X-Git-Url: http://plrg.eecs.uci.edu/git/?p=folly.git;a=blobdiff_plain;f=folly%2FExceptionWrapper.h;h=0d028714231fa5ad6fde57055c41b6734b13356f;hp=953ce4d479b1ec0127e9d2927b7d8226cd79daa6;hb=48d3f5e1691033d00ecc2ef0c3ced83ce148705a;hpb=9d4d4b000cc218ee0c9c430257222dd63ad766ff

diff --git a/folly/ExceptionWrapper.h b/folly/ExceptionWrapper.h
index 953ce4d4..0d028714 100644
--- a/folly/ExceptionWrapper.h
+++ b/folly/ExceptionWrapper.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2014 Facebook, Inc.
+ * Copyright 2017-present Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -13,349 +13,686 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/*
+ * Author: Eric Niebler <eniebler@fb.com>
+ */
 
-#ifndef FOLLY_EXCEPTIONWRAPPER_H
-#define FOLLY_EXCEPTIONWRAPPER_H
+#pragma once
 
 #include <cassert>
+#include <cstdint>
 #include <exception>
+#include <iosfwd>
 #include <memory>
-#include <folly/String.h>
-#include <folly/detail/ExceptionWrapper.h>
+#include <new>
+#include <type_traits>
+#include <typeinfo>
+#include <utility>
+
+#include <folly/CPortability.h>
+#include <folly/Demangle.h>
+#include <folly/ExceptionString.h>
+#include <folly/FBString.h>
+#include <folly/Portability.h>
+#include <folly/Traits.h>
+#include <folly/Utility.h>
+#include <folly/lang/Assume.h>
+
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpragmas"
+#pragma GCC diagnostic ignored "-Wpotentially-evaluated-expression"
+// GCC gets confused about lambda scopes and issues shadow-local warnings for
+// parameters in totally different functions.
+FOLLY_GCC_DISABLE_NEW_SHADOW_WARNINGS
+#endif
+
+#define FOLLY_EXCEPTION_WRAPPER_H_INCLUDED
 
 namespace folly {
 
-/*
- * Throwing exceptions can be a convenient way to handle errors. Storing
- * exceptions in an exception_ptr makes it easy to handle exceptions in a
- * different thread or at a later time. exception_ptr can also be used in a very
- * generic result/exception wrapper.
- *
- * However, there are some issues with throwing exceptions and
- * std::exception_ptr. These issues revolve around throw being expensive,
- * particularly in a multithreaded environment (see
- * ExceptionWrapperBenchmark.cpp).
- *
- * Imagine we have a library that has an API which returns a result/exception
- * wrapper. Let's consider some approaches for implementing this wrapper.
- * First, we could store a std::exception. This approach loses the derived
- * exception type, which can make exception handling more difficult for users
- * that prefer rethrowing the exception. We could use a folly::dynamic for every
- * possible type of exception. This is not very flexible - adding new types of
- * exceptions requires a change to the result/exception wrapper. We could use an
- * exception_ptr. However, constructing an exception_ptr as well as accessing
- * the error requires a call to throw. That means that there will be two calls
- * to throw in order to process the exception. For performance sensitive
- * applications, this may be unacceptable.
- *
- * exception_wrapper is designed to handle exception management for both
- * convenience and high performance use cases. make_exception_wrapper is
- * templated on derived type, allowing us to rethrow the exception properly for
- * users that prefer convenience. These explicitly named exception types can
- * therefore be handled without any peformance penalty.  exception_wrapper is
- * also flexible enough to accept any type. If a caught exception is not of an
- * explicitly named type, then std::exception_ptr is used to preserve the
- * exception state. For performance sensitive applications, the accessor methods
- * can test or extract a pointer to a specific exception type with very little
- * overhead.
- *
- * Example usage:
- *
- * exception_wrapper globalExceptionWrapper;
- *
- * // Thread1
- * void doSomethingCrazy() {
- *   int rc = doSomethingCrazyWithLameReturnCodes();
- *   if (rc == NAILED_IT) {
- *     globalExceptionWrapper = exception_wrapper();
- *   } else if (rc == FACE_PLANT) {
- *     globalExceptionWrapper = make_exception_wrapper<FacePlantException>();
- *   } else if (rc == FAIL_WHALE) {
- *     globalExceptionWrapper = make_exception_wrapper<FailWhaleException>();
- *   }
- * }
- *
- * // Thread2: Exceptions are ok!
- * void processResult() {
- *   try {
- *     globalExceptionWrapper.throwException();
- *   } catch (const FacePlantException& e) {
- *     LOG(ERROR) << "FACEPLANT!";
- *   } catch (const FailWhaleException& e) {
- *     LOG(ERROR) << "FAILWHALE!";
- *   }
- * }
- *
- * // Thread2: Exceptions are bad!
- * void processResult() {
- *   auto ep = globalExceptionWrapper.get();
- *   if (!ep.with_exception<FacePlantException>([&](
- *     FacePlantException& faceplant) {
- *       LOG(ERROR) << "FACEPLANT";
- *     })) {
- *     ep.with_exception<FailWhaleException>([&](
- *       FailWhaleException& failwhale) {
- *         LOG(ERROR) << "FAILWHALE!";
- *       });
- *   }
- * }
- *
- */
-class exception_wrapper {
- public:
-  exception_wrapper() : throwfn_(nullptr) { }
-
-  void throwException() const {
-    if (throwfn_) {
-      throwfn_(item_.get());
-    } else if (eptr_) {
-      std::rethrow_exception(eptr_);
-    }
-  }
+#define FOLLY_REQUIRES_DEF(...) \
+  _t<std::enable_if<static_cast<bool>(__VA_ARGS__), long>>
 
