1 //
2 // Copyright 2017 The Abseil Authors.
3 //
4 // Licensed under the Apache License, Version 2.0 (the "License");
5 // you may not use this file except in compliance with the License.
6 // You may obtain a copy of the License at
7 //
8 // http://www.apache.org/licenses/LICENSE-2.0
9 //
10 // Unless required by applicable law or agreed to in writing, software
11 // distributed under the License is distributed on an "AS IS" BASIS,
12 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 // See the License for the specific language governing permissions and
14 // limitations under the License.
15 //
16 // -----------------------------------------------------------------------------
17 // any.h
18 // -----------------------------------------------------------------------------
19 //
20 // This header file define the `absl::any` type for holding a type-safe value
21 // of any type. The 'absl::any` type is useful for providing a way to hold
22 // something that is, as yet, unspecified. Such unspecified types
23 // traditionally are passed between API boundaries until they are later cast to
24 // their "destination" types. To cast to such a destination type, use
25 // `absl::any_cast()`. Note that when casting an `absl::any`, you must cast it
26 // to an explicit type; implicit conversions will throw.
27 //
28 // Example:
29 //
30 // auto a = absl::any(65);
31 // absl::any_cast<int>(a); // 65
32 // absl::any_cast<char>(a); // throws absl::bad_any_cast
33 // absl::any_cast<std::string>(a); // throws absl::bad_any_cast
34 //
35 // `absl::any` is a C++11 compatible version of the C++17 `std::any` abstraction
36 // and is designed to be a drop-in replacement for code compliant with C++17.
37 //
38 // Traditionally, the behavior of casting to a temporary unspecified type has
39 // been accomplished with the `void *` paradigm, where the pointer was to some
40 // other unspecified type. `absl::any` provides an "owning" version of `void *`
41 // that avoids issues of pointer management.
42 //
43 // Note: just as in the case of `void *`, use of `absl::any` (and its C++17
44 // version `std::any`) is a code smell indicating that your API might not be
45 // constructed correctly. We have seen that most uses of `any` are unwarranted,
46 // and `absl::any`, like `std::any`, is difficult to use properly. Before using
47 // this abstraction, make sure that you should not instead be rewriting your
48 // code to be more specific.
49 //
50 // Abseil expects to release an `absl::variant` type shortly (a C++11 compatible
51 // version of the C++17 `std::variant), which is generally preferred for use
52 // over `absl::any`.
53 #ifndef ABSL_TYPES_ANY_H_
54 #define ABSL_TYPES_ANY_H_
55
56 #include "absl/base/config.h"
57 #include "absl/utility/utility.h"
58
59 #ifdef ABSL_HAVE_STD_ANY
60
61 #include <any>
62
63 namespace absl {
64 using std::any;
65 using std::any_cast;
66 using std::bad_any_cast;
67 using std::make_any;
68 } // namespace absl
69
70 #else // ABSL_HAVE_STD_ANY
71
72 #include <algorithm>
73 #include <cstddef>
74 #include <initializer_list>
75 #include <memory>
76 #include <stdexcept>
77 #include <type_traits>
78 #include <typeinfo>
79 #include <utility>
80
81 #include "absl/base/macros.h"
82 #include "absl/meta/type_traits.h"
83 #include "absl/types/bad_any_cast.h"
84
85 // NOTE: This macro is an implementation detail that is undefined at the bottom
86 // of the file. It is not intended for expansion directly from user code.
87 #ifdef ABSL_ANY_DETAIL_HAS_RTTI
88 #error ABSL_ANY_DETAIL_HAS_RTTI cannot be directly set
89 #elif !defined(__GNUC__) || defined(__GXX_RTTI)
90 #define ABSL_ANY_DETAIL_HAS_RTTI 1
91 #endif // !defined(__GNUC__) || defined(__GXX_RTTI)
92
93 namespace absl {
94
95 namespace any_internal {
96
97 template <typename Type>
98 struct TypeTag {
99 constexpr static char dummy_var = 0;
100 };
101
102 template <typename Type>
103 constexpr char TypeTag<Type>::dummy_var;
104
105 // FastTypeId<Type>() evaluates at compile/link-time to a unique pointer for the
106 // passed in type. These are meant to be good match for keys into maps or
107 // straight up comparisons.
108 template<typename Type>
FastTypeId()109 constexpr inline const void* FastTypeId() {
110 return &TypeTag<Type>::dummy_var;
111 }
112
113 } // namespace any_internal
114
115 class any;
116
117 // swap()
118 //
119 // Swaps two `absl::any` values. Equivalent to `x.swap(y) where `x` and `y` are
120 // `absl::any` types.
