2 * Copyright 2017-present Facebook, Inc.
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
8 * http://www.apache.org/licenses/LICENSE-2.0
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.
17 * Author: Eric Niebler <eniebler@fb.com>
28 #include <type_traits>
32 #include <folly/Assume.h>
33 #include <folly/CPortability.h>
34 #include <folly/Demangle.h>
35 #include <folly/ExceptionString.h>
36 #include <folly/FBString.h>
37 #include <folly/Traits.h>
40 #pragma GCC diagnostic push
41 #pragma GCC diagnostic ignored "-Wpragmas"
42 #pragma GCC diagnostic ignored "-Wpotentially-evaluated-expression"
43 // GCC gets confused about lambda scopes and issues shadow-local warnings for
44 // parameters in totally different functions.
45 #pragma GCC diagnostic ignored "-Wshadow-local"
46 #pragma GCC diagnostic ignored "-Wshadow-compatible-local"
49 #define FOLLY_EXCEPTION_WRAPPER_H_INCLUDED
53 #define FOLLY_REQUIRES_DEF(...) \
54 _t<std::enable_if<static_cast<bool>(__VA_ARGS__), long>>
56 #define FOLLY_REQUIRES(...) FOLLY_REQUIRES_DEF(__VA_ARGS__) = __LINE__
58 namespace exception_wrapper_detail {
60 template <template <class> class T, class... As>
61 using AllOf = StrictConjunction<T<As>...>;
63 template <bool If, class T>
64 using AddConstIf = _t<std::conditional<If, const T, T>>;
66 template <class Fn, class A>
67 FOLLY_ALWAYS_INLINE FOLLY_ATTR_VISIBILITY_HIDDEN
68 auto fold(Fn&&, A&& a) {
69 return static_cast<A&&>(a);
72 template <class Fn, class A, class B, class... Bs>
73 FOLLY_ALWAYS_INLINE FOLLY_ATTR_VISIBILITY_HIDDEN
74 auto fold(Fn&& fn, A&& a, B&& b, Bs&&... bs) {
76 // This looks like a use of fn after a move of fn, but in reality, this is
77 // just a cast and not a move. That's because regardless of which fold
78 // overload is selected, fn gets bound to a &&. Had fold taken fn by value
79 // there would indeed be a problem here.
80 static_cast<Fn&&>(fn),
81 static_cast<Fn&&>(fn)(static_cast<A&&>(a), static_cast<B&&>(b)),
82 static_cast<Bs&&>(bs)...);
85 } // namespace exception_wrapper_detail
87 //! Throwing exceptions can be a convenient way to handle errors. Storing
88 //! exceptions in an `exception_ptr` makes it easy to handle exceptions in a
89 //! different thread or at a later time. `exception_ptr` can also be used in a
90 //! very generic result/exception wrapper.
92 //! However, there are some issues with throwing exceptions and
93 //! `std::exception_ptr`. These issues revolve around `throw` being expensive,
94 //! particularly in a multithreaded environment (see
95 //! ExceptionWrapperBenchmark.cpp).
97 //! Imagine we have a library that has an API which returns a result/exception
98 //! wrapper. Let's consider some approaches for implementing this wrapper.
99 //! First, we could store a `std::exception`. This approach loses the derived
100 //! exception type, which can make exception handling more difficult for users
101 //! that prefer rethrowing the exception. We could use a `folly::dynamic` for
102 //! every possible type of exception. This is not very flexible - adding new
103 //! types of exceptions requires a change to the result/exception wrapper. We
104 //! could use an `exception_ptr`. However, constructing an `exception_ptr` as
105 //! well as accessing the error requires a call to throw. That means that there
106 //! will be two calls to throw in order to process the exception. For
107 //! performance sensitive applications, this may be unacceptable.
