2 * Copyright 2016 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.
23 #include <folly/ExceptionString.h>
24 #include <folly/detail/ExceptionWrapper.h>
29 * Throwing exceptions can be a convenient way to handle errors. Storing
30 * exceptions in an exception_ptr makes it easy to handle exceptions in a
31 * different thread or at a later time. exception_ptr can also be used in a very
32 * generic result/exception wrapper.
34 * However, there are some issues with throwing exceptions and
35 * std::exception_ptr. These issues revolve around throw being expensive,
36 * particularly in a multithreaded environment (see
37 * ExceptionWrapperBenchmark.cpp).
39 * Imagine we have a library that has an API which returns a result/exception
40 * wrapper. Let's consider some approaches for implementing this wrapper.
41 * First, we could store a std::exception. This approach loses the derived
42 * exception type, which can make exception handling more difficult for users
43 * that prefer rethrowing the exception. We could use a folly::dynamic for every
44 * possible type of exception. This is not very flexible - adding new types of
45 * exceptions requires a change to the result/exception wrapper. We could use an
46 * exception_ptr. However, constructing an exception_ptr as well as accessing
47 * the error requires a call to throw. That means that there will be two calls
48 * to throw in order to process the exception. For performance sensitive
49 * applications, this may be unacceptable.
51 * exception_wrapper is designed to handle exception management for both
52 * convenience and high performance use cases. make_exception_wrapper is
53 * templated on derived type, allowing us to rethrow the exception properly for
54 * users that prefer convenience. These explicitly named exception types can
55 * therefore be handled without any peformance penalty. exception_wrapper is
56 * also flexible enough to accept any type. If a caught exception is not of an
57 * explicitly named type, then std::exception_ptr is used to preserve the
58 * exception state. For performance sensitive applications, the accessor methods
59 * can test or extract a pointer to a specific exception type with very little
64 * exception_wrapper globalExceptionWrapper;
67 * void doSomethingCrazy() {
68 * int rc = doSomethingCrazyWithLameReturnCodes();
69 * if (rc == NAILED_IT) {
70 * globalExceptionWrapper = exception_wrapper();
71 * } else if (rc == FACE_PLANT) {
72 * globalExceptionWrapper = make_exception_wrapper<FacePlantException>();
73 * } else if (rc == FAIL_WHALE) {
74 * globalExceptionWrapper = make_exception_wrapper<FailWhaleException>();
78 * // Thread2: Exceptions are ok!
79 * void processResult() {
81 * globalExceptionWrapper.throwException();
82 * } catch (const FacePlantException& e) {
83 * LOG(ERROR) << "FACEPLANT!";
84 * } catch (const FailWhaleException& e) {
85 * LOG(ERROR) << "FAILWHALE!";
89 * // Thread2: Exceptions are bad!
90 * void processResult() {
91 * globalExceptionWrapper.with_exception(
92 * [&](FacePlantException& faceplant) {
93 * LOG(ERROR) << "FACEPLANT";
95 * globalExceptionWrapper.with_exception(
96 * [&](FailWhaleException& failwhale) {
97 * LOG(ERROR) << "FAILWHALE!";
99 * LOG(FATAL) << "Unrecognized exception";
103 class exception_wrapper {
105 template <typename Ex>
109 exception_wrapper() = default;
111 // Implicitly construct an exception_wrapper from a qualifying exception.
112 // See the optimize struct for details.
113 template <typename Ex, typename =
114 typename std::enable_if<optimize<typename std::decay<Ex>::type>::value>
116 /* implicit */ exception_wrapper(Ex&& exn) {
117 typedef typename std::decay<Ex>::type DEx;
118 item_ = std::make_shared<DEx>(std::forward<Ex>(exn));
119 throwfn_ = folly::detail::Thrower<DEx>::doThrow;
122 // The following two constructors are meant to emulate the behavior of
123 // try_and_catch in performance sensitive code as well as to be flexible
124 // enough to wrap exceptions of unknown type. There is an overload that
125 // takes an exception reference so that the wrapper can extract and store
126 // the exception's type and what() when possible.
