From 06649f434861ba1a287df67ce28486879a5cddc6 Mon Sep 17 00:00:00 2001
From: Eric Niebler <eniebler@fb.com>
Date: Thu, 29 Dec 2016 11:51:18 -0800
Subject: [PATCH] folly::TryException inherits from std::logic_error, like
 std::bad_optional_access; becomes no-throw copyable

Summary:
folly::TryException was inheriting from std::exception and managing its own message in a std::string data member.
That is suboptimal for the following reasons:

1) The analagous std:: type, std::bad_optional_access, inherits from std::logic_error. According to the Principle of Least
   Astonishment, folly::TryException should follow suit.
2) std::logic_error has a nothrow copy constructor. (This is typically implemented with a ref-counted string.)
   std::string does not. By explicitly managing its own message, folly::TryException picks up a throwing copy
   constructor. Exception classes should try to be nothrow copyable.
3) With most stdlib implementations, std::string is larger by a lot than the std:: exceptions. By changing
   folly::TryException as suggested, its size drops from 40 bytes to 16 on clang and gcc >=5.0.

Reviewed By: yfeldblum

Differential Revision: D4368760

fbshipit-source-id: 327f78f428419ccf9c7af3e0f57cc478d147fb37
---
 folly/Try.h | 29 +++++++----------------------
 1 file changed, 7 insertions(+), 22 deletions(-)

diff --git a/folly/Try.h b/folly/Try.h
index e6b6ebad..1f08e27e 100644
--- a/folly/Try.h
+++ b/folly/Try.h
@@ -16,41 +16,26 @@
 
 #pragma once
 
-#include <type_traits>
-#include <exception>
-#include <algorithm>
 #include <folly/ExceptionWrapper.h>
 #include <folly/Likely.h>
 #include <folly/Memory.h>
 #include <folly/Portability.h>
 #include <folly/Unit.h>
+#include <exception>
+#include <stdexcept>
+#include <type_traits>
+#include <utility>
 
 namespace folly {
 
-class TryException : public std::exception {
+class TryException : public std::logic_error {
  public:
-  explicit TryException(std::string message_arg) noexcept
-      : message(std::move(message_arg)) {}
-
-  const char* what() const noexcept override {
-    return message.c_str();
-  }
-
-  bool operator==(const TryException& other) const noexcept {
-    return other.message == this->message;
-  }
-
-  bool operator!=(const TryException& other) const noexcept {
-    return !(*this == other);
-  }
-
- protected:
-  std::string message;
+  using std::logic_error::logic_error;
 };
 
 class UsingUninitializedTry : public TryException {
  public:
-  UsingUninitializedTry() noexcept : TryException("Using unitialized try") {}
+  UsingUninitializedTry() : TryException("Using unitialized try") {}
 };
 
 /*
-- 
2.34.1