109 //! `exception_wrapper` is designed to handle exception management for both
110 //! convenience and high performance use cases. `make_exception_wrapper` is
111 //! templated on derived type, allowing us to rethrow the exception properly for
112 //! users that prefer convenience. These explicitly named exception types can
113 //! therefore be handled without any peformance penalty. `exception_wrapper` is
114 //! also flexible enough to accept any type. If a caught exception is not of an
115 //! explicitly named type, then `std::exception_ptr` is used to preserve the
116 //! exception state. For performance sensitive applications, the accessor
117 //! methods can test or extract a pointer to a specific exception type with very
120 //! \par Example usage:
123 //! exception_wrapper globalExceptionWrapper;
126 //! void doSomethingCrazy() {
127 //! int rc = doSomethingCrazyWithLameReturnCodes();
128 //! if (rc == NAILED_IT) {
129 //! globalExceptionWrapper = exception_wrapper();
130 //! } else if (rc == FACE_PLANT) {
131 //! globalExceptionWrapper = make_exception_wrapper<FacePlantException>();
132 //! } else if (rc == FAIL_WHALE) {
133 //! globalExceptionWrapper = make_exception_wrapper<FailWhaleException>();
137 //! // Thread2: Exceptions are ok!
138 //! void processResult() {
140 //! globalExceptionWrapper.throwException();
141 //! } catch (const FacePlantException& e) {
142 //! LOG(ERROR) << "FACEPLANT!";
143 //! } catch (const FailWhaleException& e) {
144 //! LOG(ERROR) << "FAILWHALE!";
148 //! // Thread2: Exceptions are bad!
149 //! void processResult() {
150 //! globalExceptionWrapper.handle(
151 //! [&](FacePlantException& faceplant) {
152 //! LOG(ERROR) << "FACEPLANT";
154 //! [&](FailWhaleException& failwhale) {
155 //! LOG(ERROR) << "FAILWHALE!";
158 //! LOG(FATAL) << "Unrecognized exception";
162 class exception_wrapper final {
164 struct AnyException : std::exception {
165 std::type_info const* typeinfo_;
167 /* implicit */ AnyException(T&& t) noexcept : typeinfo_(&typeid(t)) {}
173 using arg_type = _t<arg_type_<Fn>>;
175 // exception_wrapper is implemented as a simple variant over four
176 // different representations:
177 // 0. Empty, no exception.
178 // 1. An small object stored in-situ.
179 // 2. A larger object stored on the heap and referenced with a
181 // 3. A std::exception_ptr, together with either:
182 // a. A pointer to the referenced std::exception object, or
183 // b. A pointer to a std::type_info object for the referenced exception,
184 // or for an unspecified type if the type is unknown.
185 // This is accomplished with the help of a union and a pointer to a hand-
186 // rolled virtual table. This virtual table contains pointers to functions
187 // that know which field of the union is active and do the proper action.
188 // The class invariant ensures that the vtable ptr and the union stay in sync.
190 void (*copy_)(exception_wrapper const*, exception_wrapper*);
191 void (*move_)(exception_wrapper*, exception_wrapper*);
192 void (*delete_)(exception_wrapper*);
193 void (*throw_)(exception_wrapper const*);
194 std::type_info const* (*type_)(exception_wrapper const*);
195 std::exception const* (*get_exception_)(exception_wrapper const*);
196 exception_wrapper (*get_exception_ptr_)(exception_wrapper const*);
199 [[noreturn]] static void onNoExceptionError();
201 template <class Ret, class... Args>
202 static Ret noop_(Args...);
204 static std::type_info const* uninit_type_(exception_wrapper const*);
206 static constexpr VTable const uninit_{
207 &noop_<void, exception_wrapper const*, exception_wrapper*>,
208 &noop_<void, exception_wrapper*, exception_wrapper*>,
209 &noop_<void, exception_wrapper*>,
210 &noop_<void, exception_wrapper const*>,
212 &noop_<std::exception const*, exception_wrapper const*>,
213 &noop_<exception_wrapper, exception_wrapper const*>};
216 using IsStdException = std::is_base_of<std::exception, _t<std::decay<Ex>>>;
217 template <bool B, class T>
218 using AddConstIf = exception_wrapper_detail::AddConstIf<B, T>;
219 template <class CatchFn>
221 std::is_same<arg_type<_t<std::decay<CatchFn>>>, AnyException>;
225 // Sadly, with the gcc-4.9 platform, std::logic_error and std::runtime_error
226 // do not fit here. They also don't have noexcept copy-ctors, so the internal
227 // storage wouldn't be used anyway. For the gcc-5 platform, both logic_error
228 // and runtime_error can be safely stored internally.