-  explicit operator bool() const {
-    return item_ || eptr_;
-  }
+#define FOLLY_REQUIRES(...) FOLLY_REQUIRES_DEF(__VA_ARGS__) = __LINE__
 
-  // This implementation is similar to std::exception_ptr's implementation
-  // where two exception_wrappers are equal when the address in the underlying
-  // reference field both point to the same exception object.  The reference
-  // field remains the same when the exception_wrapper is copied or when
-  // the exception_wrapper is "rethrown".
-  bool operator==(const exception_wrapper& a) const {
-    if (item_) {
-      return a.item_ && item_.get() == a.item_.get();
-    } else {
-      return eptr_ == a.eptr_;
-    }
-  }
+namespace exception_wrapper_detail {
 
-  bool operator!=(const exception_wrapper& a) const {
-    return !(*this == a);
-  }
+template <template <class> class T, class... As>
+using AllOf = StrictConjunction<T<As>...>;
 
-  // This will return a non-nullptr only if the exception is held as a
-  // copy.  It is the only interface which will distinguish between an
-  // exception held this way, and by exception_ptr.  You probably
-  // shouldn't use it at all.
-  std::exception* getCopied() { return item_.get(); }
-  const std::exception* getCopied() const { return item_.get(); }
-
-  fbstring what() const {
-    if (item_) {
-      return exceptionStr(*item_.get());
-    } else if (eptr_) {
-      return estr_;
-    } else {
-      return fbstring();
-    }
-  }
+template <bool If, class T>
+using AddConstIf = _t<std::conditional<If, const T, T>>;
+
+template <class Fn, class A>
+FOLLY_ALWAYS_INLINE FOLLY_ATTR_VISIBILITY_HIDDEN
+auto fold(Fn&&, A&& a) {
+  return static_cast<A&&>(a);
+}
+
+template <class Fn, class A, class B, class... Bs>
+FOLLY_ALWAYS_INLINE FOLLY_ATTR_VISIBILITY_HIDDEN
+auto fold(Fn&& fn, A&& a, B&& b, Bs&&... bs) {
+  return fold(
+      // This looks like a use of fn after a move of fn, but in reality, this is
+      // just a cast and not a move. That's because regardless of which fold
+      // overload is selected, fn gets bound to a &&. Had fold taken fn by value
+      // there would indeed be a problem here.
+      static_cast<Fn&&>(fn),
+      static_cast<Fn&&>(fn)(static_cast<A&&>(a), static_cast<B&&>(b)),
+      static_cast<Bs&&>(bs)...);
+}
+
+} // namespace exception_wrapper_detail
+
+//! Throwing exceptions can be a convenient way to handle errors. Storing
+//! exceptions in an `exception_ptr` makes it easy to handle exceptions in a
+//! different thread or at a later time. `exception_ptr` can also be used in a
+//! very generic result/exception wrapper.
+//!
+//! However, there are some issues with throwing exceptions and
+//! `std::exception_ptr`. These issues revolve around `throw` being expensive,
+//! particularly in a multithreaded environment (see
+//! ExceptionWrapperBenchmark.cpp).
+//!
+//! Imagine we have a library that has an API which returns a result/exception
+//! wrapper. Let's consider some approaches for implementing this wrapper.
+//! First, we could store a `std::exception`. This approach loses the derived
+//! exception type, which can make exception handling more difficult for users
+//! that prefer rethrowing the exception. We could use a `folly::dynamic` for
+//! every possible type of exception. This is not very flexible - adding new
+//! types of exceptions requires a change to the result/exception wrapper. We
+//! could use an `exception_ptr`. However, constructing an `exception_ptr` as
+//! well as accessing the error requires a call to throw. That means that there
+//! will be two calls to throw in order to process the exception. For
+//! performance sensitive applications, this may be unacceptable.
+//!
+//! `exception_wrapper` is designed to handle exception management for both
+//! convenience and high performance use cases. `make_exception_wrapper` is
+//! templated on derived type, allowing us to rethrow the exception properly for
+//! users that prefer convenience. These explicitly named exception types can
+//! therefore be handled without any peformance penalty. `exception_wrapper` is
+//! also flexible enough to accept any type. If a caught exception is not of an
+//! explicitly named type, then `std::exception_ptr` is used to preserve the
+//! exception state. For performance sensitive applications, the accessor
+//! methods can test or extract a pointer to a specific exception type with very
+//! little overhead.
+//!
+//! \par Example usage:
+//! \par
+//! \code