128 // The canonical use case is to construct an all-catching exception wrapper
129 // with minimal overhead like so:
132 // // some throwing code
133 // } catch (const std::exception& e) {
134 // // won't lose e's type and what()
135 // exception_wrapper ew{std::current_exception(), e};
137 // // everything else
138 // exception_wrapper ew{std::current_exception()};
141 // try_and_catch is cleaner and preferable. Use it unless you're sure you need
142 // something like this instead.
143 template <typename Ex>
144 explicit exception_wrapper(std::exception_ptr eptr, Ex& exn) {
145 assign_eptr(eptr, exn);
148 explicit exception_wrapper(std::exception_ptr eptr) {
152 // If the exception_wrapper does not contain an exception, std::terminate()
153 // is invoked to assure the [[noreturn]] behaviour.
154 [[noreturn]] void throwException() const {
156 throwfn_(item_.get());
158 std::rethrow_exception(eptr_);
161 << "Cannot use `throwException` with an empty folly::exception_wrapper"
166 explicit operator bool() const {
167 return item_ || eptr_;
170 // This implementation is similar to std::exception_ptr's implementation
171 // where two exception_wrappers are equal when the address in the underlying
172 // reference field both point to the same exception object. The reference
173 // field remains the same when the exception_wrapper is copied or when
174 // the exception_wrapper is "rethrown".
175 bool operator==(const exception_wrapper& a) const {
177 return a.item_ && item_.get() == a.item_.get();
179 return eptr_ == a.eptr_;
183 bool operator!=(const exception_wrapper& a) const {
184 return !(*this == a);
187 // This will return a non-nullptr only if the exception is held as a
188 // copy. It is the only interface which will distinguish between an
189 // exception held this way, and by exception_ptr. You probably
190 // shouldn't use it at all.
191 std::exception* getCopied() { return item_.get(); }
192 const std::exception* getCopied() const { return item_.get(); }
194 fbstring what() const {
196 return exceptionStr(*item_);
204 fbstring class_name() const {
207 return demangle(typeid(i));
216 bool is_compatible_with() const {
218 return dynamic_cast<const Ex*>(item_.get());
221 std::rethrow_exception(eptr_);
222 } catch (typename std::decay<Ex>::type&) {
232 bool with_exception(F&& f) {
233 using arg_type = typename functor_traits<F>::arg_type_decayed;
234 return with_exception<arg_type>(std::forward<F>(f));
238 bool with_exception(F&& f) const {
239 using arg_type = typename functor_traits<F>::arg_type_decayed;
240 return with_exception<const arg_type>(std::forward<F>(f));
243 // If this exception wrapper wraps an exception of type Ex, with_exception
244 // will call f with the wrapped exception as an argument and return true, and
245 // will otherwise return false.
246 template <class Ex, class F>
247 typename std::enable_if<
248 std::is_base_of<std::exception, typename std::decay<Ex>::type>::value,
250 with_exception(F f) {
251 return with_exception1<typename std::decay<Ex>::type>(f, this);
255 template <class Ex, class F>
256 typename std::enable_if<
257 std::is_base_of<std::exception, typename std::decay<Ex>::type>::value,
259 with_exception(F f) const {
260 return with_exception1<const typename std::decay<Ex>::type>(f, this);
263 // Overload for non-exceptions. Always rethrows.
264 template <class Ex, class F>
265 typename std::enable_if<
266 !std::is_base_of<std::exception, typename std::decay<Ex>::type>::value,
268 with_exception(F f) const {
273 } catch (typename std::decay<Ex>::type& e) {
282 std::exception_ptr getExceptionPtr() const {
292 return std::current_exception();
294 return std::exception_ptr();
298 template <typename Ex>
300 static const bool value =
301 std::is_base_of<std::exception, Ex>::value &&
302 std::is_copy_assignable<Ex>::value &&
303 !std::is_abstract<Ex>::value;
306 template <typename Ex>
307 void assign_eptr(std::exception_ptr eptr, Ex& e) {
309 this->estr_ = exceptionStr(e).toStdString();
310 this->ename_ = demangle(typeid(e)).toStdString();
313 void assign_eptr(std::exception_ptr eptr) {
317 // Optimized case: if we know what type the exception is, we can
318 // store a copy of the concrete type, and a helper function so we
320 std::shared_ptr<std::exception> item_;
321 void (*throwfn_)(std::exception*){nullptr};
322 // Fallback case: store the library wrapper, which is less efficient
323 // but gets the job done. Also store exceptionPtr() the name of the
324 // exception type, so we can at least get those back out without
325 // having to rethrow.