121 void swap(any& x, any& y) noexcept;
122
123 // make_any()
124 //
125 // Constructs an `absl::any` of type `T` with the given arguments.
126 template <typename T, typename... Args>
127 any make_any(Args&&... args);
128
129 // Overload of `absl::make_any()` for constructing an `absl::any` type from an
130 // initializer list.
131 template <typename T, typename U, typename... Args>
132 any make_any(std::initializer_list<U> il, Args&&... args);
133
134 // any_cast()
135 //
136 // Statically casts the value of a `const absl::any` type to the given type.
137 // This function will throw `absl::bad_any_cast` if the stored value type of the
138 // `absl::any` does not match the cast.
139 //
140 // `any_cast()` can also be used to get a reference to the internal storage iff
141 // a reference type is passed as its `ValueType`:
142 //
143 // Example:
144 //
145 // absl::any my_any = std::vector<int>();
146 // absl::any_cast<std::vector<int>&>(my_any).push_back(42);
147 template <typename ValueType>
148 ValueType any_cast(const any& operand);
149
150 // Overload of `any_cast()` to statically cast the value of a non-const
151 // `absl::any` type to the given type. This function will throw
152 // `absl::bad_any_cast` if the stored value type of the `absl::any` does not
153 // match the cast.
154 template <typename ValueType>
155 ValueType any_cast(any& operand); // NOLINT(runtime/references)
156
157 // Overload of `any_cast()` to statically cast the rvalue of an `absl::any`
158 // type. This function will throw `absl::bad_any_cast` if the stored value type
159 // of the `absl::any` does not match the cast.
160 template <typename ValueType>
161 ValueType any_cast(any&& operand);
162
163 // Overload of `any_cast()` to statically cast the value of a const pointer
164 // `absl::any` type to the given pointer type, or `nullptr` if the stored value
165 // type of the `absl::any` does not match the cast.
166 template <typename ValueType>
167 const ValueType* any_cast(const any* operand) noexcept;
168
169 // Overload of `any_cast()` to statically cast the value of a pointer
170 // `absl::any` type to the given pointer type, or `nullptr` if the stored value
171 // type of the `absl::any` does not match the cast.
172 template <typename ValueType>
173 ValueType* any_cast(any* operand) noexcept;
174
175 // -----------------------------------------------------------------------------
176 // absl::any
177 // -----------------------------------------------------------------------------
178 //
179 // An `absl::any` object provides the facility to either store an instance of a
180 // type, known as the "contained object", or no value. An `absl::any` is used to
181 // store values of types that are unknown at compile time. The `absl::any`
182 // object, when containing a value, must contain a value type; storing a
183 // reference type is neither desired nor supported.
184 //
185 // An `absl::any` can only store a type that is copy-constructable; move-only
186 // types are not allowed within an `any` object.
187 //
188 // Example:
189 //
190 // auto a = absl::any(65); // Literal, copyable
191 // auto b = absl::any(std::vector<int>()); // Default-initialized, copyable
192 // std::unique_ptr<Foo> my_foo;
193 // auto c = absl::any(std::move(my_foo)); // Error, not copy-constructable
194 //
195 // Note that `absl::any` makes use of decayed types (`absl::decay_t` in this
196 // context) to remove const-volatile qualifiers (known as "cv qualifiers"),
197 // decay functions to function pointers, etc. We essentially "decay" a given
198 // type into its essential type.
199 //
200 // `absl::any` makes use of decayed types when determining the basic type `T` of
201 // the value to store in the any's contained object. In the documentation below,
202 // we explicitly denote this by using the phrase "a decayed type of `T`".
203 //
204 // Example:
205 //
206 // const int a = 4;
207 // absl::any foo(a); // Decay ensures we store an "int", not a "const int&".
208 //
209 // void my_function() {}
210 // absl::any bar(my_function); // Decay ensures we store a function pointer.
211 //
212 // `absl::any` is a C++11 compatible version of the C++17 `std::any` abstraction
213 // and is designed to be a drop-in replacement for code compliant with C++17.
214 class any {
215 private:
216 template <typename T>
217 struct IsInPlaceType;
218
219 public:
220 // Constructors
221
222 // Constructs an empty `absl::any` object (`any::has_value()` will return
223 // `false`).
