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