326 std::exception_ptr eptr_;
330 template <class T, class... Args>
331 friend exception_wrapper make_exception_wrapper(Args&&... args);
334 template <typename F>
335 struct functor_traits {
336 template <typename T>
338 template <typename C, typename R, typename A>
339 struct impl<R(C::*)(A)> { using arg_type = A; };
340 template <typename C, typename R, typename A>
341 struct impl<R(C::*)(A) const> { using arg_type = A; };
342 using functor_decayed = typename std::decay<F>::type;
343 using functor_op = decltype(&functor_decayed::operator());
344 using arg_type = typename impl<functor_op>::arg_type;
345 using arg_type_decayed = typename std::decay<arg_type>::type;
348 // What makes this useful is that T can be exception_wrapper* or
349 // const exception_wrapper*, and the compiler will use the
350 // instantiation which works with F.
351 template <class Ex, class F, class T>
352 static bool with_exception1(F f, T* that) {
354 if (auto ex = dynamic_cast<Ex*>(that->item_.get())) {
358 } else if (that->eptr_) {
360 std::rethrow_exception(that->eptr_);
372 template <class T, class... Args>
373 exception_wrapper make_exception_wrapper(Args&&... args) {
374 exception_wrapper ew;
375 ew.item_ = std::make_shared<T>(std::forward<Args>(args)...);
376 ew.throwfn_ = folly::detail::Thrower<T>::doThrow;
380 // For consistency with exceptionStr() functions in String.h
381 inline fbstring exceptionStr(const exception_wrapper& ew) {
386 * try_and_catch is a simple replacement for try {} catch(){} that allows you to
387 * specify which derived exceptions you would like to catch and store in an
390 * Because we cannot build an equivalent of std::current_exception(), we need
391 * to catch every derived exception that we are interested in catching.
393 * Exceptions should be listed in the reverse order that you would write your
394 * catch statements (that is, std::exception& should be first).
396 * NOTE: Although implemented as a derived class (for syntactic delight), don't
397 * be confused - you should not pass around try_and_catch objects!
401 * // This catches my runtime_error and if I call throwException() on ew, it
402 * // will throw a runtime_error
403 * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
404 * if (badThingHappens()) {
405 * throw std::runtime_error("ZOMG!");
409 * // This will catch the exception and if I call throwException() on ew, it
410 * // will throw a std::exception
411 * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
412 * if (badThingHappens()) {
413 * throw std::exception();
417 * // This will not catch the exception and it will be thrown.
418 * auto ew = folly::try_and_catch<std::runtime_error>([=]() {
419 * if (badThingHappens()) {
420 * throw std::exception();
425 template <typename... Exceptions>
428 template <typename LastException, typename... Exceptions>
429 class try_and_catch<LastException, Exceptions...> :
430 public try_and_catch<Exceptions...> {
432 template <typename F>
433 explicit try_and_catch(F&& fn) : Base() {
438 typedef try_and_catch<Exceptions...> Base;
440 try_and_catch() : Base() {}
442 template <typename Ex>
443 typename std::enable_if<!exception_wrapper::optimize<Ex>::value>::type
444 assign_exception(Ex& e, std::exception_ptr eptr) {
445 exception_wrapper::assign_eptr(eptr, e);
448 template <typename Ex>
449 typename std::enable_if<exception_wrapper::optimize<Ex>::value>::type
450 assign_exception(Ex& e, std::exception_ptr /*eptr*/) {
451 this->item_ = std::make_shared<Ex>(e);
452 this->throwfn_ = folly::detail::Thrower<Ex>::doThrow;
455 template <typename F>
456 void call_fn(F&& fn) {
458 Base::call_fn(std::move(fn));
459 } catch (LastException& e) {
460 if (typeid(e) == typeid(LastException&)) {
461 assign_exception(e, std::current_exception());
463 exception_wrapper::assign_eptr(std::current_exception(), e);
470 class try_and_catch<> : public exception_wrapper {
472 try_and_catch() = default;
475 template <typename F>
476 void call_fn(F&& fn) {