C++11中std::lock_guard的使用

互斥类的最重要成员函数是lock()和unlock()。在进入临界区时，执行lock()加锁操作，如果这时已经被其它线程锁住，则当前线程在此排队等待。退出临界区时，执行unlock()解锁操作。更好的办法是采用”资源分配时初始化”(RAII)方法来加锁、解锁，这避免了在临界区中因为抛出异常或return等操作导致没有解锁就退出的问题。极大地简化了程序员编写mutex相关的异常处理代码。C++11的标准库中提供了std::lock_guard类模板做mutex的RAII。

std::lock_guard类的构造函数禁用拷贝构造，且禁用移动构造。std::lock_guard类除了构造函数和析构函数外没有其它成员函数。

在std::lock_guard对象构造时，传入的mutex对象(即它所管理的mutex对象)会被当前线程锁住。在lock_guard对象被析构时，它所管理的mutex对象会自动解锁，不需要程序员手动调用lock和unlock对mutex进行上锁和解锁操作。lock_guard对象并不负责管理mutex对象的生命周期，lock_guard对象只是简化了mutex对象的上锁和解锁操作，方便线程对互斥量上锁，即在某个lock_guard对象的生命周期内，它所管理的锁对象会一直保持上锁状态；而lock_guard的生命周期结束之后，它所管理的锁对象会被解锁。程序员可以非常方便地使用lock_guard，而不用担心异常安全问题。

std::lock_guard在构造时只被锁定一次，并且在销毁时解锁。

关于std::mutex的基础介绍可以参考： http://blog.csdn.net/fengbingchun/article/details/73521630 

The difference is that you can lock and unlock a std::unique_lock. std::lock_guard will be locked only once on construction and unlocked on destruction.

std::unique_lock has other features that allow it to e.g.: be constructed without locking the mutex immediately but to build the RAII wrapper. However, std::unique_lock might have a tad more overhead(较多开销).

std::lock_guard also provides a convenient RAII wrapper, but cannot lock multiple mutexes safely. It can be used when you need a wrapper for a limited scope, e.g.: a member function.

One of the differences between std::lock_guard and std::unique_lock is that the programmer is able to unlock std::unique_lock, but she/he is not able to unlock std::lock_guard.

下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：

#include "lock_guard.hpp"
#include 
#include 
#include 
#include 
#include 
#include 

namespace lock_guard_ {

///////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/mutex/lock_guard/
namespace {
std::mutex mtx;

void print_even(int x) {
	if (x % 2 == 0) std::cout << x << " is even\n";
	else throw (std::logic_error("not even"));
}

void print_thread_id(int id) {
	try {
		// using a local lock_guard to lock mtx guarantees unlocking on destruction / exception:
		std::lock_guard lck(mtx);
		print_even(id);
	} catch (std::logic_error&) {
		std::cout << "[exception caught]\n";
	}
}
}

int test_lock_guard_1()
{
	std::thread threads[10];
	// spawn 10 threads:
	for (int i = 0; i<10; ++i)
		threads[i] = std::thread(print_thread_id, i + 1);

	for (auto& th : threads) th.join();

	return 0;
}

///////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/mutex/lock_guard/lock_guard/
namespace {
std::mutex mtx2;           // mutex for critical section

void print_thread_id2(int id) {
	mtx2.lock();
	std::lock_guard lck(mtx2, std::adopt_lock);
	std::cout << "thread #" << id << '\n';
}
}

int test_lock_guard_2()
{
	std::thread threads[10];
	// spawn 10 threads:
	for (int i = 0; i<10; ++i)
		threads[i] = std::thread(print_thread_id2, i + 1);

	for (auto& th : threads) th.join();

	return 0;
}

////////////////////////////////////////////////////////////
// reference: http://en.cppreference.com/w/cpp/thread/lock_guard
namespace {
int g_i = 0;
std::mutex g_i_mutex;  // protects g_i

void safe_increment()
{
	std::lock_guard lock(g_i_mutex);
	++g_i;

	std::cout << std::this_thread::get_id() << ": " << g_i << '\n';

	// g_i_mutex is automatically released when lock goes out of scope
}
}

int test_lock_guard_3()
{
	std::cout << "main: " << g_i << '\n';

	std::thread t1(safe_increment);
	std::thread t2(safe_increment);

	t1.join();
	t2.join();

	std::cout << "main: " << g_i << '\n';

	return 0;
}

//////////////////////////////////////////////////////////////////////
// reference: http://www.bogotobogo.com/cplusplus/C11/7_C11_Thread_Sharing_Memory.php
namespace {
// a global variable
std::list myList;

// a global instance of std::mutex to protect global variable
std::mutex myMutex;

void addToList(int max, int interval)
{
	// the access to this function is mutually exclusive
	std::lock_guard guard(myMutex);
	for (int i = 0; i < max; i++) {
		if ((i % interval) == 0) myList.push_back(i);
	}
}

void printList()
{
	// the access to this function is mutually exclusive
	std::lock_guard guard(myMutex);
	for (auto itr = myList.begin(), end_itr = myList.end(); itr != end_itr; ++itr) {
		std::cout << *itr << ",";
	}
}
}
int test_lock_guard_4()
{
	int max = 100;

	std::thread t1(addToList, max, 1);
	std::thread t2(addToList, max, 10);
	std::thread t3(printList);

	t1.join();
	t2.join();
	t3.join();

	return 0;
}

} // namespace lock_guard_

GitHub： https://github.com/fengbingchun/Messy_Test