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function-view.h
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1 /* Copyright (C) 2017-2018 Free Software Foundation, Inc.
2
3 This file is part of GDB.
4
5 This program 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 of the License, or
8 (at your option) any later version.
9
10 This program 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 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>. */
17
18 #ifndef COMMON_FUNCTION_VIEW_H
19 #define COMMON_FUNCTION_VIEW_H
20
21 /* function_view is a polymorphic type-erasing wrapper class that
22 encapsulates a non-owning reference to arbitrary callable objects.
23
24 A way to put it is that function_view is to std::function like
25 std::string_view is to std::string. While std::function stores a
26 type-erased callable object internally, function_view holds a
27 type-erased reference to an external callable object.
28
29 This is meant to be used as callback type of a function that:
30
31 #1 - Takes a callback as parameter.
32
33 #2 - Wants to support arbitrary callable objects as callback type
34 (e.g., stateful function objects, lambda closures, free
35 functions).
36
37 #3 - Does not store the callback anywhere; instead the function
38 just calls the callback directly or forwards it to some
39 other function that calls it.
40
41 #4 - Can't be, or we don't want it to be, a template function
42 with the callable type as template parameter. For example,
43 when the callback is a parameter of a virtual member
44 function, or when putting the function template in a header
45 would expose too much implementation detail.
46
47 Note that the C-style "function pointer" + "void *data" callback
48 parameter idiom fails requirement #2 above. Please don't add new
49 uses of that idiom. I.e., something like this wouldn't work;
50
51 typedef bool (iterate_over_foos_cb) (foo *f, void *user_data),
52 void iterate_over_foos (iterate_over_foos_cb *callback, void *user_data);
53
54 foo *find_foo_by_type (int type)
55 {
56 foo *found = nullptr;
57
58 iterate_over_foos ([&] (foo *f, void *data)
59 {
60 if (foo->type == type)
61 {
62 found = foo;
63 return true; // stop iterating
64 }
65 return false; // continue iterating
66 }, NULL);
67
68 return found;
69 }
70
71 The above wouldn't compile, because lambdas with captures can't be
72 implicitly converted to a function pointer (because a capture means
73 some context data must be passed to the lambda somehow).
74
75 C++11 gave us std::function as type-erased wrapper around arbitrary
76 callables, however, std::function is not an ideal fit for transient
77 callbacks such as the use case above. For this use case, which is
78 quite pervasive, a function_view is a better choice, because while
79 function_view is light and does not require any heap allocation,
80 std::function is a heavy-weight object with value semantics that
81 generally requires a heap allocation on construction/assignment of
82 the target callable. In addition, while it is possible to use
83 std::function in such a way that avoids most of the overhead by
84 making sure to only construct it with callables of types that fit
85 std::function's small object optimization, such as function
86 pointers and std::reference_wrapper callables, that is quite
87 inconvenient in practice, because restricting to free-function
88 callables would imply no state/capture/closure, which we need in
89 most cases, and std::reference_wrapper implies remembering to use
90 std::ref/std::cref where the callable is constructed, with the
91 added inconvenience that std::ref/std::cref have deleted rvalue-ref
92 overloads, meaning you can't use unnamed/temporary lambdas with
93 them.
94
95 Note that because function_view is a non-owning view of a callable,
96 care must be taken to ensure that the callable outlives the
97 function_view that calls it. This is not really a problem for the
98 use case function_view is intended for, such as passing a temporary
99 function object / lambda to a function that accepts a callback,
100 because in those cases, the temporary is guaranteed to be live
101 until the called function returns.
102
103 Calling a function_view with no associated target is undefined,
104 unlike with std::function, which throws std::bad_function_call.
105 This is by design, to avoid the otherwise necessary NULL check in
106 function_view::operator().
107
108 Since function_view objects are small (a pair of pointers), they
109 should generally be passed around by value.
