update ceph source to reef 18.2.0

[ceph.git] / ceph / src / common / item_history.h

// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
// vim: ts=8 sw=2 smarttab

#pragma once

#include <list>
#include <mutex>

/*

Keep a history of item values so that readers can dereference the pointer to
the latest value and continue using it as long as they want.  This container
is only appropriate for values that are updated a handful of times over their
total lifetime.

*/

template<class T>
class safe_item_history {
private:
  std::mutex lock;
  std::list<T> history;
  T *current = nullptr;

public:
  safe_item_history() {
    history.emplace_back(T());
    current = &history.back();
  }

  // readers are lock-free
  const T& operator*() const {
    return *current;
  }
  const T *operator->() const {
    return current;
  }

  // writes are serialized
  const T& operator=(const T& other) {
    std::lock_guard l(lock);
    history.push_back(other);
    current = &history.back();
    return *current;
  }

};