2 * Copyright 2015 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 #ifndef FOLLY_EXCEPTIONWRAPPER_H
18 #define FOLLY_EXCEPTIONWRAPPER_H
23 #include <folly/String.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 * auto ep = globalExceptionWrapper.get();
92 * if (!ep.with_exception<FacePlantException>([&](
93 * FacePlantException& faceplant) {
94 * LOG(ERROR) << "FACEPLANT";
96 * ep.with_exception<FailWhaleException>([&](
97 * FailWhaleException& failwhale) {
98 * LOG(ERROR) << "FAILWHALE!";
104 class exception_wrapper {
106 template <typename Ex>
110 exception_wrapper() = default;
112 // Implicitly construct an exception_wrapper from a qualifying exception.
113 // See the optimize struct for details.
114 template <typename Ex, typename =
115 typename std::enable_if<optimize<typename std::decay<Ex>::type>::value>
117 /* implicit */ exception_wrapper(Ex&& exn) {
118 typedef typename std::decay<Ex>::type DEx;
119 item_ = std::make_shared<DEx>(std::forward<Ex>(exn));
120 throwfn_ = folly::detail::Thrower<DEx>::doThrow;
123 // The following two constructors are meant to emulate the behavior of
124 // try_and_catch in performance sensitive code as well as to be flexible
125 // enough to wrap exceptions of unknown type. There is an overload that
126 // takes an exception reference so that the wrapper can extract and store
127 // the exception's type and what() when possible.
129 // The canonical use case is to construct an all-catching exception wrapper
130 // with minimal overhead like so:
133 // // some throwing code
134 // } catch (const std::exception& e) {
135 // // won't lose e's type and what()
136 // exception_wrapper ew{std::current_exception(), e};
138 // // everything else
139 // exception_wrapper ew{std::current_exception()};
142 // try_and_catch is cleaner and preferable. Use it unless you're sure you need
143 // something like this instead.
144 template <typename Ex>
145 explicit exception_wrapper(std::exception_ptr eptr, Ex& exn) {
146 assign_eptr(eptr, exn);
149 explicit exception_wrapper(std::exception_ptr eptr) {
153 void throwException() const {
155 throwfn_(item_.get());
157 std::rethrow_exception(eptr_);
161 explicit operator bool() const {
162 return item_ || eptr_;
165 // This implementation is similar to std::exception_ptr's implementation
166 // where two exception_wrappers are equal when the address in the underlying
167 // reference field both point to the same exception object. The reference
168 // field remains the same when the exception_wrapper is copied or when
169 // the exception_wrapper is "rethrown".
170 bool operator==(const exception_wrapper& a) const {
172 return a.item_ && item_.get() == a.item_.get();
174 return eptr_ == a.eptr_;
178 bool operator!=(const exception_wrapper& a) const {
179 return !(*this == a);
182 // This will return a non-nullptr only if the exception is held as a
183 // copy. It is the only interface which will distinguish between an
184 // exception held this way, and by exception_ptr. You probably
185 // shouldn't use it at all.
186 std::exception* getCopied() { return item_.get(); }
187 const std::exception* getCopied() const { return item_.get(); }
189 fbstring what() const {
191 return exceptionStr(*item_);
199 fbstring class_name() const {
202 return demangle(typeid(i));
211 bool is_compatible_with() const {
213 return dynamic_cast<const Ex*>(item_.get());
216 std::rethrow_exception(eptr_);
217 } catch (std::exception& e) {
218 return dynamic_cast<const Ex*>(&e);
226 // If this exception wrapper wraps an exception of type Ex, with_exception
227 // will call f with the wrapped exception as an argument and return true, and
228 // will otherwise return false.
229 template <class Ex, class F>
230 typename std::enable_if<
231 std::is_base_of<std::exception, typename std::decay<Ex>::type>::value,
233 with_exception(F f) {
234 return with_exception1<typename std::decay<Ex>::type>(f, this);
238 template <class Ex, class F>
239 typename std::enable_if<
240 std::is_base_of<std::exception, typename std::decay<Ex>::type>::value,
242 with_exception(F f) const {
243 return with_exception1<const typename std::decay<Ex>::type>(f, this);
246 // Overload for non-exceptions. Always rethrows.