+//! exception_wrapper globalExceptionWrapper;
+//!
+//! // Thread1
+//! void doSomethingCrazy() {
+//!   int rc = doSomethingCrazyWithLameReturnCodes();
+//!   if (rc == NAILED_IT) {
+//!     globalExceptionWrapper = exception_wrapper();
+//!   } else if (rc == FACE_PLANT) {
+//!     globalExceptionWrapper = make_exception_wrapper<FacePlantException>();
+//!   } else if (rc == FAIL_WHALE) {
+//!     globalExceptionWrapper = make_exception_wrapper<FailWhaleException>();
+//!   }
+//! }
+//!
+//! // Thread2: Exceptions are ok!
+//! void processResult() {
+//!   try {
+//!     globalExceptionWrapper.throw_exception();
+//!   } catch (const FacePlantException& e) {
+//!     LOG(ERROR) << "FACEPLANT!";
+//!   } catch (const FailWhaleException& e) {
+//!     LOG(ERROR) << "FAILWHALE!";
+//!   }
+//! }
+//!
+//! // Thread2: Exceptions are bad!
+//! void processResult() {
+//!   globalExceptionWrapper.handle(
+//!       [&](FacePlantException& faceplant) {
+//!         LOG(ERROR) << "FACEPLANT";
+//!       },
+//!       [&](FailWhaleException& failwhale) {
+//!         LOG(ERROR) << "FAILWHALE!";
+//!       },
+//!       [](...) {
+//!         LOG(FATAL) << "Unrecognized exception";
+//!       });
+//! }
+//! \endcode
+class exception_wrapper final {
+ private:
+  struct AnyException : std::exception {
+    std::type_info const* typeinfo_;
+    template <class T>
+    /* implicit */ AnyException(T&& t) noexcept : typeinfo_(&typeid(t)) {}
+  };
+
+  template <class Fn>
+  struct arg_type_;
+  template <class Fn>
+  using arg_type = _t<arg_type_<Fn>>;
+
+  // exception_wrapper is implemented as a simple variant over four
+  // different representations:
+  //  0. Empty, no exception.
+  //  1. An small object stored in-situ.
+  //  2. A larger object stored on the heap and referenced with a
+  //     std::shared_ptr.
+  //  3. A std::exception_ptr, together with either:
+  //       a. A pointer to the referenced std::exception object, or
+  //       b. A pointer to a std::type_info object for the referenced exception,
+  //          or for an unspecified type if the type is unknown.
+  // This is accomplished with the help of a union and a pointer to a hand-
+  // rolled virtual table. This virtual table contains pointers to functions
+  // that know which field of the union is active and do the proper action.
+  // The class invariant ensures that the vtable ptr and the union stay in sync.
+  struct VTable {
+    void (*copy_)(exception_wrapper const*, exception_wrapper*);
+    void (*move_)(exception_wrapper*, exception_wrapper*);
+    void (*delete_)(exception_wrapper*);
+    void (*throw_)(exception_wrapper const*);
+    std::type_info const* (*type_)(exception_wrapper const*);
+    std::exception const* (*get_exception_)(exception_wrapper const*);
+    exception_wrapper (*get_exception_ptr_)(exception_wrapper const*);
+  };
+
+  [[noreturn]] static void onNoExceptionError(char const* name);
+
+  template <class Ret, class... Args>
+  static Ret noop_(Args...);
+
+  static std::type_info const* uninit_type_(exception_wrapper const*);
+
+  static VTable const uninit_;
 
   template <class Ex>
-  bool is_compatible_with() const {
-    if (item_) {
-      return dynamic_cast<const Ex*>(getCopied());
-    } else if (eptr_) {
-      try {
-        std::rethrow_exception(eptr_);
-      } catch (std::exception& e) {
-        return dynamic_cast<const Ex*>(&e);
-      } catch (...) {
-        // fall through
-      }
-    }
-    return false;
-  }
+  using IsStdException = std::is_base_of<std::exception, _t<std::decay<Ex>>>;
+  template <bool B, class T>
+  using AddConstIf = exception_wrapper_detail::AddConstIf<B, T>;
+  template <class CatchFn>
+  using IsCatchAll =
+      std::is_same<arg_type<_t<std::decay<CatchFn>>>, AnyException>;
+
+  struct Unknown {};
+
+  // Sadly, with the gcc-4.9 platform, std::logic_error and std::runtime_error
+  // do not fit here. They also don't have noexcept copy-ctors, so the internal
+  // storage wouldn't be used anyway. For the gcc-5 platform, both logic_error
+  // and runtime_error can be safely stored internally.
+  struct Buffer {
+    using Storage =
+        _t<std::aligned_storage<2 * sizeof(void*), alignof(std::exception)>>;
+    Storage buff_;
+
+    Buffer() : buff_{} {}
+
+    template <class Ex, typename... As>
+    Buffer(in_place_type_t<Ex>, As&&... as_);
+    template <class Ex>
+    Ex& as() noexcept;
+    template <class Ex>
+    Ex const& as() const noexcept;
+  };
 