224 constexpr any() noexcept;
225
226 // Copy constructs an `absl::any` object with a "contained object" of the
227 // passed type of `other` (or an empty `absl::any` if `other.has_value()` is
228 // `false`.
any(const any & other)229 any(const any& other)
230 : obj_(other.has_value() ? other.obj_->Clone()
231 : std::unique_ptr<ObjInterface>()) {}
232
233 // Move constructs an `absl::any` object with a "contained object" of the
234 // passed type of `other` (or an empty `absl::any` if `other.has_value()` is
235 // `false`).
236 any(any&& other) noexcept = default;
237
238 // Constructs an `absl::any` object with a "contained object" of the decayed
239 // type of `T`, which is initialized via `std::forward<T>(value)`.
240 //
241 // This constructor will not participate in overload resolution if the
242 // decayed type of `T` is not copy-constructible.
243 template <
244 typename T, typename VT = absl::decay_t<T>,
245 absl::enable_if_t<!absl::disjunction<
246 std::is_same<any, VT>, IsInPlaceType<VT>,
247 absl::negation<std::is_copy_constructible<VT> > >::value>* = nullptr>
any(T && value)248 any(T&& value) : obj_(new Obj<VT>(in_place, std::forward<T>(value))) {}
249
250 // Constructs an `absl::any` object with a "contained object" of the decayed
251 // type of `T`, which is initialized via `std::forward<T>(value)`.
252 template <typename T, typename... Args, typename VT = absl::decay_t<T>,
253 absl::enable_if_t<absl::conjunction<
254 std::is_copy_constructible<VT>,
255 std::is_constructible<VT, Args...>>::value>* = nullptr>
any(in_place_type_t<T>,Args &&...args)256 explicit any(in_place_type_t<T> /*tag*/, Args&&... args)
257 : obj_(new Obj<VT>(in_place, std::forward<Args>(args)...)) {}
258
259 // Constructs an `absl::any` object with a "contained object" of the passed
260 // type `VT` as a decayed type of `T`. `VT` is initialized as if
261 // direct-non-list-initializing an object of type `VT` with the arguments
262 // `initializer_list, std::forward<Args>(args)...`.
263 template <
264 typename T, typename U, typename... Args, typename VT = absl::decay_t<T>,
265 absl::enable_if_t<
266 absl::conjunction<std::is_copy_constructible<VT>,
267 std::is_constructible<VT, std::initializer_list<U>&,
268 Args...>>::value>* = nullptr>
any(in_place_type_t<T>,std::initializer_list<U> ilist,Args &&...args)269 explicit any(in_place_type_t<T> /*tag*/, std::initializer_list<U> ilist,
270 Args&&... args)
271 : obj_(new Obj<VT>(in_place, ilist, std::forward<Args>(args)...)) {}
272
273 // Assignment operators
274
275 // Copy assigns an `absl::any` object with a "contained object" of the
276 // passed type.
277 any& operator=(const any& rhs) {
278 any(rhs).swap(*this);
279 return *this;
280 }
281
282 // Move assigns an `absl::any` object with a "contained object" of the
283 // passed type. `rhs` is left in a valid but otherwise unspecified state.
284 any& operator=(any&& rhs) noexcept {
285 any(std::move(rhs)).swap(*this);
286 return *this;
287 }
288
289 // Assigns an `absl::any` object with a "contained object" of the passed type.
290 template <typename T, typename VT = absl::decay_t<T>,
291 absl::enable_if_t<absl::conjunction<
292 absl::negation<std::is_same<VT, any>>,
293 std::is_copy_constructible<VT>>::value>* = nullptr>
294 any& operator=(T&& rhs) {
295 any tmp(in_place_type_t<VT>(), std::forward<T>(rhs));
296 tmp.swap(*this);
297 return *this;
298 }
299
300 // Modifiers
301
302 // any::emplace()
303 //
304 // Emplaces a value within an `absl::any` object by calling `any::reset()`,
305 // initializing the contained value as if direct-non-list-initializing an
306 // object of type `VT` with the arguments `std::forward<Args>(args)...`, and
307 // returning a reference to the new contained value.
308 //
309 // Note: If an exception is thrown during the call to `VT`'s constructor,
310 // `*this` does not contain a value, and any previously contained value has
311 // been destroyed.
312 template <
313 typename T, typename... Args, typename VT = absl::decay_t<T>,
314 absl::enable_if_t<std::is_copy_constructible<VT>::value &&
315 std::is_constructible<VT, Args...>::value>* = nullptr>
emplace(Args &&...args)316 VT& emplace(Args&&... args) {
317 reset(); // NOTE: reset() is required here even in the world of exceptions.