247 template <class Ex, class F>
248 typename std::enable_if<
249 !std::is_base_of<std::exception, typename std::decay<Ex>::type>::value,
251 with_exception(F f) const {
254 } catch (typename std::decay<Ex>::type& e) {
263 std::exception_ptr getExceptionPtr() const {
271 return std::current_exception();
273 return std::exception_ptr();
277 template <typename Ex>
279 static const bool value =
280 std::is_base_of<std::exception, Ex>::value &&
281 std::is_copy_assignable<Ex>::value &&
282 !std::is_abstract<Ex>::value;
285 template <typename Ex>
286 void assign_eptr(std::exception_ptr eptr, Ex& e) {
288 this->estr_ = exceptionStr(e).toStdString();
289 this->ename_ = demangle(typeid(e)).toStdString();
292 void assign_eptr(std::exception_ptr eptr) {
296 // Optimized case: if we know what type the exception is, we can
297 // store a copy of the concrete type, and a helper function so we
299 std::shared_ptr<std::exception> item_;
300 void (*throwfn_)(std::exception*){nullptr};
301 // Fallback case: store the library wrapper, which is less efficient
302 // but gets the job done. Also store exceptionPtr() the name of the
303 // exception type, so we can at least get those back out without
304 // having to rethrow.
305 std::exception_ptr eptr_;
309 template <class T, class... Args>
310 friend exception_wrapper make_exception_wrapper(Args&&... args);
313 // What makes this useful is that T can be exception_wrapper* or
314 // const exception_wrapper*, and the compiler will use the
315 // instantiation which works with F.
316 template <class Ex, class F, class T>
317 static bool with_exception1(F f, T* that) {
319 if (auto ex = dynamic_cast<Ex*>(that->item_.get())) {
323 } else if (that->eptr_) {
325 std::rethrow_exception(that->eptr_);
326 } catch (std::exception& e) {
327 if (auto ex = dynamic_cast<Ex*>(&e)) {
339 template <class T, class... Args>
340 exception_wrapper make_exception_wrapper(Args&&... args) {
341 exception_wrapper ew;
342 ew.item_ = std::make_shared<T>(std::forward<Args>(args)...);
343 ew.throwfn_ = folly::detail::Thrower<T>::doThrow;
347 // For consistency with exceptionStr() functions in String.h
348 inline fbstring exceptionStr(const exception_wrapper& ew) {
353 * try_and_catch is a simple replacement for try {} catch(){} that allows you to
354 * specify which derived exceptions you would like to catch and store in an
357 * Because we cannot build an equivalent of std::current_exception(), we need
358 * to catch every derived exception that we are interested in catching.
360 * Exceptions should be listed in the reverse order that you would write your
361 * catch statements (that is, std::exception& should be first).
363 * NOTE: Although implemented as a derived class (for syntactic delight), don't
364 * be confused - you should not pass around try_and_catch objects!
368 * // This catches my runtime_error and if I call throwException() on ew, it
369 * // will throw a runtime_error
370 * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
371 * if (badThingHappens()) {
372 * throw std::runtime_error("ZOMG!");
376 * // This will catch the exception and if I call throwException() on ew, it
377 * // will throw a std::exception
378 * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
379 * if (badThingHappens()) {
380 * throw std::exception();
384 * // This will not catch the exception and it will be thrown.
385 * auto ew = folly::try_and_catch<std::runtime_error>([=]() {
386 * if (badThingHappens()) {
387 * throw std::exception();
392 template <typename... Exceptions>
395 template <typename LastException, typename... Exceptions>
396 class try_and_catch<LastException, Exceptions...> :
397 public try_and_catch<Exceptions...> {
399 template <typename F>
400 explicit try_and_catch(F&& fn) : Base() {
405 typedef try_and_catch<Exceptions...> Base;
407 try_and_catch() : Base() {}
409 template <typename Ex>
410 typename std::enable_if<!exception_wrapper::optimize<Ex>::value>::type
411 assign_exception(Ex& e, std::exception_ptr eptr) {
412 exception_wrapper::assign_eptr(eptr, e);
415 template <typename Ex>
416 typename std::enable_if<exception_wrapper::optimize<Ex>::value>::type
417 assign_exception(Ex& e, std::exception_ptr /*eptr*/) {
418 this->item_ = std::make_shared<Ex>(e);
419 this->throwfn_ = folly::detail::Thrower<Ex>::doThrow;
422 template <typename F>
423 void call_fn(F&& fn) {
425 Base::call_fn(std::move(fn));
426 } catch (LastException& e) {
427 if (typeid(e) == typeid(LastException&)) {
428 assign_exception(e, std::current_exception());
430 exception_wrapper::assign_eptr(std::current_exception(), e);
437 class try_and_catch<> : public exception_wrapper {
439 try_and_catch() = default;
442 template <typename F>
443 void call_fn(F&& fn) {