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+//
+// detail/conditionally_enabled_mutex.hpp
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2021 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef ASIO_DETAIL_CONDITIONALLY_ENABLED_MUTEX_HPP
+#define ASIO_DETAIL_CONDITIONALLY_ENABLED_MUTEX_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+#include "asio/detail/mutex.hpp"
+#include "asio/detail/noncopyable.hpp"
+#include "asio/detail/scoped_lock.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace detail {
+
+// Mutex adapter used to conditionally enable or disable locking.
+class conditionally_enabled_mutex
+ : private noncopyable
+{
+public:
+ // Helper class to lock and unlock a mutex automatically.
+ class scoped_lock
+ : private noncopyable
+ {
+ public:
+ // Tag type used to distinguish constructors.
+ enum adopt_lock_t { adopt_lock };
+
+ // Constructor adopts a lock that is already held.
+ scoped_lock(conditionally_enabled_mutex& m, adopt_lock_t)
+ : mutex_(m),
+ locked_(m.enabled_)
+ {
+ }
+
+ // Constructor acquires the lock.
+ explicit scoped_lock(conditionally_enabled_mutex& m)
+ : mutex_(m)
+ {
+ if (m.enabled_)
+ {
+ mutex_.mutex_.lock();
+ locked_ = true;
+ }
+ else
+ locked_ = false;
+ }
+
+ // Destructor releases the lock.
+ ~scoped_lock()
+ {
+ if (locked_)
+ mutex_.mutex_.unlock();
+ }
+
+ // Explicitly acquire the lock.
+ void lock()
+ {
+ if (mutex_.enabled_ && !locked_)
+ {
+ mutex_.mutex_.lock();
+ locked_ = true;
+ }
+ }
+
+ // Explicitly release the lock.
+ void unlock()
+ {
+ if (locked_)
+ {
+ mutex_.unlock();
+ locked_ = false;
+ }
+ }
+
+ // Test whether the lock is held.
+ bool locked() const
+ {
+ return locked_;
+ }
+
+ // Get the underlying mutex.
+ asio::detail::mutex& mutex()
+ {
+ return mutex_.mutex_;
+ }
+
+ private:
+ friend class conditionally_enabled_event;
+ conditionally_enabled_mutex& mutex_;
+ bool locked_;
+ };
+
+ // Constructor.
+ explicit conditionally_enabled_mutex(bool enabled)
+ : enabled_(enabled)
+ {
+ }
+
+ // Destructor.
+ ~conditionally_enabled_mutex()
+ {
+ }
+
+ // Determine whether locking is enabled.
+ bool enabled() const
+ {
+ return enabled_;
+ }
+
+ // Lock the mutex.
+ void lock()
+ {
+ if (enabled_)
+ mutex_.lock();
+ }
+
+ // Unlock the mutex.
+ void unlock()
+ {
+ if (enabled_)
+ mutex_.unlock();
+ }
+
+private:
+ friend class scoped_lock;
+ friend class conditionally_enabled_event;
+ asio::detail::mutex mutex_;
+ const bool enabled_;
+};
+
+} // namespace detail
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_DETAIL_CONDITIONALLY_ENABLED_MUTEX_HPP