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 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<Ex>::value>::type>
116 /* implicit */ exception_wrapper(Ex&& exn) {
117 item_ = std::make_shared<Ex>(std::forward<Ex>(exn));
118 throwfn_ = folly::detail::Thrower<Ex>::doThrow;
121 // The following two constructors are meant to emulate the behavior of
122 // try_and_catch in performance sensitive code as well as to be flexible
123 // enough to wrap exceptions of unknown type. There is an overload that
124 // takes an exception reference so that the wrapper can extract and store
125 // the exception's type and what() when possible.
127 // The canonical use case is to construct an all-catching exception wrapper
128 // with minimal overhead like so:
131 // // some throwing code
132 // } catch (const std::exception& e) {
133 // // won't lose e's type and what()
134 // exception_wrapper ew{std::current_exception(), e};
136 // // everything else
137 // exception_wrapper ew{std::current_exception()};
140 // try_and_catch is cleaner and preferable. Use it unless you're sure you need
141 // something like this instead.
142 template <typename Ex>
143 explicit exception_wrapper(std::exception_ptr eptr, Ex& exn) {
144 assign_eptr(eptr, exn);
147 explicit exception_wrapper(std::exception_ptr eptr) {
151 void throwException() const {
153 throwfn_(item_.get());
155 std::rethrow_exception(eptr_);
159 explicit operator bool() const {
160 return item_ || eptr_;
163 // This implementation is similar to std::exception_ptr's implementation
164 // where two exception_wrappers are equal when the address in the underlying
165 // reference field both point to the same exception object. The reference
166 // field remains the same when the exception_wrapper is copied or when
167 // the exception_wrapper is "rethrown".
168 bool operator==(const exception_wrapper& a) const {
170 return a.item_ && item_.get() == a.item_.get();
172 return eptr_ == a.eptr_;
176 bool operator!=(const exception_wrapper& a) const {
177 return !(*this == a);
180 // This will return a non-nullptr only if the exception is held as a
181 // copy. It is the only interface which will distinguish between an
182 // exception held this way, and by exception_ptr. You probably
183 // shouldn't use it at all.
184 std::exception* getCopied() { return item_.get(); }
185 const std::exception* getCopied() const { return item_.get(); }
187 fbstring what() const {
189 return exceptionStr(*item_);
197 fbstring class_name() const {
199 return demangle(typeid(*item_));
208 bool is_compatible_with() const {
210 return dynamic_cast<const Ex*>(item_.get());
213 std::rethrow_exception(eptr_);
214 } catch (std::exception& e) {
215 return dynamic_cast<const Ex*>(&e);
223 // If this exception wrapper wraps an exception of type Ex, with_exception
224 // will call f with the wrapped exception as an argument and return true, and
225 // will otherwise return false.
226 template <class Ex, class F>
227 typename std::enable_if<
228 std::is_base_of<std::exception, typename std::decay<Ex>::type>::value,
230 with_exception(F f) {
231 return with_exception1<typename std::decay<Ex>::type>(f, this);
235 template <class Ex, class F>
236 typename std::enable_if<
237 std::is_base_of<std::exception, typename std::decay<Ex>::type>::value,
239 with_exception(F f) const {
240 return with_exception1<const typename std::decay<Ex>::type>(f, this);
243 // Overload for non-exceptions. Always rethrows.
244 template <class Ex, class F>
245 typename std::enable_if<
246 !std::is_base_of<std::exception, typename std::decay<Ex>::type>::value,
248 with_exception(F f) const {
251 } catch (typename std::decay<Ex>::type& e) {
260 std::exception_ptr getExceptionPtr() const {
268 return std::current_exception();
270 return std::exception_ptr();
274 template <typename Ex>
276 static const bool value =
277 std::is_base_of<std::exception, Ex>::value &&
278 std::is_copy_assignable<Ex>::value &&
279 !std::is_abstract<Ex>::value;
282 template <typename Ex>
283 void assign_eptr(std::exception_ptr eptr, Ex& e) {
285 this->estr_ = exceptionStr(e).toStdString();
286 this->ename_ = demangle(typeid(e)).toStdString();
289 void assign_eptr(std::exception_ptr eptr) {
293 // Optimized case: if we know what type the exception is, we can
294 // store a copy of the concrete type, and a helper function so we
296 std::shared_ptr<std::exception> item_;
297 void (*throwfn_)(std::exception*){nullptr};
298 // Fallback case: store the library wrapper, which is less efficient
299 // but gets the job done. Also store exceptionPtr() the name of the
300 // exception type, so we can at least get those back out without
301 // having to rethrow.