231 _t<std::aligned_storage<2 * sizeof(void*), alignof(std::exception)>>;
234 Buffer() : buff_{} {}
236 template <class Ex, class DEx = _t<std::decay<Ex>>>
237 Buffer(in_place_t, Ex&& ex);
241 Ex const& as() const noexcept;
244 enum class Placement { kInSitu, kOnHeap };
246 using PlacementOf = std::integral_constant<
248 sizeof(T) <= sizeof(Buffer::Storage) &&
249 alignof(T) <= alignof(Buffer::Storage) &&
250 noexcept(T(std::declval<T&&>()))
252 : Placement::kOnHeap>;
254 using InSituTag = std::integral_constant<Placement, Placement::kInSitu>;
255 using OnHeapTag = std::integral_constant<Placement, Placement::kOnHeap>;
257 static std::exception const* as_exception_or_null_(std::exception const& ex);
258 static std::exception const* as_exception_or_null_(AnyException);
260 struct ExceptionPtr {
261 std::exception_ptr ptr_;
262 std::uintptr_t exception_or_type_; // odd for type_info
264 1 < alignof(std::exception) && 1 < alignof(std::type_info),
265 "Surprise! std::exception and std::type_info don't have alignment "
266 "greater than one. as_int_ below will not work!");
268 static std::uintptr_t as_int_(std::exception const& e);
269 static std::uintptr_t as_int_(AnyException e);
270 bool has_exception_() const;
271 std::exception const* as_exception_() const;
272 std::type_info const* as_type_() const;
273 static void copy_(exception_wrapper const* from, exception_wrapper* to);
274 static void move_(exception_wrapper* from, exception_wrapper* to);
275 static void delete_(exception_wrapper* that);
276 [[noreturn]] static void throw_(exception_wrapper const* that);
277 static std::type_info const* type_(exception_wrapper const* that);
278 static std::exception const* get_exception_(exception_wrapper const* that);
279 static exception_wrapper get_exception_ptr_(exception_wrapper const* that);
280 static constexpr VTable const ops_{copy_,
291 static void copy_(exception_wrapper const* from, exception_wrapper* to);
292 static void move_(exception_wrapper* from, exception_wrapper* to);
293 static void delete_(exception_wrapper* that);
294 [[noreturn]] static void throw_(exception_wrapper const* that);
295 static std::type_info const* type_(exception_wrapper const*);
296 static std::exception const* get_exception_(exception_wrapper const* that);
297 static exception_wrapper get_exception_ptr_(exception_wrapper const* that);
298 static constexpr VTable const ops_{copy_,
309 std::type_info const* info_;
311 explicit Base(std::type_info const& info) : info_(&info) {}
313 virtual void throw_() const = 0;
314 virtual std::exception const* get_exception_() const noexcept = 0;
315 virtual exception_wrapper get_exception_ptr_() const noexcept = 0;
318 struct Impl final : public Base {
321 explicit Impl(Ex const& ex) : Base{typeid(ex)}, ex_(ex) {}
322 explicit Impl(Ex&& ex)
324 ex_(std::move(ex)){}[[noreturn]] void throw_() const override;
325 std::exception const* get_exception_() const noexcept override;
326 exception_wrapper get_exception_ptr_() const noexcept override;
328 std::shared_ptr<Base> ptr_;
330 static void copy_(exception_wrapper const* from, exception_wrapper* to);
331 static void move_(exception_wrapper* from, exception_wrapper* to);
332 static void delete_(exception_wrapper* that);
333 [[noreturn]] static void throw_(exception_wrapper const* that);
334 static std::type_info const* type_(exception_wrapper const* that);
335 static std::exception const* get_exception_(exception_wrapper const* that);
336 static exception_wrapper get_exception_ptr_(exception_wrapper const* that);
337 static constexpr VTable ops_{copy_,
351 VTable const* vptr_{&uninit_};
353 template <class Ex, class DEx = _t<std::decay<Ex>>>
354 exception_wrapper(Ex&& ex, OnHeapTag);
356 template <class Ex, class DEx = _t<std::decay<Ex>>>
357 exception_wrapper(Ex&& ex, InSituTag);
