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/**
* 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 current 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
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