ceph/src/common/item_history.h

   1 // -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
   2 // vim: ts=8 sw=2 smarttab
   3
   4 #pragma once
   5
   6 #include <list>
   7 #include <mutex>
   8
   9 /*
  10
  11 Keep a history of item values so that readers can dereference the pointer to
  12 the latest value and continue using it as long as they want.  This container
  13 is only appropriate for values that are updated a handful of times over their
  14 total lifetime.
  15
  16 */
  17
  18 template<class T>
  19 class safe_item_history {
  20 private:
  21   std::mutex lock;
  22   std::list<T> history;
  23   T *current = nullptr;
  24
  25 public:
  26   safe_item_history() {
  27     history.emplace_back(T());
  28     current = &history.back();
  29   }
  30
  31   // readers are lock-free
  32   const T& operator*() const {
  33     return *current;
  34   }
  35   const T *operator->() const {
  36     return current;
  37   }
  38
  39   // writes are serialized
  40   const T& operator=(const T& other) {
  41     std::lock_guard l(lock);
  42     history.push_back(other);
  43     current = &history.back();
  44     return *current;
  45   }
  46
  47 };