110
111 Usage:
112
113 Given this function that accepts a callback:
114
115 void
116 iterate_over_foos (gdb::function_view<void (foo *)> callback)
117 {
118 for (auto &foo : foos)
119 callback (&foo);
120 }
121
122 you can call it like this, passing a lambda as callback:
123
124 iterate_over_foos ([&] (foo *f)
125 {
126 process_one_foo (f);
127 });
128
129 or like this, passing a function object as callback:
130
131 struct function_object
132 {
133 void operator() (foo *f)
134 {
135 if (s->check ())
136 process_one_foo (f);
137 }
138
139 // some state
140 state *s;
141 };
142
143 state mystate;
144 function_object matcher {&mystate};
145 iterate_over_foos (matcher);
146
147 or like this, passing a function pointer as callback:
148
149 iterate_over_foos (process_one_foo);
150
151 You can find unit tests covering the whole API in
152 unittests/function-view-selftests.c. */
153
154 namespace gdb {
155
156 namespace fv_detail {
157 /* Bits shared by all function_view instantiations that do not depend
158 on the template parameters. */
159
160 /* Storage for the erased callable. This is a union in order to be
161 able to save both a function object (data) pointer or a function
162 pointer without triggering undefined behavior. */
163 union erased_callable
164 {
165 /* For function objects. */
166 void *data;
167
168 /* For function pointers. */
169 void (*fn) ();
170 };
171
172 } /* namespace fv_detail */
173
174 /* Use partial specialization to get access to the callable's
175 signature. */
176 template<class Signature>
177 struct function_view;
178
179 template<typename Res, typename... Args>
180 class function_view<Res (Args...)>
181 {
182 template<typename From, typename To>
183 using CompatibleReturnType
184 = Or<std::is_void<To>,
185 std::is_same<From, To>,
186 std::is_convertible<From, To>>;
187
188 /* True if Func can be called with Args, and either the result is
189 Res, convertible to Res or Res is void. */
190 template<typename Callable,
191 typename Res2 = typename std::result_of<Callable &(Args...)>::type>
192 struct IsCompatibleCallable : CompatibleReturnType<Res2, Res>
193 {};
194
195 /* True if Callable is a function_view. Used to avoid hijacking the
196 copy ctor. */
197 template <typename Callable>
198 struct IsFunctionView
199 : std::is_same<function_view, typename std::decay<Callable>::type>
200 {};
201
202 public:
203
204 /* NULL by default. */
205 constexpr function_view () noexcept
206 : m_erased_callable {},
207 m_invoker {}
208 {}
209
210 /* Default copy/assignment is fine. */
211 function_view (const function_view &) = default;
212 function_view &operator= (const function_view &) = default;
213
214 /* This is the main entry point. Use SFINAE to avoid hijacking the
215 copy constructor and to ensure that the target type is
216 compatible. */
217 template
218 <typename Callable,
219 typename = Requires<Not<IsFunctionView<Callable>>>,
220 typename = Requires<IsCompatibleCallable<Callable>>>
221 function_view (Callable &&callable) noexcept
222 {
223 bind (callable);
224 }
225
226 /* Construct a NULL function_view. */
227 constexpr function_view (std::nullptr_t) noexcept
228 : m_erased_callable {},
229 m_invoker {}
230 {}
231
232 /* Clear a function_view. */
233 function_view &operator= (std::nullptr_t) noexcept
234 {
235 m_invoker = nullptr;
236 return *this;
237 }
238
239 /* Return true if the wrapper has a target, false otherwise. Note
240 we check M_INVOKER instead of M_ERASED_CALLABLE because we don't
241 know which member of the union is active right now. */
242 constexpr explicit operator bool () const noexcept
243 { return m_invoker != nullptr; }
244
245 /* Call the callable. */
246 Res operator () (Args... args) const
247 { return m_invoker (m_erased_callable, std::forward<Args> (args)...); }
248
249 private:
250
251 /* Bind this function_view to a compatible function object
252 reference. */
253 template <typename Callable>
254 void bind (Callable &callable) noexcept
255 {
256 m_erased_callable.data = (void *) std::addressof (callable);
257 m_invoker = [] (fv_detail::erased_callable ecall, Args... args)
258 noexcept (noexcept (callable (std::forward<Args> (args)...))) -> Res
259 {
260 auto &restored_callable = *static_cast<Callable *> (ecall.data);
261 /* The explicit cast to Res avoids a compile error when Res is
262 void and the callable returns non-void. */
263 return (Res) restored_callable (std::forward<Args> (args)...);
264 };
265 }
266
267 /* Bind this function_view to a compatible function pointer.