-  template <class Ex, class F>
-  bool with_exception(F f) {
-    return with_exception1<Ex>(f, this);
-  }
+  enum class Placement { kInSitu, kOnHeap };
+  template <class T>
+  using PlacementOf = std::integral_constant<
+      Placement,
+      sizeof(T) <= sizeof(Buffer::Storage) &&
+              alignof(T) <= alignof(Buffer::Storage) &&
+              noexcept(T(std::declval<T&&>()))
+          ? Placement::kInSitu
+          : Placement::kOnHeap>;
+
+  using InSituTag = std::integral_constant<Placement, Placement::kInSitu>;
+  using OnHeapTag = std::integral_constant<Placement, Placement::kOnHeap>;
+
+  static std::exception const* as_exception_or_null_(std::exception const& ex);
+  static std::exception const* as_exception_or_null_(AnyException);
+
+  struct ExceptionPtr {
+    std::exception_ptr ptr_;
+    std::uintptr_t exception_or_type_; // odd for type_info
+    static_assert(
+        1 < alignof(std::exception) && 1 < alignof(std::type_info),
+        "Surprise! std::exception and std::type_info don't have alignment "
+        "greater than one. as_int_ below will not work!");
+
+    static std::uintptr_t as_int_(
+        std::exception_ptr const& ptr,
+        std::exception const& e);
+    static std::uintptr_t as_int_(
+        std::exception_ptr const& ptr,
+        AnyException e);
+    bool has_exception_() const;
+    std::exception const* as_exception_() const;
+    std::type_info const* as_type_() const;
+    static void copy_(exception_wrapper const* from, exception_wrapper* to);
+    static void move_(exception_wrapper* from, exception_wrapper* to);
+    static void delete_(exception_wrapper* that);
+    [[noreturn]] static void throw_(exception_wrapper const* that);
+    static std::type_info const* type_(exception_wrapper const* that);
+    static std::exception const* get_exception_(exception_wrapper const* that);
+    static exception_wrapper get_exception_ptr_(exception_wrapper const* that);
+    static VTable const ops_;
+  };
 
-  template <class Ex, class F>
-  bool with_exception(F f) const {
-    return with_exception1<const Ex>(f, this);
-  }
+  template <class Ex>
+  struct InPlace {
+    static void copy_(exception_wrapper const* from, exception_wrapper* to);
+    static void move_(exception_wrapper* from, exception_wrapper* to);
+    static void delete_(exception_wrapper* that);
+    [[noreturn]] static void throw_(exception_wrapper const* that);
+    static std::type_info const* type_(exception_wrapper const*);
+    static std::exception const* get_exception_(exception_wrapper const* that);
+    static exception_wrapper get_exception_ptr_(exception_wrapper const* that);
+    static constexpr VTable const ops_{copy_,
+                                       move_,
+                                       delete_,
+                                       throw_,
+                                       type_,
+                                       get_exception_,
+                                       get_exception_ptr_};
+  };
 
-  std::exception_ptr getExceptionPtr() const {
-    if (eptr_) {
-      return eptr_;
-    }
-
-    try {
-      throwException();
-    } catch (...) {
-      return std::current_exception();
-    }
-    return std::exception_ptr();
-  }
+  struct SharedPtr {
+    struct Base {
+      std::type_info const* info_;
+      Base() = default;
+      explicit Base(std::type_info const& info) : info_(&info) {}
+      virtual ~Base() {}
+      virtual void throw_() const = 0;
+      virtual std::exception const* get_exception_() const noexcept = 0;
+      virtual exception_wrapper get_exception_ptr_() const noexcept = 0;
+    };
+    template <class Ex>
+    struct Impl final : public Base {
+      Ex ex_;
+      Impl() = default;
+      template <typename... As>
+      explicit Impl(As&&... as)
+          : Base{typeid(Ex)}, ex_(std::forward<As>(as)...) {}
+      [[noreturn]] void throw_() const override;
+      std::exception const* get_exception_() const noexcept override;
+      exception_wrapper get_exception_ptr_() const noexcept override;
+    };
+    std::shared_ptr<Base> ptr_;
+
+    static void copy_(exception_wrapper const* from, exception_wrapper* to);
+    static void move_(exception_wrapper* from, exception_wrapper* to);
+    static void delete_(exception_wrapper* that);
+    [[noreturn]] static void throw_(exception_wrapper const* that);
+    static std::type_info const* type_(exception_wrapper const* that);
+    static std::exception const* get_exception_(exception_wrapper const* that);
+    static exception_wrapper get_exception_ptr_(exception_wrapper const* that);
+    static VTable const ops_;
+  };
 