318 Obj<VT>* const object_ptr =
319 new Obj<VT>(in_place, std::forward<Args>(args)...);
320 obj_ = std::unique_ptr<ObjInterface>(object_ptr);
321 return object_ptr->value;
322 }
323
324 // Overload of `any::emplace()` to emplace a value within an `absl::any`
325 // object by calling `any::reset()`, initializing the contained value as if
326 // direct-non-list-initializing an object of type `VT` with the arguments
327 // `initializer_list, std::forward<Args>(args)...`, and returning a reference
328 // to the new contained value.
329 //
330 // Note: If an exception is thrown during the call to `VT`'s constructor,
331 // `*this` does not contain a value, and any previously contained value has
332 // been destroyed. The function shall not participate in overload resolution
333 // unless `is_copy_constructible_v<VT>` is `true` and
334 // `is_constructible_v<VT, initializer_list<U>&, Args...>` is `true`.
335 template <
336 typename T, typename U, typename... Args, typename VT = absl::decay_t<T>,
337 absl::enable_if_t<std::is_copy_constructible<VT>::value &&
338 std::is_constructible<VT, std::initializer_list<U>&,
339 Args...>::value>* = nullptr>
emplace(std::initializer_list<U> ilist,Args &&...args)340 VT& emplace(std::initializer_list<U> ilist, Args&&... args) {
341 reset(); // NOTE: reset() is required here even in the world of exceptions.
342 Obj<VT>* const object_ptr =
343 new Obj<VT>(in_place, ilist, std::forward<Args>(args)...);
344 obj_ = std::unique_ptr<ObjInterface>(object_ptr);
345 return object_ptr->value;
346 }
347
348 // any::reset()
349 //
350 // Resets the state of the `absl::any` object, destroying the contained object
351 // if present.
reset()352 void reset() noexcept { obj_ = nullptr; }
353
354 // any::swap()
355 //
356 // Swaps the passed value and the value of this `absl::any` object.
swap(any & other)357 void swap(any& other) noexcept { obj_.swap(other.obj_); }
358
359 // Observers
360
361 // any::has_value()
362 //
363 // Returns `true` if the `any` object has a contained value, otherwise
364 // returns `false`.
has_value()365 bool has_value() const noexcept { return obj_ != nullptr; }
366
367 #if ABSL_ANY_DETAIL_HAS_RTTI
368 // Returns: typeid(T) if *this has a contained object of type T, otherwise
369 // typeid(void).
type()370 const std::type_info& type() const noexcept {
371 if (has_value()) {
372 return obj_->Type();
373 }
374
375 return typeid(void);
376 }
377 #endif // ABSL_ANY_DETAIL_HAS_RTTI
378
379 private:
380 // Tagged type-erased abstraction for holding a cloneable object.
381 class ObjInterface {
382 public:
383 virtual ~ObjInterface() = default;
384 virtual std::unique_ptr<ObjInterface> Clone() const = 0;
385 virtual const void* ObjTypeId() const noexcept = 0;
386 #if ABSL_ANY_DETAIL_HAS_RTTI
387 virtual const std::type_info& Type() const noexcept = 0;
388 #endif // ABSL_ANY_DETAIL_HAS_RTTI
389 };
390
391 // Hold a value of some queryable type, with an ability to Clone it.
392 template <typename T>
393 class Obj : public ObjInterface {
394 public:
395 template <typename... Args>
Obj(in_place_t,Args &&...args)396 explicit Obj(in_place_t /*tag*/, Args&&... args)
397 : value(std::forward<Args>(args)...) {}
398
Clone()399 std::unique_ptr<ObjInterface> Clone() const final {
400 return std::unique_ptr<ObjInterface>(new Obj(in_place, value));
401 }
402
ObjTypeId()403 const void* ObjTypeId() const noexcept final { return IdForType<T>(); }
404
405 #if ABSL_ANY_DETAIL_HAS_RTTI
Type()406 const std::type_info& Type() const noexcept final { return typeid(T); }
407 #endif // ABSL_ANY_DETAIL_HAS_RTTI
408
409 T value;
410 };
411
CloneObj()412 std::unique_ptr<ObjInterface> CloneObj() const {
413 if (!obj_) return nullptr;
414 return obj_->Clone();
415 }
416
417 template <typename T>
IdForType()418 constexpr static const void* IdForType() {
419 // Note: This type dance is to make the behavior consistent with typeid.
