ceph/src/rocksdb/util/defer.h

   1 // Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
   2 //  This source code is licensed under both the GPLv2 (found in the
   3 //  COPYING file in the root directory) and Apache 2.0 License
   4 //  (found in the LICENSE.Apache file in the root directory).
   5
   6 #pragma once
   7
   8 #include <functional>
   9
  10 namespace ROCKSDB_NAMESPACE {
  11
  12 // Defers the execution of the provided function until the Defer
  13 // object goes out of scope.
  14 //
  15 // Usage example:
  16 //
  17 // Status DeferTest() {
  18 //   Status s;
  19 //   Defer defer([&s]() {
  20 //     if (!s.ok()) {
  21 //       // do cleanups ...
  22 //     }
  23 //   });
  24 //   // do something ...
  25 //   if (!s.ok()) return;
  26 //   // do some other things ...
  27 //   return s;
  28 // }
  29 //
  30 // The above code ensures that cleanups will always happen on returning.
  31 //
  32 // Without the help of Defer, you can
  33 // 1. every time when !s.ok(), do the cleanup;
  34 // 2. instead of returning when !s.ok(), continue the work only when s.ok(),
  35 //    but sometimes, this might lead to nested blocks of "if (s.ok()) {...}".
  36 //
  37 // With the help of Defer, you can centralize the cleanup logic inside the
  38 // lambda passed to Defer, and you can return immediately on failure when necessary.
  39 class Defer final {
  40  public:
  41   Defer(std::function<void()>&& fn) : fn_(std::move(fn)) {}
  42   ~Defer() { fn_(); }
  43
  44   // Disallow copy.
  45   Defer(const Defer&) = delete;
  46   Defer& operator=(const Defer&) = delete;
  47
  48  private:
  49   std::function<void()> fn_;
  50 };
  51
  52 }  // namespace ROCKSDB_NAMESPACE