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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#pragma once
#include <functional>
#include "rocksdb/rocksdb_namespace.h"
namespace ROCKSDB_NAMESPACE {
// Defers the execution of the provided function until the Defer
// object goes out of scope.
//
// Usage example:
//
// Status DeferTest() {
// Status s;
// Defer defer([&s]() {
// if (!s.ok()) {
// // do cleanups ...
// }
// });
// // do something ...
// if (!s.ok()) return;
// // do some other things ...
// return s;
// }
//
// The above code ensures that cleanups will always happen on returning.
//
// Without the help of Defer, you can
// 1. every time when !s.ok(), do the cleanup;
// 2. instead of returning when !s.ok(), continue the work only when s.ok(),
// but sometimes, this might lead to nested blocks of "if (s.ok()) {...}".
//
// With the help of Defer, you can centralize the cleanup logic inside the
// lambda passed to Defer, and you can return immediately on failure when
// necessary.
class Defer final {
public:
explicit Defer(std::function<void()>&& fn) : fn_(std::move(fn)) {}
~Defer() { fn_(); }
// Disallow copy.
Defer(const Defer&) = delete;
Defer& operator=(const Defer&) = delete;
private:
std::function<void()> fn_;
};
// An RAII utility object that saves the current value of an object so that
// it can be overwritten, and restores it to the saved value when the
// SaveAndRestore object goes out of scope.
template <typename T>
class SaveAndRestore {
public:
// obj is non-null pointer to value to be saved and later restored.
explicit SaveAndRestore(T* obj) : obj_(obj), saved_(*obj) {}
// new_value is stored in *obj
SaveAndRestore(T* obj, const T& new_value)
: obj_(obj), saved_(std::move(*obj)) {
*obj = new_value;
}
SaveAndRestore(T* obj, T&& new_value) : obj_(obj), saved_(std::move(*obj)) {
*obj = std::move(new_value);
}
~SaveAndRestore() { *obj_ = std::move(saved_); }
// No copies
SaveAndRestore(const SaveAndRestore&) = delete;
SaveAndRestore& operator=(const SaveAndRestore&) = delete;
private:
T* const obj_;
T saved_;
};
} // namespace ROCKSDB_NAMESPACE
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