Boost库——线程锁lock_guard 、unique_lock 的对比

1 例子

多线程访问同一资源时，为了保证数据的一致性，必要时需要加锁。

1.1 直接操作 mutex，即直接调用 mutex 的 lock / unlock 函数。

#include <iostream>
#include <boost/thread/mutex.hpp>
#include <boost/thread/thread.hpp>

boost::mutex mutex;
int count = 0;

void Counter() {
  mutex.lock();

  int i = ++count;
  std::cout << "count == " << i << std::endl;

  // 前面代码如有异常，unlock 就调不到了。
  mutex.unlock();
}

int main() {
  boost::thread_group threads;
  for (int i = 0; i < 4; ++i) {
    threads.create_thread(&Counter);
  }

  threads.join_all();
  return 0;
}

1.2 使用 lock_guard 自动加锁、解锁。原理是 RAII，和智能指针类似。

#include <iostream>
#include <boost/thread/lock_guard.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/thread/thread.hpp>

boost::mutex mutex;
int count = 0;

void Counter() {
  // lock_guard 在构造函数里加锁，在析构函数里解锁。
  boost::lock_guard<boost::mutex> lock(mutex);

  int i = ++count;
  std::cout << "count == " << i << std::endl;
}

int main() {
  boost::thread_group threads;
  for (int i = 0; i < 4; ++i) {
    threads.create_thread(&Counter);
  }

  threads.join_all();
  return 0;
}

1.3 使用 unique_lock 自动加锁、解锁。

unique_lock 与 lock_guard 原理相同，但是提供了更多功能（比如可以结合条件变量使用）。
注意：mutex::scoped_lock 其实就是 unique_lock 的 typedef。

#include <iostream>
#include <boost/thread/mutex.hpp>
#include <boost/thread/thread.hpp>

boost::mutex mutex;
int count = 0;

void Counter() {
  boost::unique_lock<boost::mutex> lock(mutex);

  int i = ++count;
  std::cout << "count == " << i << std::endl;
}

int main() {
  boost::thread_group threads;
  for (int i = 0; i < 4; ++i) {
    threads.create_thread(&Counter);
  }

  threads.join_all();
  return 0;
}

2 unique_lock和lock_guard的区别

简单的说，unique_lock相对于lock_guard,会有更多特性。
unique_lock和lock_guard都遵循RAII。

unique_lock和lock_guard最大的不同是unique_lock不需要始终拥有关联的mutex，而lock_guard始终拥有mutex。这意味着unique_lock需要利用owns_lock()判断是否拥有mutex。另外，如果要结合使用条件变量，应该使用unique_lock。

• Lock doesn't have to taken right at the construction, you can pass the flag std::defer_lock during its construction to keep the mutex unlocked during construction.

• We can unlock it before the function ends and don't have to necessarily wait for destructor to release it, which can be handy.

• You can pass the ownership of the lock from a function, it is movable and not copyable.

• It can be used with conditional variables since that requires mutex to be locked, condition checked and unlocked while waiting for a condition.
  参考[StackOverflow]:https://stackoverflow.com/questions/6731027/boostunique-lock-vs-boostlock-guard