C++11多线程之std::unique_lock

原文如下：
http://en.cppreference.com/w/cpp/thread/unique_lock
http://en.cppreference.com/w/cpp/thread/unique_lock/unique_lock
http://en.cppreference.com/w/cpp/thread/unique_lock/~unique_lock
http://en.cppreference.com/w/cpp/thread/unique_lock/operator%3D
http://en.cppreference.com/w/cpp/thread/unique_lock/lock
http://en.cppreference.com/w/cpp/thread/unique_lock/try_lock
http://en.cppreference.com/w/cpp/thread/unique_lock/try_lock_for
http://en.cppreference.com/w/cpp/thread/unique_lock/try_lock_until
http://en.cppreference.com/w/cpp/thread/unique_lock/unlock
http://en.cppreference.com/w/cpp/thread/unique_lock/mutex
http://en.cppreference.com/w/cpp/thread/unique_lock/owns_lock
http://en.cppreference.com/w/cpp/thread/unique_lock/operator_bool

std::unique_lock

定义在头文件 中。

template <class Mutex>      (since C++11)class unique_lock;

类 unique_lock 是一个一般性质的 mutex 属主的封装，提供延迟锁定（deferred locking），限时尝试（time-constrained attempts），递归锁定（recursive locking）， 锁主的转换， 以及对条件变量的使用。

类 unique_lock 是 movable 的，但不是 copyable 的 – 它满足 MoveConstructible 和 MoveAssignable, 但不满足 CopyConstructible 和 CopyAssignable.

类 unique_lock 满足 BasicLockable 的要求。如果 Mutex 满足 Lockable 的要求，那么 unique_lock 也满足 Lockable 的要求（比如，可以被用于 std::lock）；如果 Mutex 满足 TimedLockable 的要求，unique_lock 也满足 TimedLockable 的要求。

模板参数

Mutex - 用于锁定的 mutex 的类型。该类型必须满足 BasicLockable 的要求。

构造函数

(1) unique_lock();  (2) unique_lock( unique_lock&& other );  (3) explicit unique_lock( mutex_type& m );(4) unique_lock( mutex_type& m, std::defer_lock_t t );(5) unique_lock( mutex_type& m, std::try_to_lock_t t );(6) unique_lock( mutex_type& m, std::adopt_lock_t t );(7) template< class Rep, class Period >    unique_lock(mutex_type& m,                 const std::chrono::duration<Rep,Period>& timeout_duration);(8) template< class Clock, class Duration >    unique_lock(mutex_type& m,                 const std::chrono::time_point<Clock,Duration>& timeout_time);

(1) unique_lock();
构造一个没有关联 mutex 的 unique_lock

(2) unique_lock( unique_lock&& other );
Move构造函数，使用 other 的内容来构造 unique_lock. 使得other变成没有mutex关联的unique_lock.

(3) - (8) 构造一个以 m 为关联的mutex的unique_lock, 另外：

(3) explicit unique_lock( mutex_type& m );
通过调用 m.lock() 来锁定相关联的 mutex. 如果当前线程已经拥有了mutex，且不是递归的mutex，那么行为未定义。

(4) unique_lock( mutex_type& m, std::defer_lock_t t );
不锁定相关的mutex.

(5) unique_lock( mutex_type& m, std::try_to_lock_t t );
通过调用 m.try_lock() 来尝试锁定相关的mutex而不会阻塞。如果当前线程已经拥有mutex且不是递归mutex，则行为未定义。

(6) unique_lock( mutex_type& m, std::adopt_lock_t t );
假设线程已经拥有m.