268
269 Making this a separate function allows avoiding one indirection,
270 by storing the function pointer directly in the storage, instead
271 of a pointer to pointer. erased_callable is then a union in
272 order to avoid storing a function pointer as a data pointer here,
273 which would be undefined. */
274 template<class Res2, typename... Args2>
275 void bind (Res2 (*fn) (Args2...)) noexcept
276 {
277 m_erased_callable.fn = reinterpret_cast<void (*) ()> (fn);
278 m_invoker = [] (fv_detail::erased_callable ecall, Args... args)
279 noexcept (noexcept (fn (std::forward<Args> (args)...))) -> Res
280 {
281 auto restored_fn = reinterpret_cast<Res2 (*) (Args2...)> (ecall.fn);
282 /* The explicit cast to Res avoids a compile error when Res is
283 void and the callable returns non-void. */
284 return (Res) restored_fn (std::forward<Args> (args)...);
285 };
286 }
287
288 /* Storage for the erased callable. */
289 fv_detail::erased_callable m_erased_callable;
290
291 /* The invoker. This is set to a capture-less lambda by one of the
292 'bind' overloads. The lambda restores the right type of the
293 callable (which is passed as first argument), and forwards the
294 args. */
295 Res (*m_invoker) (fv_detail::erased_callable, Args...);
296 };
297
298 /* Allow comparison with NULL. Defer the work to the in-class
299 operator bool implementation. */
300
301 template<typename Res, typename... Args>
302 constexpr inline bool
303 operator== (const function_view<Res (Args...)> &f, std::nullptr_t) noexcept
304 { return !static_cast<bool> (f); }
305
306 template<typename Res, typename... Args>
307 constexpr inline bool
308 operator== (std::nullptr_t, const function_view<Res (Args...)> &f) noexcept
309 { return !static_cast<bool> (f); }
310
311 template<typename Res, typename... Args>
312 constexpr inline bool
313 operator!= (const function_view<Res (Args...)> &f, std::nullptr_t) noexcept
314 { return static_cast<bool> (f); }
315
316 template<typename Res, typename... Args>
317 constexpr inline bool
318 operator!= (std::nullptr_t, const function_view<Res (Args...)> &f) noexcept
319 { return static_cast<bool> (f); }
320
321 } /* namespace gdb */
322
323 #endif
gdb::fv_detail::erased_callable::data
void * data
Definition: function-view.h:166
gdb::fv_detail::erased_callable
Definition: function-view.h:163
gdb::operator==
constexpr bool operator==(const function_view< Res(Args...)> &f, std::nullptr_t) noexcept
Definition: function-view.h:303
gdb::fv_detail::erased_callable::fn
void(* fn)()
Definition: function-view.h:169
gdb::operator!=
constexpr bool operator!=(const function_view< Res(Args...)> &f, std::nullptr_t) noexcept
Definition: function-view.h:313
gdb::function_view< Res(Args...)>::bind
void bind(Res2(*fn)(Args2...)) noexcept
Definition: function-view.h:275
gdb::Or
Definition: traits.h:55
type
Definition: gdbtypes.h:749
gdb::function_view< Res(Args...)>::function_view
function_view(Callable &&callable) noexcept
Definition: function-view.h:221
gdb
Definition: array-view.h:66
gdb::function_view< Res(Args...)>::function_view
constexpr function_view() noexcept
Definition: function-view.h:205
gdb::function_view
Definition: function-view.h:177
gdb::function_view< Res(Args...)>::function_view
constexpr function_view(std::nullptr_t) noexcept
Definition: function-view.h:227
gdb::function_view< Res(Args...)>::bind
void bind(Callable &callable) noexcept
Definition: function-view.h:254
gdb::function_view< Res(Args...)>::m_erased_callable
fv_detail::erased_callable m_erased_callable
Definition: function-view.h:289