302 std::exception_ptr eptr_;
306 template <class T, class... Args>
307 friend exception_wrapper make_exception_wrapper(Args&&... args);
310 // What makes this useful is that T can be exception_wrapper* or
311 // const exception_wrapper*, and the compiler will use the
312 // instantiation which works with F.
313 template <class Ex, class F, class T>
314 static bool with_exception1(F f, T* that) {
316 if (auto ex = dynamic_cast<Ex*>(that->item_.get())) {
320 } else if (that->eptr_) {
322 std::rethrow_exception(that->eptr_);
323 } catch (std::exception& e) {
324 if (auto ex = dynamic_cast<Ex*>(&e)) {
336 template <class T, class... Args>
337 exception_wrapper make_exception_wrapper(Args&&... args) {
338 exception_wrapper ew;
339 ew.item_ = std::make_shared<T>(std::forward<Args>(args)...);
340 ew.throwfn_ = folly::detail::Thrower<T>::doThrow;
345 * try_and_catch is a simple replacement for try {} catch(){} that allows you to
346 * specify which derived exceptions you would like to catch and store in an
349 * Because we cannot build an equivalent of std::current_exception(), we need
350 * to catch every derived exception that we are interested in catching.
352 * Exceptions should be listed in the reverse order that you would write your
353 * catch statements (that is, std::exception& should be first).
355 * NOTE: Although implemented as a derived class (for syntactic delight), don't
356 * be confused - you should not pass around try_and_catch objects!
360 * // This catches my runtime_error and if I call throwException() on ew, it
361 * // will throw a runtime_error
362 * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
363 * if (badThingHappens()) {
364 * throw std::runtime_error("ZOMG!");
368 * // This will catch the exception and if I call throwException() on ew, it
369 * // will throw a std::exception
370 * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
371 * if (badThingHappens()) {
372 * throw std::exception();
376 * // This will not catch the exception and it will be thrown.
377 * auto ew = folly::try_and_catch<std::runtime_error>([=]() {
378 * if (badThingHappens()) {
379 * throw std::exception();
384 template <typename... Exceptions>
387 template <typename LastException, typename... Exceptions>
388 class try_and_catch<LastException, Exceptions...> :
389 public try_and_catch<Exceptions...> {
391 template <typename F>
392 explicit try_and_catch(F&& fn) : Base() {
397 typedef try_and_catch<Exceptions...> Base;
399 try_and_catch() : Base() {}
401 template <typename Ex>
402 typename std::enable_if<!exception_wrapper::optimize<Ex>::value>::type
403 assign_exception(Ex& e, std::exception_ptr eptr) {
404 exception_wrapper::assign_eptr(eptr, e);
407 template <typename Ex>
408 typename std::enable_if<exception_wrapper::optimize<Ex>::value>::type
409 assign_exception(Ex& e, std::exception_ptr eptr) {
410 this->item_ = std::make_shared<Ex>(e);
411 this->throwfn_ = folly::detail::Thrower<Ex>::doThrow;
414 template <typename F>
415 void call_fn(F&& fn) {
417 Base::call_fn(std::move(fn));
418 } catch (LastException& e) {
419 if (typeid(e) == typeid(LastException&)) {
420 assign_exception(e, std::current_exception());
422 exception_wrapper::assign_eptr(std::current_exception(), e);
429 class try_and_catch<> : public exception_wrapper {
434 template <typename F>
435 void call_fn(F&& fn) {