-protected:
-  // Optimized case: if we know what type the exception is, we can
-  // store a copy of the concrete type, and a helper function so we
-  // can rethrow it.
-  std::shared_ptr<std::exception> item_;
-  void (*throwfn_)(std::exception*);
-  // Fallback case: store the library wrapper, which is less efficient
-  // but gets the job done.  Also store the the what() string, so we
-  // can at least get it back out without having to rethrow.
-  std::exception_ptr eptr_;
-  std::string estr_;
-
-  template <class T, class... Args>
-  friend exception_wrapper make_exception_wrapper(Args&&... args);
-
-private:
-  // What makes this useful is that T can be exception_wrapper* or
-  // const exception_wrapper*, and the compiler will use the
-  // instantiation which works with F.
-  template <class Ex, class F, class T>
-  static bool with_exception1(F f, T* that) {
-    if (that->item_) {
-      if (auto ex = dynamic_cast<Ex*>(that->getCopied())) {
-        f(*ex);
-        return true;
-      }
-    } else if (that->eptr_) {
-      try {
-        std::rethrow_exception(that->eptr_);
-      } catch (std::exception& e) {
-        if (auto ex = dynamic_cast<Ex*>(&e)) {
-          f(*ex);
-          return true;
-        }
-      } catch (...) {
-        // fall through
-      }
-    }
-    return false;
-  }
-};
+  union {
+    Buffer buff_{};
+    ExceptionPtr eptr_;
+    SharedPtr sptr_;
+  };
+  VTable const* vptr_{&uninit_};
 
-template <class T, class... Args>
-exception_wrapper make_exception_wrapper(Args&&... args) {
-  exception_wrapper ew;
-  ew.item_ = std::make_shared<T>(std::forward<Args>(args)...);
-  ew.throwfn_ = folly::detail::Thrower<T>::doThrow;
-  return ew;
-}
+  template <class Ex, typename... As>
+  exception_wrapper(OnHeapTag, in_place_type_t<Ex>, As&&... as);
 
-/*
- * try_and_catch is a simple replacement for try {} catch(){} that allows you to
- * specify which derived exceptions you would like to catch and store in an
- * exception_wrapper.
- *
- * Because we cannot build an equivalent of std::current_exception(), we need
- * to catch every derived exception that we are interested in catching.
- *
- * Exceptions should be listed in the reverse order that you would write your
- * catch statements (that is, std::exception& should be first).
- *
- * NOTE: Although implemented as a derived class (for syntactic delight), don't
- * be confused - you should not pass around try_and_catch objects!
- *
- * Example Usage:
- *
- * // This catches my runtime_error and if I call throwException() on ew, it
- * // will throw a runtime_error
- * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
- *   if (badThingHappens()) {
- *     throw std::runtime_error("ZOMG!");
- *   }
- * });
- *
- * // This will catch the exception and if I call throwException() on ew, it
- * // will throw a std::exception
- * auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
- *   if (badThingHappens()) {
- *     throw std::exception();
- *   }
- * });
- *
- * // This will not catch the exception and it will be thrown.
- * auto ew = folly::try_and_catch<std::runtime_error>([=]() {
- *   if (badThingHappens()) {
- *     throw std::exception();
- *   }
- * });
- */
+  template <class Ex, typename... As>
+  exception_wrapper(InSituTag, in_place_type_t<Ex>, As&&... as);
 
-template <typename... Exceptions>
-class try_and_catch;
+  template <class T>
+  struct IsRegularExceptionType
+      : StrictConjunction<
+            std::is_copy_constructible<T>,
+            Negation<std::is_base_of<exception_wrapper, T>>,
+            Negation<std::is_abstract<T>>> {};
 
-template <typename LastException, typename... Exceptions>
-class try_and_catch<LastException, Exceptions...> :
-    public try_and_catch<Exceptions...> {
- public:
-  template <typename F>
-  explicit try_and_catch(F&& fn) : Base() {
-    call_fn(fn);
-  }
+  template <class CatchFn, bool IsConst = false>
+  struct ExceptionTypeOf;
 
- protected:
-  typedef try_and_catch<Exceptions...> Base;
+  template <bool IsConst>
+  struct HandleReduce;
 
-  try_and_catch() : Base() {}
+  template <bool IsConst>
+  struct HandleStdExceptReduce;
 
-  template <typename Ex>
-  typename std::enable_if<std::is_base_of<std::exception, Ex>::value>::type
-  assign_eptr(Ex& e) {
-    this->eptr_ = std::current_exception();
-    this->estr_ = exceptionStr(e).toStdString();
-  }
+  template <class This, class... CatchFns>
+  static void handle_(std::false_type, This& this_, CatchFns&... fns);
 
-  template <typename Ex>
-  typename std::enable_if<!std::is_base_of<std::exception, Ex>::value>::type
-  assign_eptr(Ex& e) {
-    this->eptr_ = std::current_exception();
-    this->estr_ = exceptionStr(e).toStdString();
-  }
+  template <class This, class... CatchFns>
+  static void handle_(std::true_type, This& this_, CatchFns&... fns);
 
-  template <typename Ex>
-  struct optimize {
-    static const bool value =
-      std::is_base_of<std::exception, Ex>::value &&
-      std::is_copy_assignable<Ex>::value &&
-      !std::is_abstract<Ex>::value;
-  };
+  template <class Ex, class This, class Fn>
+  static bool with_exception_(This& this_, Fn fn_);
 