(7)

template< class Rep, class Period >      unique_lock(mutex_type& m, const std::chrono::duration<Rep,Period>& timeout_duration);  

通过调用 m.try_lock_for(timeout_duration) 试图锁定相关联的 mutex. 一直阻塞直到超时或锁定成功。也可能阻塞得比time_duration的时间更长一些。

(8)

template< class Clock, class Duration >      unique_lock(mutex_type& m, const std::chrono::time_point<Clock,Duration>& timeout_time);  

通过调用 m.try_lock_until(timeout_time) 来试图锁定相关联的 mutex. 一直阻塞直到指定的时间点到达或者锁定成功。可能会在指定的时间到达后仍阻塞一会儿。

示例程序：

#include <cassert>#include <iostream> // std::cout#include <thread>#include <vector>#include <mutex>class Number;std::ostream& operator<<(std::ostream& stream, const Number& number);class Number { public:  Number() : v_(1) {}  // thread-safe update of 'a' and 'b'  static void update(Number& a, Number& b, bool order) {    // do not lock 'mutex_' of 'a' and 'b' sequentially,    // two sequential lock may lead to deadlock,    // that's why 'std::lock' exists (see below)    GuardLock lock_a(a.mutex_, std::defer_lock);    GuardLock lock_b(b.mutex_, std::defer_lock);    // mutexes is not locked    assert(!lock_a.owns_lock());    assert(!lock_b.owns_lock());    // unspecified series of calls...    std::lock(lock_a, lock_b);    // Result: 'a.mutex_' and 'b.mutex_' is in locked state    // 'a' and 'b' can be modified safety    assert(lock_a.owns_lock());    assert(lock_b.owns_lock());    if (order) {      a.v_ += b.v_;      b.v_ += a.v_;      std::cout << a << b;    }    else {      b.v_ += a.v_;      a.v_ += b.v_;      std::cout << b << a;    }    // 'lock_a' and 'lock_b' will be destroyed,    // unlocking 'a.mutex_' and 'b.mutex_'  }  // not thread-safe; used before thread creation or in thread-safe 'update'  std::ostream& print(std::ostream& stream) const {    stream << v_ << " ";    return stream;  } private:  using Mutex = std::mutex;  using GuardLock = std::unique_lock<Mutex>;  Mutex mutex_;  int v_;};// not thread-safe; see 'Number::print'std::ostream& operator<<(std::ostream& stream, const Number& number) {  return number.print(stream);}int main() {  Number a, b;  std::cout << a << b;  std::vector<std::thread> threads;  for (unsigned i = 0; i < 4; ++i) {    // without 'std::lock' deadlock may occur in this situation:    //   thread #1 lock 'a.mutex_'    //   thread #2 lock 'b.mutex_'    //   thread #1 try to lock 'b.mutex_' and blocked (it's locked by #2)    //   thread #2 try to lock 'a.mutex_' and blocked (it's locked by #1)    //   ... deadlock    threads.emplace_back(Number::update, std::ref(a), std::ref(b), true); // #1    threads.emplace_back(Number::update, std::ref(b), std::ref(a), false); // #2  }  for (auto& i: threads) {    i.join();  }  std::cout << '\n';}// Output: // 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584

析构函数

销毁这个unique_lock对象。如果 *this 此时拥有一个相关联的 mutex 并且已经获得它，那么就会解锁该mutex.

赋值操作符 operator =

unique_lock& operator=( unique_lock&& other ); (since C++11)

Move赋值操作符。使用 other 中的内容来赋值给自己。
如果在此调用前，*this 已经拥有一个mutex并锁定了它，那么此调用会解锁该mutex.

std::unique_lock::lock 函数

void lock(); (since C++11)
锁定关联的mutex. 高效地调用 mutex()->lock().

异常：
- 抛出mutex()->lock()所抛出的异常。
- 如果没有相关联的 mutex, std::system_error就会抛出，所携带的错误码是std::errc::operation_not_permitted.
- 如果mutex已经被本unique_lock锁定了（换句话说，owns_lock为true），那么std::system_error就会被抛出，错误码是std::errc::resource_deadlock_would_occur.