360 struct IsRegularExceptionType
362 std::is_copy_constructible<T>,
363 Negation<std::is_base_of<exception_wrapper, T>>,
364 Negation<std::is_abstract<T>>> {};
366 template <class CatchFn, bool IsConst = false>
367 struct ExceptionTypeOf;
369 template <bool IsConst>
372 template <bool IsConst>
373 struct HandleStdExceptReduce;
375 template <class This, class... CatchFns>
376 static void handle_(std::false_type, This& this_, CatchFns&... fns);
378 template <class This, class... CatchFns>
379 static void handle_(std::true_type, This& this_, CatchFns&... fns);
381 template <class Ex, class This, class Fn>
382 static bool with_exception_(This& this_, Fn fn_);
385 //! Default-constructs an empty `exception_wrapper`
386 //! \post `type() == none()`
387 exception_wrapper() noexcept {}
389 //! Move-constructs an `exception_wrapper`
390 //! \post `*this` contains the value of `that` prior to the move
391 //! \post `that.type() == none()`
392 exception_wrapper(exception_wrapper&& that) noexcept;
394 //! Copy-constructs an `exception_wrapper`
395 //! \post `*this` contains a copy of `that`, and `that` is unmodified
396 //! \post `type() == that.type()`
397 exception_wrapper(exception_wrapper const& that);
399 //! Move-assigns an `exception_wrapper`
400 //! \pre `this != &that`
401 //! \post `*this` contains the value of `that` prior to the move
402 //! \post `that.type() == none()`
403 exception_wrapper& operator=(exception_wrapper&& that) noexcept;
405 //! Copy-assigns an `exception_wrapper`
406 //! \post `*this` contains a copy of `that`, and `that` is unmodified
407 //! \post `type() == that.type()`
408 exception_wrapper& operator=(exception_wrapper const& that);
410 ~exception_wrapper();
412 //! \pre `ptr` is empty, or it holds a reference to an exception that is not
413 //! derived from `std::exception`.
414 //! \post `!ptr || bool(*this)`
415 //! \post `hasThrownException() == true`
416 //! \post `type() == unknown()`
417 explicit exception_wrapper(std::exception_ptr ptr) noexcept;
419 //! \pre `ptr` holds a reference to `ex`.
420 //! \post `hasThrownException() == true`
421 //! \post `bool(*this)`
422 //! \post `type() == typeid(ex)`
424 exception_wrapper(std::exception_ptr ptr, Ex& ex);
426 //! \pre `typeid(ex) == typeid(typename decay<Ex>::type)`
427 //! \post `bool(*this)`
428 //! \post `hasThrownException() == false`
429 //! \post `type() == typeid(ex)`
430 //! \note Exceptions of types derived from `std::exception` can be implicitly
431 //! converted to an `exception_wrapper`.
434 class Ex_ = _t<std::decay<Ex>>,
436 Conjunction<IsStdException<Ex_>, IsRegularExceptionType<Ex_>>())>
437 /* implicit */ exception_wrapper(Ex&& ex);
439 //! \pre `typeid(ex) == typeid(typename decay<Ex>::type)`
440 //! \post `bool(*this)`
441 //! \post `hasThrownException() == false`
442 //! \post `type() == typeid(ex)`
443 //! \note Exceptions of types not derived from `std::exception` can still be
444 //! used to construct an `exception_wrapper`, but you must specify
445 //! `folly::in_place` as the first parameter.
448 class Ex_ = _t<std::decay<Ex>>,
449 FOLLY_REQUIRES(IsRegularExceptionType<Ex_>())>
450 exception_wrapper(in_place_t, Ex&& ex);
452 //! Swaps the value of `*this` with the value of `that`
453 void swap(exception_wrapper& that) noexcept;
455 //! \return `true` if `*this` is not holding an exception.
456 explicit operator bool() const noexcept;
458 //! \return `!bool(*this)`
459 bool operator!() const noexcept;
461 //! Make this `exception_wrapper` empty
465 //! \return `true` if this `exception_wrapper` holds a reference to an
466 //! exception that was thrown (i.e., if it was constructed with
467 //! a `std::exception_ptr`, or if `to_exception_ptr()` was called on a
468 //! (non-const) reference to `*this`).