+ public:
+  static exception_wrapper from_exception_ptr(
+      std::exception_ptr const& eptr) noexcept;
+
+  //! Default-constructs an empty `exception_wrapper`
+  //! \post `type() == none()`
+  exception_wrapper() noexcept {}
+
+  //! Move-constructs an `exception_wrapper`
+  //! \post `*this` contains the value of `that` prior to the move
+  //! \post `that.type() == none()`
+  exception_wrapper(exception_wrapper&& that) noexcept;
+
+  //! Copy-constructs an `exception_wrapper`
+  //! \post `*this` contains a copy of `that`, and `that` is unmodified
+  //! \post `type() == that.type()`
+  exception_wrapper(exception_wrapper const& that);
+
+  //! Move-assigns an `exception_wrapper`
+  //! \pre `this != &that`
+  //! \post `*this` contains the value of `that` prior to the move
+  //! \post `that.type() == none()`
+  exception_wrapper& operator=(exception_wrapper&& that) noexcept;
+
+  //! Copy-assigns an `exception_wrapper`
+  //! \post `*this` contains a copy of `that`, and `that` is unmodified
+  //! \post `type() == that.type()`
+  exception_wrapper& operator=(exception_wrapper const& that);
+
+  ~exception_wrapper();
+
+  //! \pre `ptr` is empty, or it holds a reference to an exception that is not
+  //!     derived from `std::exception`.
+  //! \post `!ptr || bool(*this)`
+  //! \post `hasThrownException() == true`
+  //! \post `type() == unknown()`
+  explicit exception_wrapper(std::exception_ptr ptr) noexcept;
+
+  //! \pre `ptr` holds a reference to `ex`.
+  //! \post `hasThrownException() == true`
+  //! \post `bool(*this)`
+  //! \post `type() == typeid(ex)`
+  template <class Ex>
+  exception_wrapper(std::exception_ptr ptr, Ex& ex);
+
+  //! \pre `typeid(ex) == typeid(typename decay<Ex>::type)`
+  //! \post `bool(*this)`
+  //! \post `hasThrownException() == false`
+  //! \post `type() == typeid(ex)`
+  //! \note Exceptions of types derived from `std::exception` can be implicitly
+  //!     converted to an `exception_wrapper`.
+  template <
+      class Ex,
+      class Ex_ = _t<std::decay<Ex>>,
+      FOLLY_REQUIRES(
+          Conjunction<IsStdException<Ex_>, IsRegularExceptionType<Ex_>>::value)>
+  /* implicit */ exception_wrapper(Ex&& ex);
+
+  //! \pre `typeid(ex) == typeid(typename decay<Ex>::type)`
+  //! \post `bool(*this)`
+  //! \post `hasThrownException() == false`
+  //! \post `type() == typeid(ex)`
+  //! \note Exceptions of types not derived from `std::exception` can still be
+  //!     used to construct an `exception_wrapper`, but you must specify
+  //!     `folly::in_place` as the first parameter.
+  template <
+      class Ex,
+      class Ex_ = _t<std::decay<Ex>>,
+      FOLLY_REQUIRES(IsRegularExceptionType<Ex_>::value)>
+  exception_wrapper(in_place_t, Ex&& ex);
+
+  template <
+      class Ex,
+      typename... As,
+      FOLLY_REQUIRES(IsRegularExceptionType<Ex>::value)>
+  exception_wrapper(in_place_type_t<Ex>, As&&... as);
+
+  //! Swaps the value of `*this` with the value of `that`
+  void swap(exception_wrapper& that) noexcept;
+
+  //! \return `true` if `*this` is holding an exception.
+  explicit operator bool() const noexcept;
+
+  //! \return `!bool(*this)`
+  bool operator!() const noexcept;
+
+  //! Make this `exception_wrapper` empty
+  //! \post `!*this`
+  void reset();
+
+  //! \return `true` if this `exception_wrapper` holds a reference to an
+  //!     exception that was thrown (i.e., if it was constructed with
+  //!     a `std::exception_ptr`, or if `to_exception_ptr()` was called on a
+  //!     (non-const) reference to `*this`).
+  bool has_exception_ptr() const noexcept;
+
+  //! \return a pointer to the `std::exception` held by `*this`, if it holds
+  //!     one; otherwise, returns `nullptr`.
+  //! \note This function does not mutate the `exception_wrapper` object.
+  //! \note This function never causes an exception to be thrown.
+  std::exception* get_exception() noexcept;
+  //! \overload
+  std::exception const* get_exception() const noexcept;
+
+  //! \returns a pointer to the `Ex` held by `*this`, if it holds an object
+  //!     whose type `From` permits `std::is_convertible<From*, Ex*>`;
+  //!     otherwise, returns `nullptr`.
+  //! \note This function does not mutate the `exception_wrapper` object.
+  //! \note This function may cause an exception to be thrown and immediately
+  //!     caught internally, affecting runtime performance.
   template <typename Ex>
-  typename std::enable_if<!optimize<Ex>::value>::type
-  assign_exception(Ex& e) {
-    assign_eptr(e);
-  }
-
+  Ex* get_exception() noexcept;
+  //! \overload
   template <typename Ex>
-  typename std::enable_if<optimize<Ex>::value>::type
-  assign_exception(Ex& e) {
-    this->item_ = std::make_shared<Ex>(e);
-    this->throwfn_ = folly::detail::Thrower<Ex>::doThrow;
-  }
-
-  template <typename F>
-  void call_fn(F&& fn) {
-    try {
-      Base::call_fn(std::move(fn));
-    } catch (LastException& e) {
-      if (typeid(e) == typeid(LastException&)) {
-        assign_exception(e);
-      } else {
-        assign_eptr(e);
-      }
-    }
-  }
+  Ex const* get_exception() const noexcept;
+
+  //! \return A `std::exception_ptr` that references either the exception held
+  //!     by `*this`, or a copy of same.
+  //! \note This function may need to throw an exception to complete the action.
