2 * Copyright 2014 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 exception_wrapper() : throwfn_(nullptr) { }
108 void throwException() const {
110 throwfn_(item_.get());
112 std::rethrow_exception(eptr_);
116 explicit operator bool() const {
117 return item_ || eptr_;
120 // This will return a non-nullptr only if the exception is held as a
121 // copy. It is the only interface which will distinguish between an
122 // exception held this way, and by exception_ptr. You probably
123 // shouldn't use it at all.
124 std::exception* getCopied() { return item_.get(); }
125 const std::exception* getCopied() const { return item_.get(); }
127 fbstring what() const {
129 return exceptionStr(*item_.get());
138 bool is_compatible_with() const {
140 return dynamic_cast<const Ex*>(getCopied());
143 std::rethrow_exception(eptr_);
144 } catch (std::exception& e) {
145 return dynamic_cast<const Ex*>(&e);
153 template <class Ex, class F>
154 bool with_exception(F f) {
156 if (auto ex = dynamic_cast<Ex*>(getCopied())) {
162 std::rethrow_exception(eptr_);
163 } catch (std::exception& e) {
164 if (auto ex = dynamic_cast<Ex*>(&e)) {
175 template <class Ex, class F>
176 bool with_exception(F f) const {
177 return with_exception<const Ex>(f);
180 std::exception_ptr getExceptionPtr() const {
188 return std::current_exception();
190 return std::exception_ptr();
194 // Optimized case: if we know what type the exception is, we can
195 // store a copy of the concrete type, and a helper function so we
197 std::shared_ptr<std::exception> item_;
198 void (*throwfn_)(std::exception*);
199 // Fallback case: store the library wrapper, which is less efficient
200 // but gets the job done. Also store the the what() string, so we
201 // can at least get it back out without having to rethrow.
202 std::exception_ptr eptr_;
205 template <class T, class... Args>
206 friend exception_wrapper make_exception_wrapper(Args&&... args);
209 template <class T, class... Args>
210 exception_wrapper make_exception_wrapper(Args&&... args) {
211 exception_wrapper ew;
212 ew.item_ = std::make_shared<T>(std::forward<Args>(args)...);
213 ew.throwfn_ = folly::detail::Thrower<T>::doThrow;
218 * try_and_catch is a simple replacement for try {} catch(){} that allows you to
219 * specify which derived exceptions you would like to catch and store in an
222 * Because we cannot build an equivalent of std::current_exception(), we need
223 * to catch every derived exception that we are interested in catching.
225 * Exceptions should be listed in the reverse order that you would write your
226 * catch statements (that is, std::exception& should be first).
228 * NOTE: Although implemented as a derived class (for syntactic delight), don't
229 * be confused - you should not pass around try_and_catch objects!
233 * // This catches my runtime_error and if I call throwException() on ew, it
234 * // will throw a runtime_error
235 * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
236 * if (badThingHappens()) {
237 * throw std::runtime_error("ZOMG!");
241 * // This will catch the exception and if I call throwException() on ew, it
242 * // will throw a std::exception
243 * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
244 * if (badThingHappens()) {
245 * throw std::exception();
249 * // This will not catch the exception and it will be thrown.
250 * auto ew = folly::try_and_catch<std::runtime_error>([=]() {
251 * if (badThingHappens()) {
252 * throw std::exception();
257 template <typename... Exceptions>
260 template <typename LastException, typename... Exceptions>
261 class try_and_catch<LastException, Exceptions...> :
262 public try_and_catch<Exceptions...> {
264 template <typename F>
265 explicit try_and_catch(F&& fn) : Base() {
270 typedef try_and_catch<Exceptions...> Base;
272 try_and_catch() : Base() {}
274 template <typename Ex>
275 typename std::enable_if<std::is_base_of<std::exception, Ex>::value>::type
277 this->eptr_ = std::current_exception();
278 this->estr_ = exceptionStr(e).toStdString();
281 template <typename Ex>
282 typename std::enable_if<!std::is_base_of<std::exception, Ex>::value>::type
284 this->eptr_ = std::current_exception();
285 this->estr_ = exceptionStr(e).toStdString();
288 template <typename Ex>
290 static const bool value =
291 std::is_base_of<std::exception, Ex>::value &&
292 std::is_copy_assignable<Ex>::value &&
293 !std::is_abstract<Ex>::value;
296 template <typename Ex>
297 typename std::enable_if<!optimize<Ex>::value>::type
298 assign_exception(Ex& e) {
302 template <typename Ex>
303 typename std::enable_if<optimize<Ex>::value>::type
304 assign_exception(Ex& e) {
305 this->item_ = std::make_shared<Ex>(e);
306 this->throwfn_ = folly::detail::Thrower<Ex>::doThrow;
309 template <typename F>
310 void call_fn(F&& fn) {
312 Base::call_fn(std::move(fn));
313 } catch (LastException& e) {
314 if (typeid(e) == typeid(LastException&)) {
324 class try_and_catch<> : public exception_wrapper {
329 template <typename F>
330 void call_fn(F&& fn) {