
182
183 template <typename T>
184 struct static_is_a_helper
185 {
186 template <typename U>
187 static inline T cast (U *p) { return static_cast <T> (p); }
188 };
189
190 /* A generic type conversion internal helper class. */
191
192 template <typename T>
193 struct is_a_helper : reinterpret_is_a_helper<T>
194 {
195 template <typename U>
196 static inline bool test (U *p);
197 };
198
199 /* Reuse the definition of is_a_helper<T *> to implement
200 is_a_helper<const T *>. */
201
202 template <typename T>
203 struct is_a_helper<const T *>
204 {
205 template <typename U>
206 static inline const T *cast (const U *p)
207 {
208 return is_a_helper<T *>::cast (const_cast <U *> (p));
209 }
210 template <typename U>
211 static inline bool test (const U *p)
212 {
213 return is_a_helper<T *>::test (p);
214 }
215 };
216
217 /* Note that we deliberately do not define the 'test' member template. Not
218 doing so will result in a build-time error for type relationships that have
219 not been defined, rather than a run-time error. See the discussion above
220 for when to define this member. */
221
222 /* The public interface. */
223
224 /* A generic test for a type relationship. See the discussion above for when
225 to use this function. The question answered is "Is type T a derived type of
226 type U?". */
227
228 template <typename T, typename U>
229 inline bool
230 is_a (U *p)
231 {
232 return is_a_helper<T>::test (p);
233 }
234
235 /* A generic conversion from a base type U to a derived type T. See the
236 discussion above for when to use this function. */
237
238 template <typename T, typename U>
239 inline T
240 as_a (U *p)
241 {
242 gcc_checking_assert (is_a <T> (p));
243 return is_a_helper <T>::cast (p);
244 }
245
246 /* Similar to as_a<>, but where the pointer can be NULL, even if
247 is_a_helper<T> doesn't check for NULL. */
248
249 template <typename T, typename U>
250 inline T
251 safe_as_a (U *p)
252 {
253 if (p)
254 {
255 gcc_checking_assert (is_a <T> (p));
256 return is_a_helper <T>::cast (p);
257 }
258 else
259 return NULL;
260 }
261
262 /* A generic checked conversion from a base type U to a derived type T. See
263 the discussion above for when to use this function. */
264
265 template <typename T, typename U>
266 inline T
267 dyn_cast (U *p)
268 {
269 if (is_a <T> (p))
270 return is_a_helper <T>::cast (p);
271 else
272 return static_cast <T> (0);
273 }
274
275 /* Similar to dyn_cast, except that the pointer may be null. */
276
277 template <typename T, typename U>
278 inline T
279 safe_dyn_cast (U *p)
280 {
281 return p ? dyn_cast <T> (p) : 0;
282 }
283
284 #endif /* GCC_IS_A_H */