scope_guard.h

#pragma once

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

namespace caffe2 {

// Copied from folly/ScopeGuard.h

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 FunctionType>
ScopeGuardImpl<typename std::decay<FunctionType>::type>
MakeGuard(FunctionType&& fn) noexcept(
    std::is_nothrow_constructible<typename std::decay<FunctionType>::type,
                                  FunctionType>::value) {
  return ScopeGuardImpl<typename std::decay<FunctionType>::type>(
      std::forward<FunctionType>(fn));
}

typedef ScopeGuardImplBase&& ScopeGuard;

}  // namespaces