469 bool has_exception_ptr() const noexcept;
471 //! \return a pointer to the `std::exception` held by `*this`, if it holds
472 //! one; otherwise, returns `nullptr`.
473 //! \note This function does not mutate the `exception_wrapper` object.
474 //! \note This function never causes an exception to be thrown.
475 std::exception* get_exception() noexcept;
477 std::exception const* get_exception() const noexcept;
479 //! \return A `std::exception_ptr` that references either the exception held
480 //! by `*this`, or a copy of same.
481 //! \note This function may need to throw an exception to complete the action.
482 //! \note The non-const overload of this function mutates `*this` to cache the
483 //! computed `std::exception_ptr`; that is, this function may cause
484 //! `has_exception_ptr()` to change from `false` to `true`.
485 std::exception_ptr const& to_exception_ptr() noexcept;
487 std::exception_ptr to_exception_ptr() const noexcept;
489 //! \return the `typeid` of an unspecified type used by
490 //! `exception_wrapper::type()` to denote an empty `exception_wrapper`.
491 static std::type_info const& none() noexcept;
492 //! \return the `typeid` of an unspecified type used by
493 //! `exception_wrapper::type()` to denote an `exception_wrapper` that
494 //! holds an exception of unknown type.
495 static std::type_info const& unknown() noexcept;
497 //! Returns the `typeid` of the wrapped exception object. If there is no
498 //! wrapped exception object, returns `exception_wrapper::none()`. If
499 //! this instance wraps an exception of unknown type not derived from
500 //! `std::exception`, returns `exception_wrapper::unknown()`.
501 std::type_info const& type() const noexcept;
503 //! \return If `get_exception() != nullptr`, `class_name() + ": " +
504 //! get_exception()->what()`; otherwise, `class_name()`.
505 folly::fbstring what() const;
507 //! \return If `!*this`, the empty string; otherwise, if
508 //! `type() == unknown()`, the string `"<unknown exception>"`; otherwise,
509 //! the result of `type().name()` after demangling.
510 folly::fbstring class_name() const;
512 //! \tparam Ex The expression type to check for compatibility with.
513 //! \return `true` if and only if `*this` wraps an exception that would be
514 //! caught with a `catch(Ex const&)` clause.
515 //! \note If `*this` is empty, this function returns `false`.
517 bool is_compatible_with() const noexcept;
519 //! \pre `bool(*this)`
520 //! Throws the wrapped expression.
521 [[noreturn]] void throwException() const;
523 //! Call `fn` with the wrapped exception (if any), if `fn` can accept it.
526 //! exception_wrapper ew{std::runtime_error("goodbye cruel world")};
528 //! assert( ew.with_exception([](std::runtime_error& e){/*...*/}) );
530 //! assert( !ew.with_exception([](int& e){/*...*/}) );
532 //! assert( !exception_wrapper{}.with_exception([](int& e){/*...*/}) );
534 //! \tparam Ex Optionally, the type of the exception that `fn` accepts.
535 //! \tparam Fn The type of a monomophic function object.
536 //! \param fn A function object to call with the wrapped exception
537 //! \return `true` if and only if `fn` was called.
538 //! \note Optionally, you may explicitly specify the type of the exception
539 //! that `fn` expects, as in
541 //! ew.with_exception<std::runtime_error>([](auto&& e) { /*...*/; });
543 //! \note The handler may or may not be invoked with an active exception.
544 //! **Do not try to rethrow the exception with `throw;` from within your
545 //! handler -- that is, a throw expression with no operand.** This may
546 //! cause your process to terminate. (It is perfectly ok to throw from
547 //! a handler so long as you specify the exception to throw, as in
549 template <class Ex = void const, class Fn>
550 bool with_exception(Fn fn);
552 template <class Ex = void const, class Fn>
553 bool with_exception(Fn fn) const;
555 //! Handle the wrapped expression as if with a series of `catch` clauses,
556 //! propagating the exception if no handler matches.
