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#ifndef QPID_SYS_LOCKPTR_H
#define QPID_SYS_LOCKPTR_H
/*
*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
* software distributed under the License is distributed on an
* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
* KIND, either express or implied. See the License for the
* specific language governing permissions and limitations
* under the License.
*
*/
#include "qpid/sys/Mutex.h"
#include <boost/noncopyable.hpp>
namespace qpid {
namespace sys {
class Mutex;
/**
* LockPtr is a smart pointer to T. It is constructed from a volatile
* T* and a Lock (by default a Mutex). It const_casts away the
* volatile qualifier and locks the Lock for the duration of its
*
* Used in conjuntion with the "volatile" keyword to get the compiler
* to help enforce correct concurrent use of mutli-threaded objects.
* See ochttp://www.ddj.com/cpp/184403766 for a detailed discussion.
*
* To summarize the convention:
* - Declare thread-safe member functions as volatile.
* - Declare instances of the class that may be called concurrently as volatile.
* - Use LockPtr to cast away the volatile qualifier while taking a lock.
*
* This means that code calling on a concurrently-used object
* (declared volatile) can only call thread-safe (volatile) member
* functions. Code that needs to use thread-unsafe members must use a
* LockPtr, thereby acquiring the lock and making it safe to do so.
*
* A good type-safe pattern is the internally-locked object:
* - It has it's own private lock member.
* - All public functions are thread safe and declared volatile.
* - Any thread-unsafe, non-volatile functions are private.
* - Only member function implementations use LockPtr to access private functions.
*
* This encapsulates all the locking logic inside the class.
*
* One nice feature of this convention is the common case where you
* need a public, locked version of some function foo() and also a
* private unlocked version to avoid recursive locks. They can be declared as
* volatile and non-volatile overloads of the same function:
*
* // public
* void Thing::foo() volatile { LockPtr<Thing>(this, myLock)->foo(); }
* // private
* void Thing::foo() { ... do stuff ...}
*/
template <class T, class Lock> class LockPtr : public boost::noncopyable {
public:
LockPtr(volatile T* p, Lock& l) : ptr(const_cast<T*>(p)), lock(l) { lock.lock(); }
LockPtr(volatile T* p, volatile Lock& l) : ptr(const_cast<T*>(p)), lock(const_cast<Lock&>(l)) { lock.lock(); }
~LockPtr() { lock.unlock(); }
T& operator*() { return *ptr; }
T* operator->() { return ptr; }
private:
T* ptr;
Lock& lock;
};
}} // namespace qpid::sys
#endif /*!QPID_SYS_LOCKPTR_H*/
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