+  //! \note The non-const overload of this function mutates `*this` to cache the
+  //!     computed `std::exception_ptr`; that is, this function may cause
+  //!     `has_exception_ptr()` to change from `false` to `true`.
+  std::exception_ptr const& to_exception_ptr() noexcept;
+  //! \overload
+  std::exception_ptr to_exception_ptr() const noexcept;
+
+  //! \return the `typeid` of an unspecified type used by
+  //!     `exception_wrapper::type()` to denote an empty `exception_wrapper`.
+  static std::type_info const& none() noexcept;
+  //! \return the `typeid` of an unspecified type used by
+  //!     `exception_wrapper::type()` to denote an `exception_wrapper` that
+  //!     holds an exception of unknown type.
+  static std::type_info const& unknown() noexcept;
+
+  //! Returns the `typeid` of the wrapped exception object. If there is no
+  //!     wrapped exception object, returns `exception_wrapper::none()`. If
+  //!     this instance wraps an exception of unknown type not derived from
+  //!     `std::exception`, returns `exception_wrapper::unknown()`.
+  std::type_info const& type() const noexcept;
+
+  //! \return If `get_exception() != nullptr`, `class_name() + ": " +
+  //!     get_exception()->what()`; otherwise, `class_name()`.
+  folly::fbstring what() const;
+
+  //! \return If `!*this`, the empty string; otherwise, if
+  //!     `type() == unknown()`, the string `"<unknown exception>"`; otherwise,
+  //!     the result of `type().name()` after demangling.
+  folly::fbstring class_name() const;
+
+  //! \tparam Ex The expression type to check for compatibility with.
+  //! \return `true` if and only if `*this` wraps an exception that would be
+  //!     caught with a `catch(Ex const&)` clause.
+  //! \note If `*this` is empty, this function returns `false`.
+  template <class Ex>
+  bool is_compatible_with() const noexcept;
+
+  //! \pre `bool(*this)`
+  //! Throws the wrapped expression.
+  [[noreturn]] void throw_exception() const;
+
+  //! Call `fn` with the wrapped exception (if any), if `fn` can accept it.
+  //! \par Example
+  //! \code
+  //! exception_wrapper ew{std::runtime_error("goodbye cruel world")};
+  //!
+  //! assert( ew.with_exception([](std::runtime_error& e){/*...*/}) );
+  //!
+  //! assert( !ew.with_exception([](int& e){/*...*/}) );
+  //!
+  //! assert( !exception_wrapper{}.with_exception([](int& e){/*...*/}) );
+  //! \endcode
+  //! \tparam Ex Optionally, the type of the exception that `fn` accepts.
+  //! \tparam Fn The type of a monomophic function object.
+  //! \param fn A function object to call with the wrapped exception
+  //! \return `true` if and only if `fn` was called.
+  //! \note Optionally, you may explicitly specify the type of the exception
+  //!     that `fn` expects, as in
+  //! \code
+  //! ew.with_exception<std::runtime_error>([](auto&& e) { /*...*/; });
+  //! \endcode
+  //! \note The handler may or may not be invoked with an active exception.
+  //!     **Do not try to rethrow the exception with `throw;` from within your
+  //!     handler -- that is, a throw expression with no operand.** This may
+  //!     cause your process to terminate. (It is perfectly ok to throw from
+  //!     a handler so long as you specify the exception to throw, as in
+  //!     `throw e;`.)
+  template <class Ex = void const, class Fn>
+  bool with_exception(Fn fn);
+  //! \overload
+  template <class Ex = void const, class Fn>
+  bool with_exception(Fn fn) const;
+
+  //! Handle the wrapped expression as if with a series of `catch` clauses,
+  //!     propagating the exception if no handler matches.
+  //! \par Example
+  //! \code
+  //! exception_wrapper ew{std::runtime_error("goodbye cruel world")};
+  //!
+  //! ew.handle(
+  //!   [&](std::logic_error const& e) {
+  //!      LOG(DFATAL) << "ruh roh";
+  //!      ew.throw_exception(); // rethrow the active exception without
+  //!                           // slicing it. Will not be caught by other
+  //!                           // handlers in this call.
+  //!   },
+  //!   [&](std::exception const& e) {
+  //!      LOG(ERROR) << ew.what();
+  //!   });
+  //! \endcode
+  //! In the above example, any exception _not_ derived from `std::exception`
+  //!     will be propagated. To specify a catch-all clause, pass a lambda that
+  //!     takes a C-style elipses, as in:
+  //! \code
+  //! ew.handle(/*...* /, [](...) { /* handle unknown exception */ } )
+  //! \endcode
+  //! \pre `!*this`
+  //! \tparam CatchFns... A pack of unary monomorphic function object types.
+  //! \param fns A pack of unary monomorphic function objects to be treated as
+  //!     an ordered list of potential exception handlers.
+  //! \note The handlers may or may not be invoked with an active exception.
+  //!     **Do not try to rethrow the exception with `throw;` from within your
+  //!     handler -- that is, a throw expression with no operand.** This may
+  //!     cause your process to terminate. (It is perfectly ok to throw from
+  //!     a handler so long as you specify the exception to throw, as in
+  //!     `throw e;`.)
+  template <class... CatchFns>
+  void handle(CatchFns... fns);
+  //! \overload
+  template <class... CatchFns>
+  void handle(CatchFns... fns) const;
 };
 