420 using NormalizedType =
421 typename std::remove_cv<typename std::remove_reference<T>::type>::type;
422
423 return any_internal::FastTypeId<NormalizedType>();
424 }
425
GetObjTypeId()426 const void* GetObjTypeId() const {
427 return obj_ ? obj_->ObjTypeId() : any_internal::FastTypeId<void>();
428 }
429
430 // `absl::any` nonmember functions //
431
432 // Description at the declaration site (top of file).
433 template <typename ValueType>
434 friend ValueType any_cast(const any& operand);
435
436 // Description at the declaration site (top of file).
437 template <typename ValueType>
438 friend ValueType any_cast(any& operand); // NOLINT(runtime/references)
439
440 // Description at the declaration site (top of file).
441 template <typename T>
442 friend const T* any_cast(const any* operand) noexcept;
443
444 // Description at the declaration site (top of file).
445 template <typename T>
446 friend T* any_cast(any* operand) noexcept;
447
448 std::unique_ptr<ObjInterface> obj_;
449 };
450
451 // -----------------------------------------------------------------------------
452 // Implementation Details
453 // -----------------------------------------------------------------------------
454
455 constexpr any::any() noexcept = default;
456
457 template <typename T>
458 struct any::IsInPlaceType : std::false_type {};
459
460 template <typename T>
461 struct any::IsInPlaceType<in_place_type_t<T>> : std::true_type {};
462
463 inline void swap(any& x, any& y) noexcept { x.swap(y); }
464
465 // Description at the declaration site (top of file).
466 template <typename T, typename... Args>
467 any make_any(Args&&... args) {
468 return any(in_place_type_t<T>(), std::forward<Args>(args)...);
469 }
470
471 // Description at the declaration site (top of file).
472 template <typename T, typename U, typename... Args>
473 any make_any(std::initializer_list<U> il, Args&&... args) {
474 return any(in_place_type_t<T>(), il, std::forward<Args>(args)...);
475 }
476
477 // Description at the declaration site (top of file).
478 template <typename ValueType>
479 ValueType any_cast(const any& operand) {
480 using U = typename std::remove_cv<
481 typename std::remove_reference<ValueType>::type>::type;
482 static_assert(std::is_constructible<ValueType, const U&>::value,
483 "Invalid ValueType");
484 auto* const result = (any_cast<U>)(&operand);
485 if (result == nullptr) {
486 any_internal::ThrowBadAnyCast();
487 }
488 return static_cast<ValueType>(*result);
489 }
490
491 // Description at the declaration site (top of file).
492 template <typename ValueType>
493 ValueType any_cast(any& operand) { // NOLINT(runtime/references)
494 using U = typename std::remove_cv<
495 typename std::remove_reference<ValueType>::type>::type;
496 static_assert(std::is_constructible<ValueType, U&>::value,
497 "Invalid ValueType");
498 auto* result = (any_cast<U>)(&operand);
499 if (result == nullptr) {
500 any_internal::ThrowBadAnyCast();
501 }
502 return static_cast<ValueType>(*result);
503 }
504
505 // Description at the declaration site (top of file).
506 template <typename ValueType>
507 ValueType any_cast(any&& operand) {
508 using U = typename std::remove_cv<
509 typename std::remove_reference<ValueType>::type>::type;
510 static_assert(std::is_constructible<ValueType, U>::value,
511 "Invalid ValueType");
512 return static_cast<ValueType>(std::move((any_cast<U&>)(operand)));
513 }
514
515 // Description at the declaration site (top of file).
516 template <typename T>
517 const T* any_cast(const any* operand) noexcept {
518 return operand && operand->GetObjTypeId() == any::IdForType<T>()
519 ? std::addressof(
520 static_cast<const any::Obj<T>*>(operand->obj_.get())->value)
521 : nullptr;
522 }
523
524 // Description at the declaration site (top of file).
525 template <typename T>
526 T* any_cast(any* operand) noexcept {
527 return operand && operand->GetObjTypeId() == any::IdForType<T>()
528 ? std::addressof(
529 static_cast<any::Obj<T>*>(operand->obj_.get())->value)
530 : nullptr;
531 }
532
533 } // namespace absl
534
535 #undef ABSL_ANY_DETAIL_HAS_RTTI
536
537 #endif // ABSL_HAVE_STD_ANY
538
539 #endif // ABSL_TYPES_ANY_H_
540