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-rw-r--r--src/common/item_history.h47
1 files changed, 47 insertions, 0 deletions
diff --git a/src/common/item_history.h b/src/common/item_history.h
new file mode 100644
index 000000000..87512a28c
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+++ b/src/common/item_history.h
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+// -*- 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;
+ }
+
+};