scope_guard.h

/**
 * Copyright 2016 Facebook
 * @author Tudor Bosman (tudorb@fb.com)
 */

#pragma once

#include <cstddef>
#include <functional>
#include <new>
#include <type_traits>
#include <utility>

namespace caffe2 {

// Copied from folly/ScopeGuard.h

namespace detail {

class ScopeGuardImplBase {
 public:
  void dismiss() noexcept {
    dismissed_ = true;
  }

 protected:
  ScopeGuardImplBase() noexcept : dismissed_(false) {}

  static ScopeGuardImplBase makeEmptyScopeGuard() noexcept {
    return ScopeGuardImplBase{};
  }

  template <typename T>
  static const T& asConst(const T& t) noexcept {
    return t;
  }

  bool dismissed_;
};

template <typename FunctionType>
class ScopeGuardImpl : public ScopeGuardImplBase {
 public:
  explicit ScopeGuardImpl(FunctionType& fn) noexcept(
      std::is_nothrow_copy_constructible<FunctionType>::value)
      : ScopeGuardImpl(
            asConst(fn),
            makeFailsafe(std::is_nothrow_copy_constructible<FunctionType>{},
                         &fn)) {}

  explicit ScopeGuardImpl(const FunctionType& fn) noexcept(
      std::is_nothrow_copy_constructible<FunctionType>::value)
      : ScopeGuardImpl(
            fn,
            makeFailsafe(std::is_nothrow_copy_constructible<FunctionType>{},
                         &fn)) {}

  explicit ScopeGuardImpl(FunctionType&& fn) noexcept(
      std::is_nothrow_move_constructible<FunctionType>::value)
      : ScopeGuardImpl(
            std::move_if_noexcept(fn),
            makeFailsafe(std::is_nothrow_move_constructible<FunctionType>{},
                         &fn)) {}

  ScopeGuardImpl(ScopeGuardImpl&& other) noexcept(
      std::is_nothrow_move_constructible<FunctionType>::value)
      : function_(std::move_if_noexcept(other.function_)) {
    // If the above line attempts a copy and the copy throws, other is
    // left owning the cleanup action and will execute it (or not) depending
    // on the value of other.dismissed_. The following lines only execute
    // if the move/copy succeeded, in which case *this assumes ownership of
    // the cleanup action and dismisses other.
    dismissed_ = other.dismissed_;
    other.dismissed_ = true;
  }

  ~ScopeGuardImpl() noexcept {
    if (!dismissed_) {
      execute();
    }
  }

 private:
  static ScopeGuardImplBase makeFailsafe(std::true_type, const void*) noexcept {
    return makeEmptyScopeGuard();
  }

  template <typename Fn>
  static auto makeFailsafe(std::false_type, Fn* fn) noexcept
      -> ScopeGuardImpl<decltype(std::ref(*fn))> {
    return ScopeGuardImpl<decltype(std::ref(*fn))>{std::ref(*fn)};
  }

  template <typename Fn>
  explicit ScopeGuardImpl(Fn&& fn, ScopeGuardImplBase&& failsafe)
      : ScopeGuardImplBase{}, function_(std::forward<Fn>(fn)) {
    failsafe.dismiss();
  }

  void* operator new(std::size_t) = delete;

  void execute() noexcept { function_(); }

  FunctionType function_;
};

template <typename F>
using ScopeGuardImplDecay = ScopeGuardImpl<typename std::decay<F>::type>;

} // namespace detail

/**
 * ScopeGuard is a general implementation of the "Initialization is
 * Resource Acquisition" idiom.  Basically, it guarantees that a function
 * is executed upon leaving the currrent scope unless otherwise told.
 *
 * The MakeGuard() function is used to create a new ScopeGuard object.
 * It can be instantiated with a lambda function, a std::function<void()>,
 * a functor, or a void(*)() function pointer.
 *
 *
 * Usage example: Add a friend to memory iff it is also added to the db.
 *
 * void User::addFriend(User& newFriend) {
 *   // add the friend to memory
 *   friends_.push_back(&newFriend);
 *
 *   // If the db insertion that follows fails, we should
 *   // remove it from memory.
 *   auto guard = MakeGuard([&] { friends_.pop_back(); });
 *
 *   // this will throw an exception upon error, which
 *   // makes the ScopeGuard execute UserCont::pop_back()
 *   // once the Guard's destructor is called.
 *   db_->addFriend(GetName(), newFriend.GetName());
 *
 *   // an exception was not thrown, so don't execute
 *   // the Guard.
 *   guard.dismiss();
 * }
 *
 * Examine ScopeGuardTest.cpp for some more sample usage.
 *
 * Stolen from:
 *   Andrei's and Petru Marginean's CUJ article:
 *     http://drdobbs.com/184403758
 *   and the loki library:
 *     http://loki-lib.sourceforge.net/index.php?n=Idioms.ScopeGuardPointer
 *   and triendl.kj article:
 *     http://www.codeproject.com/KB/cpp/scope_guard.aspx
 */
template <typename F>
detail::ScopeGuardImplDecay<F> MakeGuard(F&& f) noexcept(
    noexcept(detail::ScopeGuardImplDecay<F>(static_cast<F&&>(f)))) {
  return detail::ScopeGuardImplDecay<F>(static_cast<F&&>(f));
}

}  // namespaces