-template<>
-class try_and_catch<> : public exception_wrapper {
- public:
-  try_and_catch() {}
+template <class Ex>
+constexpr exception_wrapper::VTable exception_wrapper::InPlace<Ex>::ops_;
+
+/**
+ * \return An `exception_wrapper` that wraps an instance of type `Ex`
+ *     that has been constructed with arguments `std::forward<As>(as)...`.
+ */
+template <class Ex, typename... As>
+exception_wrapper make_exception_wrapper(As&&... as) {
+  return exception_wrapper{in_place_type<Ex>, std::forward<As>(as)...};
+}
+
+/**
+ * Inserts `ew.what()` into the ostream `sout`.
+ * \return `sout`
+ */
+template <class Ch>
+std::basic_ostream<Ch>& operator<<(
+    std::basic_ostream<Ch>& sout,
+    exception_wrapper const& ew) {
+  return sout << ew.what();
+}
+
+/**
+ * Swaps the value of `a` with the value of `b`.
+ */
+inline void swap(exception_wrapper& a, exception_wrapper& b) noexcept {
+  a.swap(b);
+}
+
+// For consistency with exceptionStr() functions in ExceptionString.h
+fbstring exceptionStr(exception_wrapper const& ew);
 
- protected:
-  template <typename F>
-  void call_fn(F&& fn) {
-    fn();
+namespace detail {
+template <typename F>
+inline exception_wrapper try_and_catch_(F&& f) {
+  return (f(), exception_wrapper());
+}
+
+template <typename F, typename Ex, typename... Exs>
+inline exception_wrapper try_and_catch_(F&& f) {
+  try {
+    return try_and_catch_<F, Exs...>(std::forward<F>(f));
+  } catch (Ex& ex) {
+    return exception_wrapper(std::current_exception(), ex);
   }
-};
 }
+} // namespace detail
+
+//! `try_and_catch` is a simple replacement for `try {} catch(){}`` that allows
+//! you to specify which derived exceptions you would like to catch and store in
+//! an `exception_wrapper`.
+//!
+//! Because we cannot build an equivalent of `std::current_exception()`, we need
+//! to catch every derived exception that we are interested in catching.
+//!
+//! Exceptions should be listed in the reverse order that you would write your
+//! catch statements (that is, `std::exception&` should be first).
+//!
+//! \par Example Usage:
+//! \code
+//! // This catches my runtime_error and if I call throw_exception() on ew, it
+//! // will throw a runtime_error
+//! auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
+//!   if (badThingHappens()) {
+//!     throw std::runtime_error("ZOMG!");
+//!   }
+//! });
+//!
+//! // This will catch the exception and if I call throw_exception() on ew, it
+//! // will throw a std::exception
+//! auto ew = folly::try_and_catch<std::exception, std::runtime_error>([=]() {
+//!   if (badThingHappens()) {
+//!     throw std::exception();
+//!   }
+//! });
+//!
+//! // This will not catch the exception and it will be thrown.
+//! auto ew = folly::try_and_catch<std::runtime_error>([=]() {
+//!   if (badThingHappens()) {
+//!     throw std::exception();
+//!   }
+//! });
+//! \endcode
+template <typename... Exceptions, typename F>
+exception_wrapper try_and_catch(F&& fn) {
+  return detail::try_and_catch_<F, Exceptions...>(std::forward<F>(fn));
+}
+} // namespace folly
+
+#include <folly/ExceptionWrapper-inl.h>
+
+#undef FOLLY_REQUIRES
+#undef FOLLY_REQUIRES_DEF
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
 #endif