559 //! exception_wrapper ew{std::runtime_error("goodbye cruel world")};
562 //! [&](std::logic_error const& e) {
563 //! LOG(DFATAL) << "ruh roh";
564 //! ew.throwException(); // rethrow the active exception without
565 //! // slicing it. Will not be caught by other
566 //! // handlers in this call.
568 //! [&](std::exception const& e) {
569 //! LOG(ERROR) << ew.what();
572 //! In the above example, any exception _not_ derived from `std::exception`
573 //! will be propagated. To specify a catch-all clause, pass a lambda that
574 //! takes a C-style elipses, as in:
576 //! ew.handle(/*...* /, [](...) { /* handle unknown exception */ } )
579 //! \tparam CatchFns... A pack of unary monomorphic function object types.
580 //! \param fns A pack of unary monomorphic function objects to be treated as
581 //! an ordered list of potential exception handlers.
582 //! \note The handlers may or may not be invoked with an active exception.
583 //! **Do not try to rethrow the exception with `throw;` from within your
584 //! handler -- that is, a throw expression with no operand.** This may
585 //! cause your process to terminate. (It is perfectly ok to throw from
586 //! a handler so long as you specify the exception to throw, as in
588 template <class... CatchFns>
589 void handle(CatchFns... fns);
591 template <class... CatchFns>
592 void handle(CatchFns... fns) const;
596 constexpr exception_wrapper::VTable exception_wrapper::InPlace<Ex>::ops_;
599 * \return An `exception_wrapper` that wraps an instance of type `Ex`
600 * that has been constructed with arguments `std::forward<As>(as)...`.
602 template <class Ex, typename... As>
603 exception_wrapper make_exception_wrapper(As&&... as) {
604 return exception_wrapper{Ex{std::forward<As>(as)...}};
608 * Inserts `ew.what()` into the ostream `sout`.
612 std::basic_ostream<Ch>& operator<<(
613 std::basic_ostream<Ch>& sout,
614 exception_wrapper const& ew) {
615 return sout << ew.what();
619 * Swaps the value of `a` with the value of `b`.
621 inline void swap(exception_wrapper& a, exception_wrapper& b) noexcept {
625 // For consistency with exceptionStr() functions in ExceptionString.h
626 fbstring exceptionStr(exception_wrapper const& ew);
629 template <typename F>
630 inline exception_wrapper try_and_catch_(F&& f) {
631 return (f(), exception_wrapper());
634 template <typename F, typename Ex, typename... Exs>
635 inline exception_wrapper try_and_catch_(F&& f) {
637 return try_and_catch_<F, Exs...>(std::forward<F>(f));
639 return exception_wrapper(std::current_exception(), ex);
644 //! `try_and_catch` is a simple replacement for `try {} catch(){}`` that allows
645 //! you to specify which derived exceptions you would like to catch and store in
646 //! an `exception_wrapper`.
648 //! Because we cannot build an equivalent of `std::current_exception()`, we need
649 //! to catch every derived exception that we are interested in catching.
651 //! Exceptions should be listed in the reverse order that you would write your
652 //! catch statements (that is, `std::exception&` should be first).
654 //! \par Example Usage:
656 //! // This catches my runtime_error and if I call throwException() on ew, it
657 //! // will throw a runtime_error
658 //! auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
659 //! if (badThingHappens()) {
660 //! throw std::runtime_error("ZOMG!");
664 //! // This will catch the exception and if I call throwException() on ew, it
665 //! // will throw a std::exception
666 //! auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
667 //! if (badThingHappens()) {
668 //! throw std::exception();
672 //! // This will not catch the exception and it will be thrown.
673 //! auto ew = folly::try_and_catch<std::runtime_error>([=]() {
674 //! if (badThingHappens()) {
675 //! throw std::exception();
679 template <typename... Exceptions, typename F>
680 exception_wrapper try_and_catch(F&& fn) {
681 return detail::try_and_catch_<F, Exceptions...>(std::forward<F>(fn));
685 #include <folly/ExceptionWrapper-inl.h>
687 #undef FOLLY_REQUIRES
688 #undef FOLLY_REQUIRES_DEF
690 #pragma GCC diagnostic pop