Boost庫——線程鎖lock_guard 、unique_lock 的對比

1 例子

多線程訪問同一資源時，為了保證數據的一致性，必要時需要加鎖。

1.1 直接操作 mutex，即直接調用 mutex 的 lock / unlock 函數。

#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;

  // 前面代碼如有異常，unlock 就調不到了。
  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 自動加鎖、解鎖。原理是 RAII，和智能指針類似。

#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 在構造函數里加鎖，在析構函數里解鎖。
  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 自動加鎖、解鎖。

unique_lock 與 lock_guard 原理相同，但是提供了更多功能（比如可以結合條件變量使用）。
注意：mutex::scoped_lock 其實就是 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的區別

簡單的說，unique_lock相對於lock_guard,會有更多特性。
unique_lock和lock_guard都遵循RAII。

unique_lock和lock_guard最大的不同是unique_lock不需要始終擁有關聯的mutex，而lock_guard始終擁有mutex。這意味着unique_lock需要利用owns_lock()判斷是否擁有mutex。另外，如果要結合使用條件變量，應該使用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.
  參考[StackOverflow]:https://stackoverflow.com/questions/6731027/boostunique-lock-vs-boostlock-guard