参见示例程序：

#include <mutex>#include <thread>#include <iostream>#include <vector>#include <chrono>int main(){    int counter = 0;    std::mutex counter_mutex;    std::vector<std::thread> threads;    auto worker_task = [&](int id) {        // Note, this is just lock! See the document of the constructor.         std::unique_lock<std::mutex> lock(counter_mutex);          ++counter;        std::cout << id << ", initial counter: " << counter << '\n';        lock.unlock();        // don't hold the lock while we simulate an expensive operation        std::this_thread::sleep_for(std::chrono::seconds(1));        lock.lock();        ++counter;        std::cout << id << ", final counter: " << counter << '\n';    };    for (int i = 0; i < 10; ++i) {        // vector::push_back() cannot work due to std::thread is not copyable.        threads.emplace_back(worker_task, i);    }    for (auto &thread : threads) thread.join();}/**Possible Output:0, initial counter: 11, initial counter: 22, initial counter: 33, initial counter: 44, initial counter: 55, initial counter: 66, initial counter: 77, initial counter: 88, initial counter: 99, initial counter: 101, final counter: 110, final counter: 123, final counter: 135, final counter: 142, final counter: 154, final counter: 167, final counter: 179, final counter: 186, final counter: 198, final counter: 20**/

std::unique_lock::try_lock 函数

bool try_lock(); (since C++11)
试图锁定相关联的mutex，而不会阻塞。高效地调用 mutex()->try_lock().
如果没有相关联的mutex或者该mutex已经被该unique_lock锁定，那么 std::system_error 就会被抛出。

异常：
- 任何 mutex()->try_lock() 抛出的异常都会被抛出 （Mutex类型不会被try_lock抛出，但一个自定义的Lockable类型可能会被抛出）。
- 如果没有相关联的mutex，那么std::system_error就会被抛出，其错误码是std::errc::operation_not_permitted.
- 如果mutex已经被本unique_lock锁定了，std::system_error也会被抛出，错误码是std::errc::resource_deadlock_would_occur.

std::unique_lock::try_lock_for 函数

template

std::unique_lock::unlock 函数

void unlock(); (since C++11)
解锁相关的mutex.
如果没有相关联的mutex或该mutex已经被锁定，则std::system_error就会被抛出。

异常：
- 任何被mutex()->unlock()抛出的异常都会被抛出。
- 如果没有相关联的mutex或者mutex并没有被锁定，则std::system_error就会被抛出，错误码是std::errc::operation_not_permitted.

std::unique_lock::mutex 函数

mutex_type* mutex() const; (since C++11)
返回一个指向所关联的mutex的指针，或者如果没有相关联的mutex的话就返回一个空指针。

std::unique_lock::owns_lock 函数

bool owns_lock() const; (since C++11)
检查 *this 是否已经锁住mutex.
是，则返回true；否则返回false.

std::unique_lock::operator bool 函数

explicit operator bool() const; (since C++11)

检查 *this 是否已锁定一个mutex. 高效地调用owns_lock().
是，则返回true；否则返回false.

示例程序

#include <mutex>#include <thread>#include <chrono>#include <iostream>struct Box {    explicit Box(int num) : num_things{num} {}    int num_things;    std::mutex m;};void transfer(Box &a, Box &b, int num){    // don't actually take the locks yet    std::unique_lock<std::mutex> lock1(a.m, std::defer_lock);    std::unique_lock<std::mutex> lock2(b.m, std::defer_lock);    // lock both unique_locks without deadlock    std::lock(lock1, lock2);    a.num_things -= num;    b.num_things += num;    // 'a.m' and 'b.m' mutexes unlocked outside of 'unique_lock'}int main(){    Box acc1(100);    Box acc2(50);    std::thread t1(transfer, std::ref(acc1), std::ref(acc2), 10);    std::thread t2(transfer, std::ref(acc2), std::ref(acc1), 5);    t1.join();    t2.join();    std::cout << "acc1: " << acc1.num_things << std::endl;    std::cout << "acc2: " << acc2.num_things << std::endl;}/** Output: acc1: 95acc2: 55**/

(译注： 原示例程序没有任何输出且变量命名比较令人费解，故略作修改。)

(完）